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rev. a 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 which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of analog devices. a ad7891 one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 781/329-4700 world wide web site: http://www.analog.com fax: 781/326-8703 ? analog devices, inc., 1998 lc 2 mos 8-channel, 12-bit high speed data acquisition system functional block diagram +2.5v reference track/hold v in1a v in1b v in2a v in2b v in3a v in3b v in4a v in4b v in5a v in5b v in6a v in6b v in7a v in7b v in8a v in8b control logic wr cs rd eoc convst mode agnd agnd dgnd clock v dd v dd ad7891 12-bit adc address decode ref out/ ref in ref gnd standby m u x data/ control lines features fast 12-bit adc with 1.6 m s conversion time eight single-ended analog input channels overvoltage protection on each channel selection of input ranges: 6 5 v, 6 10 v for ad7891-1 0 to +2.5 v, 0 to +5 v, 6 2.5 v for ad7891-2 parallel and serial interface on-chip track/hold amplifier on-chip reference single supply, low power operation (85 mw max) power-down mode (75 m w typ) applications data acquisition systems motor control mobile communication base stations instrumentation general description the ad7891 is an eight-channel 12-bit data acquisition system with a choice of either parallel or serial interface structure. the part contains an input multiplexer, an on-chip track/hold ampli- fier, a high speed 12-bit adc, a +2.5 v reference and a high speed interface. the part operates from a single +5 v supply and accepts a variety of analog input ranges across two models, the ad7891-1 ( 5 v and 10 v) and the ad7891-2 (0 v to +2.5 v, 0 v to +5 v and 2.5 v). the ad7891 provides the option of either a parallel interface or serial interface structure determined by the mode pin. the part has standard control inputs and fast data access times for both the serial and parallel interfaces which ensures easy inter- facing to modern microprocessors, microcontrollers and digital signal processors. in addition to the traditional dc accuracy specifications such as linearity, full-scale and offset errors, the part is also specified for dynamic performance parameters including harmonic distortion and signal-to-noise ratio. power dissipation in normal mode is 90 mw typical while in the standby mode this is reduced to 75 m w typ. the part is available in a 44-terminal plastic quad flatpack (pqfp) and a 44-lead plastic leaded chip carrier (plcc). product highlights 1. the ad7891 is a complete monolithic 12-bit data acquisition system combining an eight-channel multiplexer, 12-bit adc, +2.5 v reference and track/hold amplifier on a single chip. 2. the ad7891-2 features a conversion time of 1.6 m s and an acquisition time of 0.4 m s. this allows a sample rate of 500 ksps when sampling one channel and 62.5 ksps when channel hopping. these sample rates can be achieved using either a software or hardware convert start. the ad7891-1 has an acquisition time of 0.6 m s when using a hardware convert start and an acquisition time of 0.7 m s when using a software convert start. these acquisition times allow sample rates of 454.5 ksps and 435 ksps respectively for hardware and software convert start. 3. each channel on the ad7891 has overvoltage protection. this means that an overvoltage on an unselected channel does not affect the conversion on a selected channel. the ad7891-1 can withstand overvoltages of 17 v.
