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8-channel, 200 ksps, 12-bit adc with sequencer in 20-lead tssop ad7927 rev. b information furnished by analog devices is believed to be accurate and reliable. however, no responsibility is assumed by analog devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. specifications subject to change without notice. no license is granted by implication or otherwise under any patent or patent rights of analog devices. trademarks and registered trademarks are the property of their respective owners. one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 781.329.4700 www.analog.com fax: 781.461.3113 ?2003C2008 analog devices, inc. all rights reserved. features fast throughput rate: 200 ksps specified for av dd of 2.7 v to 5.25 v low power 3.6 mw maximum at 200 ksps with 3 v supply 7.5 mw maximum at 200 ksps with 5 v supply 8 (single-ended) inputs with sequencer wide input bandwidth 70 db minimum sinad at 50 khz input frequency flexible power/serial clock speed management no pipeline delays high speed serial interface spi?-, qspi?-, microwire?-, dsp-compatible shutdown mode: 0.5 a maximum 20-lead tssop general description the ad7927 is a 12-bit, high speed, low power, 8-channel, successive approximation adc. the part operates from a single 2.7 v to 5.25 v power supply and features throughput rates up to 200 ksps. the part contains a low noise, wide bandwidth track-and-hold amplifier that can handle input frequencies in excess of 8 mhz. the conversion process and data acquisition are controlled using cs and the serial clock signal, allowing the device to easily interface with microprocessors or dsps. the input signal is sampled on the falling edge of cs and the conversion is also initiated at this point. there are no pipeline delays associated with the part. the ad7927 uses advanced design techniques to achieve very low power dissipation at maximum throughput rates. at maximum throughput rates, the ad7927 consumes 1.2 ma maximum with 3 v supplies; with 5 v supplies, the current consumption is 1.5 ma maximum. through the configuration of the control register, the analog input range for the part can be selected as 0 v to ref in or 0 v to 2 ref in , with either straight binary or twos complement output coding. the ad7927 features eight single-ended analog inputs with a channel sequencer to allow a preprogrammed selection of channels to be converted sequentially. the conversion time for the ad7927 is determined by the sclk frequency, as this is also used as the master clock to control the conversion. the conversion time may be as short as 800 ns with a 20 mhz sclk. functional block diagram control logic agnd sclk dout din cs ad7927 v drive a v dd ref in v in 0 v in 7 input mux 03088-001 sequencer 12-bit successive approximation adc t/h figure 1. product highlights 1. high throughput with low power consumption. the ad7927 offers up to 200 ksps throughput rates. at the maximum throughput rate with 3 v supplies, the ad7927 dissipates 3.6 mw of power maximum. 2. eight single-ended inputs with a channel sequencer. a consecutive sequence of channels can be selected on which the adc cycles and converts. 3. single-supply operation with v drive function. the ad7927 operates from a single 2.7 v to 5.25 v supply. the v drive function allows the serial interface to connect directly to either 3 v or 5 v processor systems independent of av dd . 4. flexible power/serial clock speed management. the conversion rate is determined by the serial clock, allowing the conversion time to be reduced through the serial clock speed increase. the part also features various shutdown modes to maximize power efficiency at lower throughput rates. current consumption is 0.5 a maxi- mum when in full shutdown. 5. no pipeline delay. the part features a standard successive approximation adc with a cs input pin, which allows for accurate control of each sampling instant.
ad7927 rev. b | page 2 of 28 table of contents features .............................................................................................. 1 ? general description ......................................................................... 1 ? functional block diagram .............................................................. 1 ? product highlights ........................................................................... 1 ? revision history ............................................................................... 2 ? specifications ..................................................................................... 3 ? timing specifications .................................................................. 5 ? absolute maximum ratings ............................................................ 6 ? esd caution .................................................................................. 6 ? pin configuration and function descriptions ............................. 7 ? terminology ...................................................................................... 8 ? typical performance characteristics ........................................... 10 ? control register .............................................................................. 12 ? sequencer operation ...................................................................... 13 ? shadow register .............................................................................. 14 ? circuit information .................................................................... 15 ? converter operation .................................................................. 15 ? analog input ............................................................................... 15 ? adc transfer function ............................................................. 16 ? handling bipolar input signals ................................................ 16 ? typical connecti on diagram ........................................................ 17 ? analog input selection .............................................................. 17 ? digital inputs .............................................................................. 17 ? v drive ............................................................................................ 18 ? the reference ............................................................................. 18 ? modes of operation ....................................................................... 19 ? normal mode (pm1 = pm0 = 1) ............................................. 19 ? full shutdown (pm1 = 1, pm0 = 0) ........................................ 19 ? auto shutdown (pm1 = 0, pm0 = 1) ....................................... 20 ? powering up the ad7927 ......................................................... 21 ? power vs. throughput rate ....................................................... 21 ? serial interface ................................................................................ 22 ? writing between conversions .................................................. 22 ? microprocessor interfacing ........................................................... 24 ? ad7927 to tms320c541 .......................................................... 24 ? ad7927 to adsp-21xx .............................................................. 24 ? ad7927 to dsp563xx ................................................................ 25 ? application hints ........................................................................... 26 ? grounding and layout .............................................................. 26 ? evaluating the ad7927 performance ...................................... 26 ? outline dimensions ....................................................................... 27 ? ordering guide .......................................................................... 27 ? revision history 12/08rev. a to rev. b changes to esd parameter, table 3 ............................................... 6 changes to ordering guide .......................................................... 27 1/07rev. 0 to rev. a updated format .................................................................. universal updated layout ................................................................................. 8 updated layout ............................................................................... 10 changes to figure 12 caption ....................................................... 14 changes to figure 13 caption ....................................................... 15 changes to ordering guide .......................................................... 27 1/03revision 0: initial version
ad7927 rev. b | page 3 of 28 specifications av dd = v drive = 2.7 v to 5.25 v, ref in = 2.5 v, f sclk = 20 mhz; t a = t min to t max , unless otherwise noted. table 1. parameter b version 1 unit test conditions/comments dynamic performance f in = 50 khz sine wave, f sclk = 20 mhz signal-to-(noise + distortion) (sinad) 2 70 db min @ 5 v 69 db min @ 3 v typically 70 db signal-to-noise ratio (snr) 2 70 db min total harmonic distortion (thd) 2 ?77 db max @ 5 v typically ?84 db ?73 db max @ 3 v typically ?77 db peak harmonic or spurious noise ? 78 db max @ 5 v typically ?86 db (sfdr) 2 ?76 db max @ 3 v typically ?80 db intermodulation distortion (imd) 2 f a = 40.1 khz, f b = 41.5 khz second-order terms ?90 db typ third-order terms ?90 db typ aperture delay 10 ns typ aperture jitter 50 ps typ channel-to-channel isolation 2 ?82 db typ f in = 400 khz full power bandwidth 8.2 mhz typ @ 3 db 1.6 mhz typ @ 0.1 db dc accuracy 2 resolution 12 bits integral nonlinearity 1 lsb max differential nonlinearity ?0.9/+1.5 lsb max guaranteed no missed codes to 12 bits 0 v to ref in input range straight binary output coding offset error 8 lsb max typically 0.5 lsb offset error match 0.5 lsb max gain error 1.5 lsb max gain error match 0.5 lsb max 0 v to 2 ref in input range ?ref in to +ref in biased about ref in with positive gain error 1.5 lsb max twos complement output coding positive gain error match 0.5 lsb max zero code error 8 lsb max typically 0.8 lsb zero code error match 0.5 lsb max negative gain error 1 lsb max negative gain error match 0.5 lsb max analog input input voltage ranges 0 to ref in v range bit set to 1 0 to 2 ref in v range bit set to 0, av dd /v drive = 4.75 v to 5.25 v dc leakage current 1 a max input capacitance 20 pf typ f sample = 200 ksps reference input ref in input voltage 2.5 v 1% specified performance dc leakage current 1 a max ref in input impedance 36 k typ logic inputs input high voltage, v inh 0.7 v drive v min input low voltage, v inl 0.3 v drive v max input current, i in 1 a max typically 10 na, v in = 0 v or v drive input capacitance, c in 3 10 pf max
ad7927 rev. b | page 4 of 28 parameter b version 1 unit test conditions/comments logic outputs output high voltage, v oh v drive ? 0.2 v min i source = 200 a, av dd = 2.7 v to 5.25 v output low voltage, v ol 0.4 v max i sink = 200 a floating-state leakage current 1 a max floating-state output capacitance 3 10 pf max output coding straight (natural) binary coding bit set to 1 twos complement coding bit set to 0 conversion rate conversion time 800 ns max 16 sclk cycles with sclk at 20 mhz track-and-hold acquisition time 300 ns max sine wave input 300 ns max full-scale step input throughput rate 200 ksps max see serial interface section power requirements av dd 2.7/5.25 v min/max v drive 2.7/5.25 v min/max i dd 4 digital inputs = 0 v or v drive during conversion 2.7 ma max av dd = 4.75 v to 5.25 v, f sclk = 20 mhz 2 ma max av dd = 2.7 v to 3.6 v, f sclk = 20 mhz normal mode (static) 600 a typ av dd = 2.7 v to 5.25 v, sclk on or off normal mode (operational) f sample = 200 ksps 1.5 ma max av dd = 4.75 v to 5.25 v, f sclk = 20 mhz 1.2 ma max av dd = 2.7 v to 3.6 v, f sclk = 20 mhz using auto shutdown mode f sample = 200 ksps 900 a typ av dd = 4.75 v to 5.25 v, f sclk = 20 mhz 650 a typ av dd = 2.7 v to 3.6 v, f sclk = 20 mhz auto shutdown (static) 0.5 a max sclk on or off (20 na typ) full shutdown mode 0.5 a max sclk on or off (20 na typ) power dissipation 4 normal mode (operational) 7.5 mw max av dd = 5 v, f sclk = 20 mhz 3.6 mw max av dd = 3 v, f sclk = 20 mhz auto shutdown (static) 2.5 w max av dd = 5 v 1.5 w max av dd = 3 v full shutdown mode 2.5 w max av dd = 5 v 1.5 w max av dd = 3 v 1 temperature ranges as follows: b version: ?40c to +85c. 2 see terminology section. 3 sample tested @ 25c to ensure compliance. 4 see power vs. throughput rate section.