C2C rev. a ad7891Cspecifications (v dd = +5 v 6 5%, agnd = dgnd = 0 v, ref in = +2.5 v. all specifications t min to t max unless otherwise noted.) aby parameter version 1 version version units test conditions/comments dynamic performance 2 sample rate = 454.5 ksps 3 (ad7891-1), 500 ksps 3 (ad7891-2). any channel signal to (noise + distortion) ratio 4 @ +25 c 707070db min t min to t max 70 70 70 db min total harmonic distortion 4 C78 C78 C78 db max peak harmonic or spurious noise 4 C80 C80 C80 db max intermodulation distortion 4 fa = 9 khz, fb = 9.5 khz 2nd order terms C80 C80 C80 db typ 3rd order terms C80 C80 C80 db typ channel-to-channel isolation 4 C80 C80 C80 db max dc accuracy any channel resolution 12 12 12 bits minimum resolution for which no missing codes are guaranteed 12 12 12 bits relative accuracy 4 1 0.75 1 lsb max differential nonlinearity 4 1 1 1 lsb max positive full-scale error 4 3 3 3 lsb max positive full-scale error match 4, 5 0.6 0.6 0.6 lsb typ 1.5 lsb max unipolar offset error 3 3 3 lsb max input ranges of 0 v to +2.5 v, 0 v to +5 v unipolar offset error match 5 0.1 0.1 0.1 lsb typ 1 lsb max negative full-scale error 4 3 3 3 lsb max input ranges of 2.5 v, 5 v, 10 v negative full-scale error match 4, 5 0.6 0.6 0.6 lsb typ 1.5 lsb max bipolar zero error 4 4 4 lsb max input ranges of 2.5 v, 5 v, 10 v bipolar zero error match 5 0.2 0.2 0.2 lsb typ 1.5 lsb max analog inputs ad7891-1 input voltage range 5 5 5 volts input applied to both v inxa and v inxb 10 10 10 volts input applied to v inxa , v inxb = agnd ad7891-1 v inxa input resistance 7.5 7.5 7.5 k w min input range of 5 v ad7891-1 v inxa input resistance 15 15 15 k w min input range of 10 v ad7891-2 input voltage range 0 to +2.5 0 to +2.5 0 to +2.5 volts input applied to both v inxa and v inxb 0 to +5 0 to +5 0 to +5 volts input applied to v inxa , v inxb = agnd 2.5 2.5 2.5 volts input applied to v inxa , v inxb = ref in 6 ad7891-2 v inxa input resistance 1.5 1.5 1.5 k w min input ranges of 2.5 v and 0 to +5 v ad7891-2 v inxa input current 50 50 50 na max input range of 0 v to +2.5 v reference input/output ref in input voltage range 2.375/2.625 2.375/2.625 2.375/2.625 v min/v max 2.5 v 5% input impedance 1.6 1.6 1.6 k w min resistor connected to internal reference node input capacitance 5 10 10 10 pf max ref out output voltage 2.5 2.5 2.5 v nom ref out error @ +25 c 10 10 10 mv max t min to t max 20 20 20 mv max ref out temperature coefficient 25 25 25 ppm/ c typ ref out output impedance 5 5 5 k w nom see ref in input impedance logic inputs input high voltage, v inh 2.4 2.4 2.4 v min v dd = 5 v 5% input low voltage, v inl 0.8 0.8 0.8 v max v dd = 5 v 5% input current, i inh 10 10 10 m a max input capacitance, 5 c in 10 10 10 pf max
C3C rev. a ad7891 aby parameter version 1 version version units test conditions/comments logic outputs output high voltage, v oh 4.0 4.0 4.0 v min i source = 200 m a output low voltage, v ol 0.4 0.4 0.4 v max i sink = 1.6 ma db11Cdb0 floating-state leakage current 10 10 10 m a max floating-state capacitance 5 15 15 15 pf max output coding straight (natural) binary data format bit of control register = 0 2s complement data format bit of control register = 1 conversion rate conversion time 1.6 1.6 1.6 m s max track/hold acquisition time 0.6 0.6 0.6 m s max ad7891-1 hardware conversion 0.7 0.7 0.7 m s max ad7891-1 software conversion 0.4 0.4 0.4 m s max ad7891-2 power requirements v dd +5 +5 +5 v nom 5% for specified performance i dd normal mode 17 17 18 ma max standby mode 80 80 80 m a max logic inputs = 0 v or v dd power dissipation v dd = 5 v normal mode 85 85 90 mw max typically 70 mw standby mode 400 400 400 m w max typically 75 m w notes 1 temperature ranges for the a and b versions: C40 c to +85 c. temperature range for the y version: C55 c to +105 c. 2 the ad7891-1s dynamic performance (thd and snr) and the ad7891-2s thd are measured with an input frequency of 10 khz. the ad7 891-2s snr is evaluated with an input frequency of 100 khz. 3 this throughput rate can only be achieved when the part is operated in the parallel interface mode. maximum achievable throughp ut rate in the serial interface mode is 357 ksps. 4 see terminology. 