ad7927 rev. b | page 5 of 28 timing specifications 1 av dd = 2.7 v to 5.25 v, v drive av dd , ref in = 2.5 v; t a = t min to t max , unless otherwise noted. table 2. parameter limit at t min , t max ad7927 description av dd = 3 v av dd = 5 v unit f sclk 2 10 10 khz min 20 20 mhz max t convert 16 t sclk 16 t sclk t quiet 50 50 ns min minimum quiet time required between cs rising edge and start of next conversion t 2 10 10 ns min cs to sclk setup time t 3 3 35 30 ns max delay from cs until dout three-state disabled t 4 3 40 40 ns max data access time after sclk falling edge t 5 0.4 t sclk 0.4 t sclk ns min sclk low pulsewidth t 6 0.4 t sclk 0.4 t sclk ns min sclk high pulsewidth t 7 10 10 ns min sclk to dout valid hold time t 8 4 15/45 15/35 ns min/max sclk falli ng edge to dout high impedance t 9 10 10 ns min din setup time prior to sclk falling edge t 10 5 5 ns min din hold time after sclk falling edge t 11 20 20 ns min sixteenth sclk falling edge to cs high t 12 1 1 s max power-up time from full power-down/auto shutdown mode 1 sample tested at 25c to ensure compliance. all input signals are specified with tr = tf = 5 ns (10% to 90% of av dd ) and timed from a voltage level of 1.6 v, (see figure 2). the 3 v operating range sp ans from 2.7 v to 3.6 v. the 5 v operat ing range spans from 4.75 v to 5.25 v. 2 mark/space ratio for the sc lk input is 40/60 to 60/40. 3 measured with the load circuit of figure 2 and defined as the time required for the output to cross 0.4 v or 0.7 v drive . 4 t 8 is derived from the measured time taken by the data outputs to change 0.5 v when loaded with the circuit of figure 2. the meas ured number is then extrapolated back to remove the effects of charging or discharging the 50 pf capacitor. this means the time, quoted in the t 8 timing characteristics, is the true bus relinquish time of the part and is independent of the bus loading. 200a i ol 200a i oh 1.6v to output pin c l 50pf 03088-002 figure 2. load circuit for digita l output timing specifications
ad7927 rev. b | page 6 of 28 absolute maximum ratings t a = 25c, unless otherwise noted. table 3. parameter rating av dd to agnd ?0.3 v to +7 v v drive to agnd ?0.3 v to av dd + 0.3 v analog input voltage to agnd ?0.3 v to av dd + 0.3 v digital input voltage to agnd ?0.3 v to +7 v digital output voltage to agnd ?0.3 v to av dd + 0.3 v ref in to agnd ?0.3 v to av dd + 0.3 v input current to any pin except supplies 1 10 ma operating temperature range commercial (b version) ?40c to +85c storage temperature range ?65c to +150c junction temperature 150c tssop, power dissipation 450 mw ja thermal impedance 143c/w (tssop) jc thermal impedance 45c/w (tssop) lead temperature, soldering vapor phase (60 sec) 215c infrared (15 sec) 220c esd 1.5 kv 1 transient currents of up to 100 ma do not cause scr latch-up. stresses above those listed under absolute maximum ratings may cause permanent damage to the device. this is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. esd caution
ad7927 rev. b | page 7 of 28 pin configuration and fu nction descriptions 1 2 3 4 5 6 7 8 9 10 din agnd ref in av dd av dd sclk v in 6 v in 7 agnd 20 19 18 17 16 15 14 13 12 11 v drive dout agnd v in 2 v in 1 v in 0 v in 5 v in 4 v in 3 agnd ad7927 top view (not to scale) 03088-003 cs figure 3. 20-lead tssop pin configuration table 4. pin function descriptions pin no. mnemonic description 1 sclk serial clock. logic input. sclk provides the serial cloc k for accessing data from the part. this clock input is also used as the clock source for the ad7927 conversion process. 2 din data in. logic input. data to be written to the ad7927 cont rol register is provided on this input and is clocked into the register on the falling edge of sclk (see the control register section). 3 cs chip select. active low logic input. this input provid es the dual function of initiating conversions on the ad7927 and framing the serial data transfer. 4, 8, 17, 20 agnd analog ground. ground reference poi nt for all analog circuitry on th e ad7927. all analog input signals and any external reference signal should be referred to this agnd voltage. all agnd pins should be connected together. 5, 6 av dd analog power supply input. the av dd range for the ad7927 is from 2.7 v to 5.25 v. for the 0 v to 2 ref in range, av dd should be from 4.75 v to 5.25 v. 7 ref in reference input for the ad7927. an external reference mu st be applied to this input. the voltage range for the external reference is 2.5 v 1% for specified performance. 16 to 9 v in 0 to v in 7 analog input 0 through analog input 7. eight single-e nded analog input channels that are multiplexed into the on-chip track-and-hold. the analog input channel to be converted is selected by using the address bits (add2 through add0) of the control register. add2 through add0, in conjunction with the seq and shadow bits, allow the sequencer to be programmed. the input range for all input channels can extend from 0 v to ref in or 0 v to 2 ref in , as selected via the range bit in the control register. any unused input channels should be connected to agnd to avoid noise pickup. 18 dout data out. logic output. the conversion result from th e ad7927 is provided on this output as a serial data stream. the bits are clocked out on the falling edge of the sclk input. the data stream from the ad7927 consists of two leading zeros, two address bits indicating which channel the conversion result corresponds to, followed by the 12 bits of conversion data (msb first). the output coding may be selected as straight binary or twos complement via the coding bit in the control register. 19 v drive logic power supply input. the voltage supplied at this pin determines at what voltage the serial interface of the ad7927 operates.
ad7927 rev. b | page 8 of 28 terminology integral nonlinearity (inl) inl is the maximum deviation from a straight line passing through the endpoints of the adc transfer function. the end- points of the transfer function are zero scale, a point 1 lsb below the first code transition, and full scale, a point 1 lsb above the last code transition. figure 9 shows a typical inl plot for the ad7927. differential nonlinearity (dnl) dnl is the difference between the measured and the ideal 1 lsb change between any two adjacent codes in the adc. figure 10 shows a typical dnl plot for the ad7927. offset error this is the deviation of the first code transition (00 . . . 000) to (00 . . . 001) from the ideal, that is, agnd + 1 lsb. offset error match this is the difference in offset error between any two channels. gain error this is the deviation of the last code transition (111 . . . 110) to (111 . . . 111) from the ideal (that is, ref in ? 1 lsb) after the offset error has been adjusted out. gain error match this is the difference in gain error between any two channels. zero code error this applies when using the twos complement output coding option, in particular to the 2 ref in input range with ?ref in to +ref in biased about the ref in point. it is the deviation of the midscale transition (all 0s to all 1s) from the ideal v in voltage, that is, ref in ? 1 lsb. zero code error match this is the difference in zero code error between any two channels. positive gain error this applies when using the twos complement output coding option, in particular to the 2 ref in input range with ?ref in to +ref in biased about the ref in point. it is the deviation of the last code transition (011. . .110) to (011 . . . 111) from the ideal (that is, +ref in ? 1 lsb) after the zero code error has been adjusted out. positive gain error match this is the difference in positive gain error between any two channels. negative gain error this applies when using the twos complement output coding option, in particular to the 2 ref in input range with ?ref in to +ref in biased about the ref in point. it is the deviation of the first code transition (100 . . . 000) to (100 . . . 001) from the ideal (that is, ?ref in + 1 lsb) after the zero code error has been adjusted out. negative gain error match this is the difference in negative gain error between any two channels. channel-to-channel isolation channel-to-channel isolation is a measure of the level of crosstalk between channels. it is measured by applying a full- scale 400 khz sine wave signal to all seven nonselected input channels and determining how much that signal is attenuated in the selected channel with a 50 khz signal. the figure given is the worst case across all eight channels for the ad7927. power supply rejection (psr) variations in power supply affect the full-scale transition, but not the converters linearity. power supply rejection is the maximum change in full-scale transition point due to a change in power supply voltage from the nominal value (see the typical performance characteristics section). power supply rejection ration (psrr) the power supply rejection ratio is defined as the ratio of the power in the adc output at full-scale frequency (f) to the power of a 200 mv p-p sine wave applied to the adc av dd supply of frequency (f s ): psrr( db ) = 10log( pf/pf s ) where: pf is equal to the power at frequency f in adc output. pf s is equal to the power at frequency f s coupled onto the adc av dd . here a 200 mv p-p sine wave is coupled onto the av dd supply. figure 6 shows the power supply rejection ratio vs. supply ripple frequency for the ad7927 with no decoupling. track-and-hold acquisition time the track-and-hold amplifier returns into track mode at the end of conversion. track-and-hold acquisition time is the time required for the output of the track-and-hold amplifier to reach its final value, within 1 lsb, after the end of conversion.
ad7927 rev. b | page 9 of 28 signal-to-(noise + distortion) ratio (sinad) this is the measured ratio of signal-to-(noise + distortion) at the output of the adc. the signal is the rms amplitude of the fundamental. noise is the sum of all nonfundamental signals up to half the sampling frequency (f s /2), excluding dc. the ratio is dependent on the number of quantization levels in the digiti- zation process; the more levels, the smaller the quantization noise. the theoretical signal-to-(noise + distortion) ratio for an ideal n-bit converter with a sine wave input is given by signal-to-(noise + distortion) = (6.02 n + 1.76)db thus, for a 12-bit converter, this is 74 db. figure 5 shows the signal-to-(noise + distortion) ratio performance vs. input fre- quency for various supply voltages while sampling at 200 ksps with an sclk of 20 mhz. total harmonic distortion (thd) thd is the ratio of the rms sum of harmonics to the fundamental. for the ad7927, it is defined as: 1 2 6 2 5 2 4 2 3 2 2 log20)db( v vvvvv thd ++++ = where: v 1 is the rms amplitude of the fundamental. v 2 , v 3 , v 4 , v 5 , and v 6 are the rms amplitudes of the second through the sixth harmonics. figure 7 shows a graph of total harmonic distortion vs. analog input frequency for various supply voltages, and figure 8 shows a graph of total harmonic distortion vs. analog input frequency for various source impedances (see the analog input section).
ad7927 rev. b | page 10 of 28 typical performance characteristics frequency (khz) 0 snr (db) ?90 100 ?50 ?70 80 60 40 20 ?10 ?30 ?110 10 30 50 70 90 4096 point fft av dd = 4.75v f sample = 200ksps f in = 50khz sinad = 70.714db thd = ?82.853db sfdr = ?84.815db 03088-004 figure 4. dynamic performance at 200 ksps input frequency (khz) 75 01 0 0 sinad (db) 70 65 60 av dd =v drive = 5.25v av dd =v drive = 4.75v av dd =v drive = 3.6v av dd =v drive = 2.7v f sample = 200ksps t a = 25c range = 0 to ref in 03088-005 figure 5. sinad vs. analog input frequency for various supply voltages at 200 ksps supply ripple frequency (khz) 0 0 200 psrr (db) ?40 ?60 ?80 ?90 100 ?20 ?50 ?70 180160140120 80604020 ?10 ?30 av dd = 5v 200mv p-p sine wave on av dd ref in = 2.5v, 1f capacitor t a = 25c 03088-006 figure 6. psrr vs. supply ripple frequency input frequency (khz) ? 50 001 01 thd (db) ?65 ?75 ?85 ?90 ?55 ?70 ?80 ?60 f sample = 200ksps t a = 25c range = 0 to ref in 03088-007 av dd =v drive = 5.25v av dd =v drive = 4.75v av dd =v drive = 2.7v av dd =v drive = 3.6v figure 7. thd vs. analog input frequency for various supply voltages at 200 ksps
ad7927 rev. b | page 11 of 28 input frequency (khz) ? 55 thd (db) ?70 ?80 ?90 ?95 ?60 ?75 ?85 ?65 10 100 r in = 1000 ? r in = 100 ? r in = 10 ? r in = 50 ? 03088-008 f sample = 200ksps t a = 25c av dd = 5.25v range = 0 to ref in figure 8. thd vs. analog input frequency for various source impedances code 1.0 0 4096 inl error (lsb) 0 ?0.4 ?0.8 ?1.0 0.2 ?0.2 ?0.6 2048 0.6 0.4 0.8 2560 3072 3584 512 1024 1536 av dd =v drive = 5v t a = 25c 03088-009 figure 9. typical inl code 1.0 0 4096 dnl e r r or (lsb) 0 ?0.4 ?0.8 ?1.0 0.2 ?0.2 ?0.6 2048 0.6 0.4 0.8 2560 3072 3584 512 1024 1536 av dd =v drive = 5v t a = 25c 03088-010 figure 10. typical dnl
ad7927 rev. b | page 12 of 28 control register the control register on the ad7927 is a 12-bit, write-only register. data is loaded from the din pin of the ad7927 on the falli ng edge of sclk. the data is transferred on the din line at the same time that the conversion result is read from the part. the data trans ferred on the din line corresponds to the ad7927 configuration for the next conversion. this requires 16 serial clocks for every data tra nsfer. only the information provided on the first 12 falling clock edges (after cs falling edge) is loaded to the control register. msb denotes the first bit in the data stream. the bit functions are outlined in . table 5 table 5. control register bit functions msb lsb write seq dontc add2 add1 add0 pm1 pm0 shadow dontc range coding table 6. control register bit function description bit mnemonic description 11 write the value written to this bit of the control register de termines whether the following 11 bits are loaded to the control register. if this bit is a 1, the following 11 bits are written to the control register; if it is a 0, then the remaining 11 bits are not loaded to the control register and it remains unchanged. 10 seq the seq bit in the control register is used in conjunctio n with the shadow bit to cont rol the use of the sequencer function and access the shadow register (see table 10 ). 9 dontc dont care. 8 to 6 add2 to add0 these three address bits are loaded at the end of the pr esent conversion and select which analog input channel is to be converted in the next serial transfer, or they may select the final channel in a consecutive sequence as described in table 9 . the selected input channel is decoded as shown in table 7 . the address bits corresponding to the conversion result are also output on dout prior to the 12 bits of data (see the serial interface section). the next channel to be converted on is selected by the mux on the 14 th sclk falling edge. 5, 4 pm1, pm0 power management bits. these two bits decode the mode of operation of the ad7927 as shown in table 8 . 3 shadow the shadow bit in the control register is used in conjun ction with the seq bit to control the use of the sequencer function and access the shadow register (see table 10 ). 2 dontc dont care. 1 range this bit selects the analog input range to be used on the ad7927. if it is set to 0, the analog input range extends from 0 v to 2 ref in . if it is set to 1, the analog input range extends from 0 v to ref in (for the next conversion). for the 0 v to 2 ref in range, av dd = 4.75 v to 5.25 v. 0 coding this bit selects the type of output coding the ad7927 uses for the conversion result. if this bit is set to 0, the output coding for the part is twos comple ment. if this bit is set to 1, the o utput coding from the part is straight binary (for the next conversion). table 7. channel selection add2 add1 add0 analog input channel 0 0 0 v in 0 0 0 1 v in 1 0 1 0 v in 2 0 1 1 v in 3 1 0 0 v in 4 1 0 1 v in 5 1 1 0 v in 6 1 1 1 v in 7 table 8. power mode selection pm1 pm0 mode 1 1 normal operation. in this mode, the ad7927 remains in full power mode, regardless of the status of any of the logic inputs. this mode allows the fastest possible throughput rate from the ad7927. 1 0 full shutdown. in this mode, the ad7927 is in full shutdown mode with all circuitry on the ad7927 powering down. the ad7927 retains the information in the control register while in full shutdown. the part remains in full shutdown until these bits are changed. 0 1 auto shutdown. in this mode, the ad7927 au tomatically enters full shutdown mode at the end of each conversion when the control register is updated. wake-up time from full shutdown is 1 s and the user should ensure that 1 s has elapsed before attempting to perform a valid conversion on the part in this mode. 0 0 invalid selection. this configuration is not allowed.