5 sample tested during initial release and after any redesign or process change that may affect this parameter. 6 ref in must be buffered before being applied to v inxb . specifications subject to change without notice. absolute maximum ratings* (t a = +25 c unless otherwise noted) v dd to agnd . . . . . . . . . . . . . . . . . . . . . . . . . C0.3 v to +7 v v dd to dgnd . . . . . . . . . . . . . . . . . . . . . . . . . C0.3 v to +7 v analog input voltage to agnd ad7891-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 v ad7891-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . C5 v, +10 v reference input voltage to agnd . . . . C0.3 v to v dd + 0.3 v digital input voltage to dgnd . . . . . . C0.3 v to v dd + 0.3 v digital output voltage to dgnd . . . . . C0.3 v to v dd + 0.3 v operating temperature range commercial (a, b version) . . . . . . . . . . . . C 40 c to +85 c automotive (y version) . . . . . . . . . . . . . . C55 c to +105 c storage temperature range . . . . . . . . . . . . C65 c to +150 c junction temperature . . . . . . . . . . . . . . . . . . . . . . . . . +150 c pqfp package, power dissipation . . . . . . . . . . . . . . 450 mw q ja thermal impedance . . . . . . . . . . . . . . . . . . . . . . 95 c/w lead temperature, soldering vapor phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . . +215 c infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . +220 c plcc package, power dissipation . . . . . . . . . . . . . . 500 mw q ja thermal impedance . . . . . . . . . . . . . . . . . . . . . . 55 c/w lead temperature, soldering vapor phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . . +215 c infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . +220 c *stresses above those listed under absolute maximum ratings may cause perma- nent damage to the device. this is a stress rating only; functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. caution esd (electrostatic discharge) sensitive device. electrostatic charges as high as 4000 v readily accumulate on the human body and test equipment and can discharge without detection. although the ad7891 features proprietary esd protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. therefore, proper esd precautions are recommended to avoid performance degradation or loss of functionality. warning! esd sensitive device
ad7891 C4C rev. a timing characteristics 1, 2 parameter a, b, y versions units test conditions/comments t conv 1.6 m s max conversion time parallel interface t 1 0 ns min cs to rd / wr setup time t 2 35 ns min write pulsewidth t 3 25 ns min data valid to write setup time t 4 5 ns min data valid to write hold time t 5 0 ns min cs to rd / wr hold time t 6 35 ns min convst pulsewidth t 7 55 ns min eoc pulsewidth t 8 35 ns min read pulsewidth t 9 3 25 ns min data access time after falling edge of rd t 10 4 5 ns min bus relinquish time after rising edge of rd 30 ns max serial interface t 11 30 ns min rfs low to sclk falling edge setup time t 12 3 20 ns max rfs low to data valid delay t 13 25 ns min sclk high pulsewidth t 14 25 ns min sclk low pulsewidth t 15 3 5 ns min sclk rising edge to data valid hold time t 16 3 15 ns max sclk rising edge to data valid delay t 17 20 ns min rfs to sclk falling edge hold time t 18 4 0 ns min bus relinquish time after rising edge of rfs 30 ns max t 18a 4 0 ns min bus relinquish time after rising edge of sclk 30 ns max t 19 20 ns min tfs low to sclk falling edge setup time t 20 15 ns min data valid to sclk falling edge setup time t 21 10 ns min data valid to sclk falling edge hold time t 22 30 ns min tfs low to sclk falling edge hold time notes 1 sample tested during initial release and after any redesign or process change that may affect this parameter. all input signals are measured with tr = tf = 1 ns (10% to 90% of +5 v) and timed from a voltage level of +1.6 v. 2 see figures 2, 3 and 4. 