ad7927 rev. b | page 13 of 28 sequencer operation the configuration of the seq and shadow bits in the control register allows the user to select a particular mode of operation o f the sequencer function. tabl e 9 outlines the four modes of operation of the sequencer. table 9. sequence selection seq shadow sequence type 0 0 this configuration means that the sequence function is not used. the analog input channel selected for each individual conversion is determined by the contents of the channel address bits, add0 through add2, in each prior write operation. this mode of operation reflects th e traditional operation of a multichannel adc, without the sequencer function being used, where each write to the ad7927 selects the next channel for conversion (see figure 11 ). 0 1 this configuration selects the shadow register for progra mming. the following write operation loads the contents of the shadow register. this programs the sequence of cha nnels converted on continuously with each successive valid cs falling edge (see the section, , and ). the channels selected need not be consecutive. shadow register table 10 figure 12 1 0 if the seq and shadow bits are set in this way, the sequ ence function is not interr upted upon completion of the write operation. this allows other bits in the control register to be altered between conversions while in a sequence, without terminating the cycle. 1 1 this configuration is used in conjunction with the chan nel address bits, add2 to add0, to program continuous conversions on a consecutive sequence of channels from channel 0 to a selected final channel as determined by the channel address bits in the control register (see figure 13 ).
ad7927 rev. b | page 14 of 28 shadow register table 10. shadow register bit functions msb lsb v in 0 v in 1 v in 2 v in 3 v in 4 v in 5 v in 6 v in 7 v in 0 v in 1 v in 2 v in 3 v in 4 v in 5 v in 6 v in 7 --------------------------------sequence one-------------------------------- --------------------------------sequence two------ -------------------------- the shadow register on the ad7927 is a 16-bit, write-only register. data is loaded from the din pin of the ad7927 on the falling edge of sclk. the data is transferred on the din line at the same time that a conversion result is read from the part. this requires 16 serial clock falling edges for the data transfer. the information is clocked into the shadow register, provided that the seq and shadow bits were set to 0, 1, respectively, in the previous write to the control register. msb denotes the first bit in the data stream. each bit represents an analog input from channel 0 to channel 7. through programming the shadow register, two sequences of channels may be selected, through which the ad7927 cycles with each consecutive conversion after the write to the shadow register. sequence one is per- formed first and then sequence two. if the user does not wish to perform a second sequence option, then all 0s must be written to the last eight lsbs of the shadow register. to select a sequence of channels, the associated channel bit must be set for each analog input. the ad7927 continuously cycles through the selected channels in ascending order, beginning with the lowest channel, until a write operation occurs (that is, the write bit is set to 1) with the seq and shadow bits configured in any way except 1, 0 (see table 9 ). the bit functions are outlined in table 10 . cs cs d o u t : c o n v e r s i o n r e s u l t f r o m p r e v i o u s l y s e l e c t e d c h a n n e l a 2 t o c h a n n e l a 0 . d i n : w r i t e t o c o n t r o l r e g i s t e r , w r i t e b i t = 1 , s e l e c t c o d i n g , r a n g e , a n d p o w e r m o d e . s e l e c t c h a n n e l a 2 t o c h a n n e l a 0 f o r c o n v e r s i o n . s e q = s h a d o w = 0 write bit = 1, seq = shadow = 0 dummy conversion din = all 1s 03088-011 din: write to control register, write bit = 1, select coding, range, and power mode. select channel a2 to channel a0 for conversion. seq = shadow = 0 power-on figure 11. seq bit = 0, shadow bit = 0 flowchart figure 11 reflects the traditional operation of a multichannel adc, where each serial transfer selects the next channel for conversion. in this mode of operation, the sequencer function is not used. figure 12 shows how to program the ad7927 to continuously convert on a particular sequence of channels. to exit this mode of operation and revert back to the traditional mode of opera- tion of a multichannel adc (as outlined in figure 11 ), ensure that the write bit = 1 and the seq = shadow = 0 on the next serial transfer. figure 13 shows how a sequence of consecu- tive channels can be converted on without having to program the shadow register or write to the part on each serial transfer. again, to exit this mode of operation and revert back to the traditional mode of operation of a multichannel adc (as outlined in figure 11 ), ensure the write bit = 1 and the seq = shadow = 0 on the next serial transfer. write bit = 0 write bit = 0 write bit = 1, seq = 1 shadow = 0 03088-012 write bit = 0 continuously converts on the selected sequence of channels but allows range, coding, and so on, to change in the control register without interrupt- ing the sequence, provided seq = 1, shadow = 0 write bit = 1, seq = 1, shadow = 0 continuously converts on the selected sequence of channels dout: conversion result from previously selected channel a2 to channel a0. din: write to shadow register, selecting which channels to convert on; channels selected need not be consecutive channels din: write to control register, write bit = 1, select coding, range, and power mode. select channel a2 to channel a0 for conversion. seq = 0, shadow = 1 dummy conversion din = all 1s cs cs cs power-on figure 12. seq and shadow conversion flowchart to continuously convert a sequence of channels
ad7927 rev. b | page 15 of 28 write bit = 0 03088-013 cs cs cs dummy conversion din = all 1s write bit = 1, seq = 1, shadow = 0 din: write to control register, write bit = 1, select coding, range, and power mode. select channel a2 to channel a0 for conversion. seq = 1, shadow = 1 dout: conversion result from channel 0 continuously converts on a consecutive sequence of channels from channel 0 up to and including the previously selected a2 to channel a0 in the control register continuously converts on the selected sequence of channels but allows range, coding and so on, to change in the control register without interrupting the sequence, provided seq = 1, shadow = 0 power-on figure 13. seq and shadow conversion flowchart to convert a sequence of consecutive channels circuit information the ad7927 is a high speed, 8-channel, 12-bit, single-supply adc. the part can be operated from a 2.7 v to 5.25 v supply. when operated from either a 5 v or 3 v supply, the ad7927 is capable of throughput rates of 200 ksps. the conversion time may be as short as 800 ns when provided with a 20 mhz clock. the ad7927 provides the user with an on-chip, track-and-hold adc and a serial interface housed in a 20-lead tssop. the ad7927 has eight single-ended input channels with a channel sequencer, allowing the user to select a channel sequence through which the adc can cycle with each consecutive cs falling edge. the serial clock input accesses data from the part, controls the transfer of data written to the adc, and provides the clock source for the successive approximation adc. the analog input range for the ad7927 is 0 v to ref in or 0 v to 2 ref in , depending on the status of bit 1 in the control register. for the 0 to 2 ref in range, the part must be operated from a 4.75 v to 5.25 v supply. the ad7927 provides flexible power management options to allow the user to achieve the best power performance for a given throughput rate. these options are selected by program- ming the power management bits, pm1 and pm0, in the control register. converter operation the ad7927 is a 12-bit successive approximation adc based around a capacitive dac. the ad7927 can convert analog input signals in the range 0 v to ref in or 0 v to 2 ref in . figure 14 and figure 15 show simplified schematics of the adc. the adc is comprised of control logic, sar, and a capacitive dac that are used to add and subtract fixed amounts of charge from the sampling capacitor to bring the comparator back into a bal- anced condition. figure 14 shows the adc during its acquisition phase. sw2 is closed and sw1 is in position a. the comparator is held in a balanced condition and the sampling capacitor acquires the signal on the selected v in channel. a b sw1 sw2 comparator 4k? capacitive dac control logic agnd v in 0 v in 7 03088-014 figure 14. adc acquisition phase when the adc starts a conversion (see figure 15 ), sw2 opens and sw1 moves to position b, causing the comparator to become unbalanced. the control logic and the capacitive dac are used to add and subtract fixed amounts of charge balanced condition. when the comparator is rebalanced, the conversion is complete. the control logic generates the adc output code. figure 17 and figure 18 show the adc transfer functions. a b sw1 sw2 comparator 4k ? capacitive dac control logic agnd v in 0 v in 7 03088-015 figure 15. adc conversion phase analog input figure 16 shows an equivalent circuit of the analog input struc- ture of the ad7927. the two diodes, d1 and d2, provide esd protection for the analog inputs. care must be taken to ensure that the analog input signal never exceeds the supply rails by more than 300 mv. this causes these diodes to become forward biased and start conducting current into the substrate. 