3 measured with the load circuit of figure 1 and defined as the time required for an output to cross 0.8 v or 2.4 v. 4 these times are derived from the measured time taken by the data outputs to change 0.5 v when loaded with the circuit of figure 1. the measured number is then extrapolated back to remove the effects of charging or discharging the 50 pf capacitor. this means that the times quoted in the timing characteristics are the true bus relinquish times of the part and as such are independent of external bus loading capacitances. specifications subject to change without notice. 1.6ma 200 m a +1.6v to output pin 50pf figure 1. load circuit for access time and bus relinquish time
ad7891 C5C rev. a ordering guide model input ranges sample rate relative accuracy temperature range package options* AD7891AS-1 5 v or 10 v 454 ksps 1 lsb C40 c to +85 cs-44 ad7891ap-1 5 v or 10 v 454 ksps 1 lsb C40 c to +85 c p-44a ad7891bs-1 5 v or 10 v 454 ksps 0.75 lsb C40 c to +85 cs-44 ad7891bp-1 5 v or 10 v 454 ksps 0.75 lsb C40 c to +85 c p-44a ad7891ys-1 5 v or 10 v 454 ksps 1 lsb C55 c to +105 cs-44 ad7891yp-1 5 v or 10 v 454 ksps 1 lsb C55 c to +105 c p-44a ad7891as-2 0 v to +5 v, 0 v to +2.5 v 500 ksps 1 lsb C40 c to +85 cs-44 or 2.5 v ad7891ap-2 0 v to +5 v, 0 v to +2.5 v 500 ksps 1 lsb C40 c to +85 c p-44a or 2.5 v ad7891bs-2 0 v to +5 v, 0 v to +2.5 v 500 ksps 0.75 lsb C40 c to +85 cs-44 or 2.5 v ad7891bp-2 0 v to +5 v, 0 v to +2.5 v 500 ksps 0.75 lsb C40 c to +85 c p-44a or 2.5 v ad7891ys-2 0 v to +5 v, 0 v to +2.5 v 500 ksps 1 lsb C55 c to +105 cs-44 or 2.5 v *s = plastic quad flatpack (pqfp); p = plastic leaded chip carrier (plcc). pin configurations plcc 7 8 9 10 11 12 13 14 15 16 17 6 5 4 3 2 1 44 43 42 41 40 39 38 37 36 35 34 33 32 31 30 29 18 19 20 21 22 23 24 25 26 27 28 pin 1 identifier top view (not to scale) nc = no connect v in6a agnd eoc nc convst cs ref gnd nc ref out/ref in v dd agnd mode db11/test db10/test db9/ tfs db8/ rfs db7/data in standby v in1a db6/sclk v dd dgnd db5/a2/data out db3/a0 db2/swcon db1/swstby db0/format wr rd db4/a1 ad7891 plcc v in1b v in2a v in2b v in3a v in3b v in4a v in4b v in5a v in5b v in6b v in7a v in7b v in8a v in8b pqfp v in6a agnd eoc nc convst cs ref gnd nc ref out/ref in v dd agnd mode db11/test db10/test db9/ tfs db8/ rfs db7/data in standby v in1a db6/sclk v dd dgnd db5/a2/data out db3/a0 db2/swcon db1/swstby db0/format wr rd db4/a1 v in1b v in2a v in2b v in3a v in3b v in4a v in4b v in5a v in5b v in6b v in7a v in7b v in8a v in8b nc = no connect 44 43 42 41 40 39 38 37 36 35 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 33 32 31 30 29 28 27 26 25 24 23 pin 1 identifier top view (not to scale) ad7891 pqfp
ad7891 C6C rev. a terminology signal to (noise + distortion) ratio this is the measured ratio of signal to (noise + distortion) at the output of the a/d converter. the signal is the rms amplitude of the fundamental. noise is the rms sum of all nonfundamental signals up to half the sampling frequency (f s /2), excluding dc. the ratio is dependent upon the number of quantization levels in the digitization process; the more levels, the smaller the quan- tization 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) = (6.02n + 1.76) db thus for a 12-bit converter, this is 74 db. total harmonic distortion total harmonic distortion (thd) is the ratio of the rms sum of harmonics to the fundamental. for the ad7891 it is defined as: thd db vvvvv v () = ++++ 20 2 2 3 2 4 2 5 2 6 2 1 log where v 1 is the rms amplitude of the fundamental and v 2 , v 3 , v 4 , v 5 and v 6 are the rms amplitudes of the second through the sixth harmonics. peak harmonic or spurious noise peak harmonic or spurious noise is defined as the ratio of the rms value of the next largest component in the