10 ma is the maximum current these diodes can conduct without caus- ing irreversible damage to the part. capacitor c1, in figure 16 is typically about 4 pf and can primarily be attributed to pin capacitance. the resistor r1 is a lumped component made up of the on resistance of a switch (track-and-hold switch) and also includes the on resistance of the input multiplexer. the total resistance is typically about 400 . the capacitor, c2, is the adc sampling capacitor and has a capacitance of 30 pf typically.
ad7927 rev. b | page 16 of 28 000?000 0v analog input 111?111 000?001 000?010 111?110 ? ? 111?000 ? 011?111 ? ? 1lsb 1lsb = v ref /4096 +v ref ? 1lsb notes v ref is either ref in or 2 ref in . 03088-017 for ac applications, removing high frequency components from the analog input signal is recommended by use of an rc low- pass filter on the relevant analog input pin. in applications where harmonic distortion and signal-to-noise ratio are critical, the analog input should be driven from a low impedance source. large source impedances significantly affect the ac performance of the adc. this may necessitate the use of an input buffer amplifier. the choice of the op amp is a function of the particular application. when no amplifier is used to drive the analog input, limit the source impedance to low values. the maximum source impedance depends on the amount of thd that can be tolerated. the thd increases as the source impedance increases, and performance degrades (see figure 8 ). figure 17. straight binary transfer characteristic adc code analog input 100?000 011?111 100?001 100?010 011?110 ? ? 000?001 111?111 ? ? 000?000 1lsb = 2 v ref /4096 +v ref ? 1lsb ?v ref + 1lsb v ref ? 1lsb 03088-018 r1 d1 d2 a v dd v in c2 30pf c1 4pf conversion phase: switch open track phase: switch closed 03088-016 figure 16. equivalent analog input circuit adc transfer function the output coding of the ad7927 is either straight binary or twos complement, depending on the status of the lsb in the control register. the designed code transitions occur at successive lsb values (that is, 1 lsb, 2 lsbs, and so forth). the lsb size is ref in /4096 for the ad7927. the ideal transfer characteristic for the ad7927 when straight binary coding is selected is shown in figure 17 , and the ideal transfer characteristic for the ad7927 when twos complement coding is selected is shown in figure 18 . figure 18. twos complement transfer characteristic with ref in ref in input range handling bipolar input signals figure 19 shows how useful the combination of the 2 ref in input range and the twos complement output coding scheme is for handling bipolar input signals. if the bipolar input signal is biased about ref in and twos complement output coding is selected, then ref in becomes the zero code point, ?ref in is negative full scale and +ref in becomes positive full scale, with a dynamic range of 2 ref in . r3 r2 r4 ad7927 v dout 011?111 000?000 100?000 (= 0v) 0 v v v ref 0.1f r1 r1 = r2 = r3 = r4 av dd ref in v in 0 v in 7 v drive twos complement v dd v dd dsp/ microprocessor +ref in ref in ?ref in (= 2 ref in ) 03088-019 figure 19. handling bipolar signals
ad7927 rev. b | page 17 of 28 typical connection diagram figure 20 shows a typical connection diagram for the ad7927. in this setup, the agnd pin is connected to the analog ground plane of the system. in figure 20 , ref in is connected to a decoup- led 2.5 v supply from a reference source, the ad780, to provide an analog input range of 0 v to 2.5 v (if the range bit is 1) or 0 v to 5 v (if the range bit is 0). although the ad7927 is connected to a av dd of 5 v, the serial interface is connected to a 3 v microprocessor. the v drive pin of the ad7927 is connected to the same 3 v supply of the microprocessor to allow a 3 v logic interface (see the digital inputs section). the conversion result is output in a 16-bit word. this 16-bit data stream consists of one leading zero, three address bits indicating which channel the conversion result corresponds to, followed by the 12 bits of conversion data. for applications where power consumption is of concern, the power-down modes should be used between conversions or bursts of several conversions to improve power performance (see the modes of operation section). ad780 2.5v ad7927 av dd sclk dout din notes all unused input channels should be connected to agnd. 5 v supply 10f 0.1f serial interface microcontroller/ microprocessor v drive ref in v in 0 v in 7 ? ? ? ? ? agnd 0.1f 10f 0.1f 3v supply 0v to ref in cs 03088-020 figure 20. typical connection diagram analog input selection any one of eight analog input channels may be selected for conversion by programming the multiplexer with the address bits (add2 though add0) in the control register. the channel configurations are shown in table 7 . the ad7927 may also be configured to automatically cycle through a number of channels as selected. the sequencer feature is accessed via the seq and shadow bits in the control register (see table 9 ). the ad7927 can be programmed to continuously convert on a selection of channels in ascending order. the analog input channels to be converted on are selected through program- ming the relevant bits in the shadow register (see table 10 ). the next serial transfer then acts on the sequence programmed by executing a conversion on the lowest channel in the selection. the next serial transfer results in the conversion on the next highest channel in the sequence, and so on. it is not necessary to write to the control register once a sequencer operation has been initiated. the write bit must be set to zero or the din line tied low to ensure that the control register is not accidentally overwritten, or the sequence opera- tion interrupted. if the control register is written to at any time during the sequence, the user must ensure that the seq and shadow bits are set to 1, 0, respectively to avoid interrupting the automatic conversion sequence. this pattern continues until such time as the ad7927 is written to and the seq and shadow bits are configured with any bit combination except 1, 0. on completion of the sequence, the ad7927 sequencer returns to the first selected channel in the shadow register and commence the sequence again. rather than selecting a particular sequence of channels, a number of consecutive channels beginning with channel 0 may also be programmed via the control register alone without needing to write to the shadow register. this is possible if the seq and shadow bits are set to 1, 1, respectively. the channel address bits, add2 through add0, then determine the final channel in the consecutive sequence. the next conversion is on channel 0, then channel 1, and so on until the channel selected via the address bit add2 through address bit add0 is reached. the cycle begins again on the next serial transfer provided the write bit is set to low, or if high, that the seq and shadow bits are set to 1, 0, respectively; then the adc continues its pre- programmed automatic sequence uninterrupted. regardless of which channel selection method is used, the 16-bit word output from the ad7927 during each conversion always contains one leading zero, three channel address bits that the conversion result corresponds to, followed by the 12-bit conversion result (see the serial interface section). digital inputs the digital inputs applied to the ad7927 are not limited by the maximum ratings that limit the analog inputs. instead, the digital inputs applied can go to 7 v and are not restricted by the av dd + 0.3 v limit as on the analog inputs. another advantage of sclk, din, and cs not being restricted by the av dd + 0.3 v limit is that possible power supply sequenc- ing issues are avoided. if cs , din, or sclk are applied before av dd , there is no risk of latch-up as there would be on the analog inputs if a signal greater than 0.3 v was applied prior to av dd .