adc output spectrum (up to f s /2 and excluding dc) to the rms value of the fundamental. normally, the value of this specification is deter- mined by the largest harmonic in the spectrum, but for parts where the harmonics are buried in the noise floor, it will be a noise peak. intermodulation distortion with inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities will create distortion products at sum and difference frequencies of mfa nfb where m, n = 0, 1, 2, 3, etc. intermodulation terms are those for which neither m nor n are equal to zero. for example, the second order terms include (fa + fb) and (fa C fb), while the third order terms include (2fa + fb), (2fa C fb), (fa + 2fb) and (fa C 2fb). the ad7891 is tested using the ccif standard where two input frequencies near the top end of the input bandwidth are used. in this case, the second and third order terms are of differ- ent significance. the second order terms are usually distanced in frequency from the original sine waves while the third order terms are usually at a frequency close to the input frequencies. as a result, the second and third order terms are specified sepa- rately. the calculation of the intermodulation distortion is as per the thd specification where it is the ratio of the rms sum of the individual distortion products to the rms amplitude of the fundamental expressed in dbs. 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 20 khz (ad7891-1) or 100 khz (ad7891-2) sine wave signal to one input channel and determining how much that signal is attenuated in each of the other channels. the figure given is the worst case across all eight channels. relative accuracy relative accuracy or endpoint nonlinearity is the maximum deviation from a straight line passing through the endpoints of the adc transfer function. differential nonlinearity this is the difference between the measured and the ideal 1 lsb change between any two adjacent codes in the adc. positive full-scale error (ad7891-1, 10 v and 5 v, ad7891-2, 2.5 v) this is the deviation of the last code transition (01. . .110 to 01. . .111) from the ideal 4 ref in C 3/2 lsb (ad7891-1 10 v range), 2 ref in C 3/2 lsb (ad7891-1 5 v range) or ref in C 3/2 lsb (ad7891-2, 2.5 v range), after the bipolar zero error has been adjusted out. positive full-scale error (ad7891-2, 0 v to 5 v and 0 v to 2.5 v) this is the deviation of the last code transition (11. . .110 to 11. . .111) from the ideal 2 ref in C 3/2 lsb (0 v to 5 v range) or ref in C 3/2 lsb (0 v to 2.5 v range), after the unipolar offset error has been adjusted out. bipolar zero error (ad7891-1, 10 v and 5 v, ad7891-2 , 2.5 v) this is the deviation of the midscale transition (all 0s to all 1s) from the ideal agnd C 1/2 lsb. unipolar offset error (ad7891-2, 0 v to 5 v and 0 v to 2.5 v) this is the deviation of the first code transition (00. . .000 to 00. . .001) from the ideal agnd + 1/2 lsb. negative full-scale error (ad7891-1, 10 v and 5 v, ad7891-2, 2.5 v) this is the deviation of the first code transition (10. . .000 to 10. . .001) from the ideal C4 ref in + 1/2 lsb (ad7891-1 10 v range), C2 ref in + 1/2 lsb (ad7891-1 5 v range) or Cref in + 1/2 lsb (ad7891-2, 2.5 v range), after bipolar zero error has been adjusted out. track/hold acquisition time track/hold acquisition time is the time required for the output of the track/hold amplifier to reach its final value, within 1/2 lsb, after the end of conversion (the point at which the track/hold returns to track mode). it also applies to situations where a change in the selected input channel takes place or where there is a step input change on the input voltage applied to the selected v in input of the ad7891. it means that the user must wait for the duration of the track/hold acquisition time after the end of conversion or after a channel change/step input change to v in before starting another conversion, to ensure that the part operates to specification.