ad7927 rev. b | page 18 of 28 v drive the ad7927 also has the v drive feature. v drive controls the volt- age at which the serial interface operates. v drive allows the adc to easily interface to both 3 v and 5 v processors. for example, if the ad7927 were operated with an av dd of 5 v, the v drive pin could be powered from a 3 v supply. the ad7927 has a larger dynamic range with an av dd of 5 v while still being able to interface to 3 v processors. take care to ensure v drive does not exceed av dd by more than 0.3 v (see the absolute maximum ratings section). the reference an external reference source should be used to supply the 2.5 v reference to the ad7927. errors in the reference source result in gain errors in the ad7927 transfer function and add to the specified full-scale errors of the part. a capacitor of at least 0.1 f should be placed on the ref in pin. suitable refer- ence sources for the ad7927 include the ad780, ref192, and the ad1582. if 2.5 v is applied to the ref in pin, the analog input range can be either 0 v to 2.5 v or 0 v to 5 v, depending on the setting of the range bit in the control register.
ad7927 rev. b | page 19 of 28 modes of operation the ad7927 has a number of different modes of operation, which are designed to provide flexible power management options. these options can be chosen to optimize the power dissipation/throughput rate ratio for differing application requirements. the mode of operation of the ad7927 is con- trolled by the power management bits, pm1 and pm0, in the control register, as detailed in table 8 . when power supplies are first applied to the ad7927, care should be taken to ensure that the part is placed in the required mode of operation (see the powering up the ad7927 section). normal mode (pm1 = pm0 = 1) this mode is intended for the fastest throughput rate perform- ance because the user does not have to worry about any power- up times with the ad7927 remaining fully powered at all times. figure 21 shows the general diagram of the operation of the ad7927 in this mode. the conversion is initiated on the falling edge of cs and the track- and-hold enters hold mode as described in the section. the data presented to the ad7927 on the din line during the first 12 clock cycles of the data transfer are loaded into the control register (provided the write bit is 1). if data is to be written to the shadow register (seq = 0, shadow = 1 on the previous write), data presented on the din line during the first 16 sclk cycles is loaded into the shadow register. the part remains fully powered up in normal mode at the end of the conversion as long as pm1 and pm0 are set to 1 in the write transfer during that conversion. to ensure continued operation in normal mode, pm1 and pm0 are both loaded with 1 on every data transfer. sixteen serial clock cycles are required to complete the conversion and access the conversion result. the track-and-hold goes back into track on the 14 th sclk falling edge. serial interface cs may then idle high until the next conversion or may idle low until sometime prior to the next conversion (effectively idling cs low). for specified performance, the throughput rate should not exceed 200 ksps, which means there should be no less than 5 s between consecutive falling edges of cs when converting. the actual frequency of sclk used determines the duration of the conversion within this 5 s cycle; however, once a conversion is complete and cs has returned high, a minimum of the quiet time, t quiet , must elapse before bringing cs low again to initiate another conversion. 1 12 sclk dout din 16 1 leading zero + 3 channel identifier bits + conversion result data into control register/ shadow register notes 1. control register data is loaded on first 12 sclk cycles. 2. shadow register data is loaded on first 16 sclk cycles. cs 03088-021 figure 21. normal mode operation full shutdown (pm1 = 1, pm0 = 0) in this mode, all internal circuitry on the ad7927 is powered down. the part retains information in the control register during full shutdown. the ad7927 remains in full shutdown until the power management bits, pm1 and pm0, in the control register are changed. if a write to the control register occurs while the part is in full shutdown, with the power management bits changed to pm0 = cs rising edge. the track-and-hold that was in hold while the part was in full shutdown returns to track on the 14 th sclk falling edge. a full 16-sclk transfer must occur to ensure the control register contents are updated; however, the dout line is not driven during this wake-up transfer. to ensure that the part is fully powered up, t power up should have elapsed before the next cs falling edge; otherwise, invalid data is read if a conversion is initiated before this time. shows the general diagram for this sequence. figure 22 sclk dout din 1 1 channel identifier bits + conversion result p art begins to p o wer up on cs rising edge as pm1 = pm0 = 1 the p a rt is full y p o wered up once t power up has elapsed t 12 14 16 14 16 data in to control register/shadow register to keep the part in normal mode, load pm1 = pm0 = 1 in control register data in to control register control register is loaded on the first 12 clocks. pm1 = 1, pm0 = 1 part is in full shutdown cs 03088-022 figure 22. full shut down mode operation
ad7927 rev. b | page 20 of 28 auto shutdown (pm1 = 0, pm0 = 1) in this mode, the ad7927 automatically enters shutdown at the end of each conversion when the control register is updated. when the part is in shutdown, the track-and-hold is in hold mode. figure 23 shows the general diagram of the operation of the ad7927 in this mode. in shutdown mode all internal circuitry on the ad7927 is powered down. the part retains information in the control register during shutdown. the ad7927 remains in shutdown until the next cs falling edge it receives. on this cs falling edge, the track-and-hold that was in hold while the part was in shutdown returns to track. wake-up time from auto shutdown is 1 s maximum, and the user should ensure that 1 s has elapsed before attempting a valid conversion. when running the ad7927 with a 20 mhz clock, one dummy 16 sclk transfer should be sufficient to ensure the part is fully powered up. during this dummy transfer, the contents of the control register should remain unchanged; therefore, the write bit should be 0 on the din line. depending on the sclk frequency used, this dummy transfer may affect the achievable throughput rate of the part, with every other data transfer being a valid conversion result. if, for example, the maximum sclk frequency of 20 mhz was used, the auto shutdown mode could be used at the full throughput rate of 200 ksps without affecting the throughput rate at all. only a portion of the cycle time is taken up by the conversion time and the dummy transfer for wake-up. in this mode, the power consumption of the part is greatly reduced with the part entering shutdown at the end of each conversion. when the control register is programmed to move into auto shutdown, it does so at the end of the conversion. the user can move the adc in and out of the low power state by controlling the cs signal. 1 sclk dout din 16 1 1 16 dummy conversion channel identifier bits + conversion resu lt invalid data channel identifier bits + conversion resu lt 12 12 12 03088-023 control register is loaded on the first 12 clocks, pm1 = 0, pm0 = 1 control register should not change, write bit = 0 to keep part in this mode, load pm1 = 0, pm0 = 1 in control register or set write bit = 0 data into control/shadow register data in to control/shadow register 16 cs part enters shutdown on cs rising edge as pm1 = 0, pm0 = 1 pa r t begins to power up on cs falling edge p a r t is fully powered up pa r t enters shutdown on cs rising edge as pm1 = 0, pm0 = 1 figure 23. auto shut down mode operation
ad7927 rev. b | page 21 of 28 1 sclk dout din 16 1 1 16 dummy conversion invalid data keep din line tied high for first two dummy conversions 12 12 12 dummy conversion invalid data invalid data 16 correct v alue in cont r ol register, valid data from next conversion, user can write to shadow register in next conversion control register is loaded on the first 12 clock edges data into control register cs 03088-024 figure 24. three-dummy-conversions to place ad7927 into th e required operating mode after power supplies are applied powering up the ad7927 when supplies are first applied to the ad7927, the adc may power up in any of the operating modes of the part. to ensure that the part is placed into the required operating mode, the user should perform a dummy cycle operation as outlined in figure 24 . the three-dummy-conversion operation outlined in figure 24 must be performed to place the part into the auto shutdown mode. the first two conversions of this dummy cycle operation are performed with the din line tied high, and for the third conversion of the dummy cycle operation, the user should write the desired control register configuration to the ad7927 to place the part into the auto shutdown mode. on the third cs rising edge after the supplies are applied, the control register contains the correct information and valid data results from the next conversion. therefore, to ensure the part is placed into the correct operating mode, when supplies are first applied to the ad7927, the user must first issue two serial write operations with the din line tied high, and on the third conversion cycle the user can then write to the control register to place to part into any of the oper- ating modes. the user should not write to the shadow register until the fourth conversion cycle after the supplies are applied to the adc, to guarantee the control register contains the correct data. if the user wishes to place the part into either the normal or full shutdown mode, the second dummy cycle with din tied high can be omitted from the three-dummy-conversion operation outlined in figure 24 . power vs. throughput rate in auto shutdown mode, the average power consumption of the adc may be reduced at any given throughput rate. the power saving depends on the sclk frequency used, that is, conversion time. in some cases where the conversion time is quite a proportion of the cycle time, the throughput rate needs to be reduced to take advantage of the power-down modes. assuming a 20 mhz sclk is used, the conversion time is 800 ns, but the cycle time is 5 s when the sampling rate is at a maximum of 200 ksps. if the ad7927 is placed into shutdown for the remainder of the cycle time, then on average far less power is consumed in every cycle compared to leaving the device in normal mode. furthermore, figure 25 shows how as the through- put rate is reduced, the part remains in its shutdown longer and the average power consumption drops accordingly over time. for example, if the ad7927 is operated in a continuous sampling mode, with a throughput rate of 200 ksps and an sclk of 20 mhz (av dd = 5 v), and the device is placed in auto shutdown mode, that is, if pm1 = 0 and pm0 = 1, then the power consumption is calculated as follows. the maximum power dissipation during the conversion time is 13.5 mw (i dd = 2.7 ma maximum, av dd = 5 v). if the power- up time from auto shutdown is 1 s and the remaining conversion time is another cycle, that is, 800 ns, the ad7927 can be said to dissipate 13.5 mw for 1.8 s during each conversion cycle. for the remainder of the conversion cycle, 3.2 s, the part remains in shutdown. the ad7927 can be said to dissipate 2.5 w for the remaining 3.2 s of the conversion cycle. if the throughput rate is 200 ksps, the cycle time is 5 s and the average power dissipated during each cycle is (1.8/5) (13.5 mw) + (3.2/5) (2.5 w) = 4.8616 mw. figure 25 shows the maximum power vs. throughput rate when using the auto shutdown mode with 3 v and 5 v supplies. throughput (ksps) 10 0 200 power (mw) 0.1 0.01 80 1 100 140 180 20 40 60 120 160 av dd = 5v av dd = 3v 03088-025 figure 25. power vs. throughput rate
ad7927 rev. b | page 22 of 28 serial interface figure 26 shows the detailed timing diagram for serial inter- facing to the ad7927. the serial clock provides the conversion clock and also controls the transfer of information to and from the ad7927 during each conversion. the cs signal initiates the data transfer and conversion process. the falling edge of cs puts the track-and-hold into hold mode and takes the bus out of three-state; the analog input is sampled at this point. the conversion is also initiated at this point and requires 16 sclk cycles to complete. the track-and-hold goes back into track on the 14 th sclk falling edge as shown in at point b, except when the write is to the shadow register, in which case the track-and-hold does not return to track until the rising edge of figure 26 cs , that is, point c in . on the 16 th sclk falling edge the dout line goes back into three-state. if the rising edge of figure 27 cs occurs before 16 sclks have elapsed, the conversion is terminated and the dout line goes back into three-state and the control register is not be updated; otherwise dout returns to three-state on the 16 th sclk falling edge, as shown in . sixteen serial clock cycles are required to perform the conversion process and to access data from the ad7927. for the ad7927, the 12 bits of data are preceded by a leading zero and the three-channel address bits (add2 to add0) identifying which channel the result corresponds to. figure 26 cs going low provides the leading zero to be read in by the microcontroller or dsp. the three remaining address bits and data bits are then clocked out by subsequent sclk falling edges beginning with the first address bit (add2) thus the first falling clock edge on the serial clock has a leading zero provided and also clocks out address bit add2. the final bit in the data transfer is valid on the 16 th falling edge, having been clocked out on the previous (15 th ) falling edge. writing of information to the control register takes place on the first 12 falling edges of sclk in a data transfer, assuming the msb (that is, the write bit) has been set to 1. if the control register is programmed to use the shadow register, then the writing of information to the shadow register takes place on all 16 sclk falling edges in the next serial transfer as shown for example on the ad7927 in figure 27 . two sequence options can be pro- grammed in the shadow register. if the user does not want to program a second sequence, then the eight lsbs should be filled with zeros. the shadow register is updated upon the rising edge of cs and the track-and-hold begins to track the first channel selected in the sequence. the 16-bit word read from the ad7927 always contains a leading zero and three-channel address bits that the conversion result corresponds to, followed by the 12-bit conversion result. writing between conversions as outlined in the modes of operation section, no less than 5 s should be left between consecutive valid conversions. however, there is one case where this does not necessarily mean that at least 5 s should always be left between cs falling edges. con- sider the prior to a valid conversion. the user must write to the part to tell it to power up before it can convert successfully. once the serial write to power up has finished, it may be desirable to perform the conversion as soon as possible and not have to wait a further 5 s before bringing cs low for the conversion. in this case, as long as there is a minimum of 5 s between each valid conversion, then only the quiet time between the cs rising edge at the end of the write to power up and the next cs falling edge for a valid conversion needs to be met (see ). note that when writing to the ad7927 between these valid conversions, the dout line is not driven during the extra write operation, as shown in . figure 28 figure 28 it is critical that an extra write operation as outlined previously is never issued between valid conversions when the ad7927 is executing through a sequence function, as the falling edge of cs in the extra write would move the mux on to the next channel in the sequence. this means when the next valid conversion takes place, a channel result would have been missed. sclk dout din b zero 3 identification bits write seq dontc add2 add1 add0 dontc dontc dontc add2 add1 add0 db11 db10 db2 db1 db0 three-state t quiet 16 15 14 13 5 4 3 2 1 three- state t convert cs t 2 t 3 t 9 t 6 t 4 t 10 t 7 t 5 t 8 t 11 03088-026 figure 26. serial interface timing diagram
ad7927 rev. b | page 23 of 28 sclk dout din zero 3 identification bits c cs t 2 t 3 t 9 three- state v in 0 sequence 1 sequence 2 v in 1v in 2v in 3v in 4v in 5v in 5v in 6v in 7 t 4 t 6 t 10 t convert t 7 t 5 t 8 t 11 three-state db0 db1 db2 db10 db11 add0 add1 add2 16 15 14 13 5 4 3 2 1 03088-027 figure 27. writing to shadow register timing diagram 1 1 sclk dout power-up din va l i d data va l i d data 16 16 1 16 t quiet minimum t cycle 5s minimum cs 03088-028 figure 28. general timing diagram
ad7927 rev. b | page 24 of 28 microprocessor interfacing the serial interface on the ad7927 allows the part to be directly connected to a range of many different microprocessors. this section explains how to interface the ad7927 with some of the more common microcontroller and dsp serial interface protocols. the sport0 control register should be set up as follows: tfsw = rfsw = 1, alternate framing invrfs = invtfs = 1, active low frame signal ad7927 to tms320c541 dtype = 00, right justify data the serial interface on the tms320c541 uses a continuous serial clock and frame synchronization signals to synchronize the data transfer operations with peripheral devices like the ad7927. the cs input allows easy interfacing between the tms320c541 and the ad7927 without any glue logic required. the serial port of the tms320c541 is set up to operate in burst mode with inter- nal clkx0 (tx serial clock on serial port 0) and fsx0 (tx frame sync from serial port 0). the serial port control register (spc) must have the following setup: fo = 0, fsm = 1, mcm = 1, and txm = 1. the connection diagram is shown in . it should be noted that for signal processing applications, it is imperative that the frame synchronization signal from the tms320c541 provides equidistant sampling. the v drive pin of the ad7927 takes the same supply voltage as that of the tms320c541. this allows the adc to operate at a higher voltage than the serial interface, that is, tms320c541, if necessary. figure 29 slen = 1111, 16-bit data-words isclk = 1, internal serial clock tfsr = rfsr = 1, frame every word irfs = 0 itfs = 1 the connection diagram is shown in figure 30 . the adsp-218x has the tfs and rfs of the sport0 tied together, with tfs set as an output and rfs set as an input. the dsp operates in alter- nate framing mode and the sport0 control register is set up as described. the frame synchronization signal generated on the tfs is tied to cs , and as with all signal processing applications equidistant sampling is necessary. however, in this example, the timer interrupt is used to control the sampling rate of the adc, and under certain conditions, equidistant sampling may not be achieved. clkx clkr dr dt fsx fsr sclk dout din v drive tms320c541* v dd *additional pins removed for clarity. ad7927 * cs 03088-029 sclk dr rfs tfs dt sclk dout din adsp-218x* ad7927 * cs v drive v dd *additional pins removed for clarity. 03088-030 figure 29. interfacing to the tms320c541 ad7927 to adsp-21xx figure 30. interfacing to the adsp-218x the adsp-21xx family of dsps is interfaced directly to the ad7927 without any glue logic required. the v drive pin of the ad7927 takes the same supply voltage as that of the adsp-218x. this allows the adc to operate at a higher voltage than the serial interface, that is, adsp-218x, if necessary. the timer register, for instance, is loaded with a value that provides an interrupt at the required sample interval. when an interrupt is received, a value is transmitted with tfs or dt, (adc control word). the tfs is used to control the rfs and therefore the reading of data. the frequency of the serial clock is set in the sclkdiv register. when the instruction to transmit with tfs is given (that is, ax0 = tx0), the state of the sclk is checked. the dsp waits until the sclk has gone high, low, and high before transmission starts. if the timer and sclk values are chosen such that the instruction to transmit occurs on or near the rising edge of sclk, then the data may be transmitted or it may wait until the next clock edge.
ad7927 rev. b | page 25 of 28 for example, if the adsp-2189 had a 20 mhz crystal such that it had a master clock frequency of 40 mhz, then the master cycle time would be 25 ns. if the sclkdiv register is loaded with the value of 3, then an sclk of 5 mhz is obtained and eight master clock periods elapse for every one sclk period. depending on the throughput rate selected, if the timer registers are loaded with the value, of 803, for example, then 100.5 sclks occur between interrupts and subsequently between transmit instructions. this situation results in sampling that is not equi- distant as the transmit instruction is occurring on a sclk edge. if the number of sclks between interrupts is a whole integer figure of n, then equidistant sampling is implemented by the dsp. ad7927 to dsp563xx the connection diagram in figure 31 shows how the ad7927 can be connected to the enhanced synchronous serial interface (essi) of the dsp563xx family of dsps from motorola. each essi (two on board) is operated in synchronous mode (syn bit in crb = 1) with internally generated word length frame sync for both tx and rx (bit fsl1 = 0 and bit fsl0 = 0 in crb). normal operation of the essi is selected by making mod = 0 in the crb. set the word length to 16 by setting bits wl1 = 1 and wl0 = 0 in cra. the fsp bit in the crb should be set to 1 so the frame sync is negative. it should be noted that for signal processing applications, it is imperative that the frame synchronization signal from the dsp563xx provides equidistant sampling. in the example shown in figure 31 , the serial clock is taken from the essi so the sck0 pin must be set as an output, sckd = 1. the v drive pin of the ad7927 takes the same supply voltage as that of the dsp563xx. this allows the adc to operate at a higher voltage than the serial interface, that is, dsp563xx, if necessary. sck srd std sc2 sclk dout din ad7927 * dsp563xx* cs v drive v dd *additional pins removed for clarity. 03088-031 figure 31. interfacing to the dsp563xx
ad7927 rev. b | page 26 of 28 application hints grounding and layout the ad7927 has very good immunity to noise on the power supplies as can be seen in figure 6 . however, care should still be taken with regard to grounding and layout. the printed circuit board that houses the ad7927 should be designed such that the analog and digital sections are separated and confined to certain areas of the board. this facilitates the use of ground planes that can be separated easily. a minimum etch technique is generally best for ground planes as it gives the best shielding. all three agnd pins of the ad7927 should be sunk in the agnd plane. digital and analog ground planes should be joined at only one place. if the ad7927 is in a system where multiple devices require an agnd to dgnd connec- tion, the connection should still be made at one point only, a star ground point that should be established as close as possible to the ad7927. avoid running digital lines under the device as these couple noise onto the die. the analog ground plane should be allowed to run under the ad7927 to avoid noise coupling. the power supply lines to the ad7927 should use as large a trace as possi- ble to provide low impedance paths and reduce the effects of glitches on the power supply line. fast switching signals, like clocks, should be shielded with digital ground to avoid radiating noise to other sections of the board, and clock signals should never be run near the analog inputs. avoid crossover of digital and analog signals. traces on opposite sides of the board should run at right angles to each other. this reduces the effects of feedthrough through the board. a microstrip technique is by far the best, but is not always possible with a double-sided board. in this technique, the component side of the board is dedicated to ground planes while signals are placed on the solder side. good decoupling is also important. all analog supplies should be decoupled with 10 f tantalum in parallel with 0.1 f capaci- tors to agnd. to achieve the best from these decoupling components, they must be placed as close as possible to the device, ideally right up against the device. the 0.1 f capacitors should have low effective series resistance (esr) and effective series inductance (esi), such as the common ceramic types or surface mount types, which provide a low impedance path to ground at high frequencies to handle transient currents due to internal logic switching. evaluating the ad7927 performance the recommended layout for the ad7927 is outlined in the evaluation board for the ad7927. the evaluation board pack- age includes a fully assembled and tested evaluation board, documentation, and software for controlling the board from the pc via the evaluation-board controller. the evaluation- board controller can be used in conjunction with the ad7927 evaluation board as well as many other analog devices, inc. evaluation boards ending in the cb designator to demonstrate/ evaluate the ac and dc performance of the ad7927. the software allows the user to perform ac (fast fourier transform) and dc (histogram of codes) tests on the ad7927. the software and documentation are on a cd shipped with the evaluation board.
ad7927 rev. b | page 27 of 28 outline dimensions compliant to jedec standards mo-153-ac 20 1 11 10 6.40 bsc 4.50 4.40 4.30 pin 1 6.60 6.50 6.40 seating plane 0.15 0.05 0.30 0.19 0.65 bsc 1.20 max 0.20 0.09 0.75 0.60 0.45 8 0 coplanarit y 0.10 figure 32. 20-lead thin shrink small outline package [tssop] (ru-20) dimensions shown in millimeters ordering guide model temperature range linearity error (lsb) 1 package description package option ad7927bru ?40c to +85c 1 20-lead tssop ru-20 ad7927bru-reel ?40c to +85c 1 20-lead tssop ru-20 ad7927bruCreel7 ?40c to +85c 1 20-lead tssop ru-20 AD7927BRUZ 2 ?40c to +85c 1 20-lead tssop ru-20 AD7927BRUZ-reel 2 ?40c to +85c 1 20-lead tssop ru-20 AD7927BRUZCreel7 2 ?40c to +85c 1 20-lead tssop ru-20 eval-ad7927cbz 2 , 3 evaluation board eval-control brd2 4 controller board 1 linearity error refers to integral linearity error. 2 z = rohs compliant part. 3 this can be used as a standalone evaluation board or in conjunction with the evaluation controller board for evaluation/demons tration purposes. 4 this board is a complete unit allowing a pc to control and communicate with all analog devices evaluation boards ending in the cb designators. to order a complete evaluation kit, please order the particular adc evaluation board, for example, eval-ad7927cb, the eval-c ontrol brd2, and a 12 v ac transformer. see the relevant evaluation board application note or data sheet for more information.
ad7927 rev. b | page 28 of 28 notes ?2003C2008 analog devices, inc. all rights reserved. trademarks and registered trademarks are the prop erty of their respective owners. d03088-0-12/08(b)

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