ad7891 C7C rev. a pin function descriptions mnemonic description v inxa , v inxb analog input channels. the ad7891 contains eight pairs of analog input channels. each channel con- tains two input pins to allow a number of different input ranges to be used with the ad7891. there are two possible input voltage ranges on the ad7891-1. the 5 v input range is selected by connecting the input voltage to both v inxa and v inxb , while the 10 v input range is selected by applying the input voltage to v inxa and connecting v inxb to agnd. the ad7891-2 has three possible input ranges. the 0 v to +2.5 v input range is selected by connecting the analog input voltage to both v inxa and v inxb ; the 0 v to +5 v input range is selected by applying the input voltage to v inxa and connecting v inxb to agnd while the 2.5 v input range is selected by connecting the analog input voltage to v inxa and connecting v inxb to ref in (provided this ref in voltage comes from a low impedance source). the channel to be converted is selected by the a2, a1 and a0 bits of the control register. in the parallel interface mode, these bits are available as three data input lines (db3 to db5) in a parallel write operation while in the serial interface mode, these three bits are accessed via the data in line in a serial write operation. the multiplexer has guaranteed break-before-make operation. v dd positive supply voltage, +5 v 5%. agnd analog ground. ground reference for track/hold, comparator and dac. dgnd digital ground. ground reference for digital circuitry. standby standby mode input. ttl-compatible input which is used to put the device into the power save or standby mode. the standby input is high for normal operation and low for standby operation. ref out/ref in voltage reference output/input. the part can either be used with its own internal reference or with an external reference source. the on-chip +2.5 v reference voltage is provided at this pin. when using this internal reference as the reference source for the part, ref out should be decoupled to ref gnd with a 0.1 m f disc ceramic capacitor. the output impedance of the reference source is typically 2 k w . when using an external reference source as the reference voltage for the part, the reference source should be connected to this pin. this overdrives the internal reference and provides the reference source for the part. the reference pin is buffered on-chip but must be able to sink or source current through this 2 k w resistor to the output of the on-chip reference. the nominal reference voltage for correct operation of the ad7891 is +2.5 v. ref gnd reference ground. ground reference for the parts on-chip reference buffer. the ref out pin of the part should be decoupled with a 0.1 m f capacitor to this ref gnd pin. if the ad7891 is used with an external reference, the external reference should also be decoupled to this pin. the ref gnd pin should be connected to the agnd pin or the systems agnd plane. convst convert start. edge-triggered logic input. a low to high transition on this input puts the track/hold into hold and initiates conversion. when changing channels on the part, sufficient time should be given for multiplexer settling and track/hold acquisition between the channel change and the rising edge of convst . eoc end-of-conversion. active low logic output indicating converter status. the end of conversion is signified by a low-going pulse on this line. the duration of this eoc pulse is nominally 80 ns. mode interface mode. control input which determines the interface mode for the part. with this pin at a logic low, the ad7891 is in its serial interface mode; with this pin at a logic high, the device is in its parallel interface mode. nc no connect. the two nc pins on the device can be left unconnected. if they are to be connected to a voltage it should be to ground potential. to ensure correct operation of the ad7891, neither of the nc pins should be connected to a logic high potential.
ad7891 C8C rev. a parallel interface mode functions mnemonic description cs chip select input. active low logic input which is used in conjunction with rd to enable the data outputs and with wr to allow input data to be written to the part. rd read input. active low logic input which is used in conjunction with cs low to enable the data outputs. wr write input. active low, logic input used in conjunction with cs to latch the multiplexer address and software control information. the rising edge of this input also initiates an internal pulse. when using the software start facility, this pulse delays the point at which the track/hold goes into hold and con- version is initiated. this allows the multiplexer to settle and acquisition time of the track/hold to elapse when a channel address is changed. if the swcon bit of the control register is set to 1, when this pulse times out, the track/hold then goes into hold and conversion is initiated. if the swcon bit of the control register is set to 0 the track/hold and conversion sequence are unaffected by the wr operation. data i/o lines there are 12 data input/output lines on the ad7891. when the part is configured for parallel mode (mode = 1), the output data from the part is provided at these 12 pins during a read operation. for a write operation in parallel mode, these lines provide access to the parts control register. parallel read operation during a parallel read operation the 12 lines become the 12 data bits containing the conversion result from the ad7891. these d ata bits are labelled data bit 0 (lsb) to data bit 11 (msb). they are three-state ttl-compatible outputs. output data coding is two s complement when the data format bit of the control register is 1 and straight binary when the data format bit of the control register is 0. mnemonic description db0Cdb11 data bit 0 (lsb) to data bit 11 (msb). three-state ttl-compatible outputs which are controlled by the cs and rd inputs. parallel write operation during a parallel write operation the following functions can be written to the control register via the 12 data input/output p ins. mnemonic description a0 address input. the status of this input during a parallel write operation is latched to the a0 bit of the control register (see control register section). a1 address input. the status of this input during a parallel write operation is latched to the a1 bit of the control register (see control register section). a2 address input. the status of this input during a parallel write operation is latched to the a2 bit of the control register (see control register section). swcon software conversion start. the status of this input during a parallel write operation is latched to the swconv bit of the control register (see control register section). swstby software standby control. the status of this input during a parallel write operation is latched to the swstby bit of the control register (see control register section). format data format selection. the status of this input during a parallel write operation is latched to the format bit of the control register (see control register section).
ad7891 C9C rev. a serial interface mode functions when the part is configured for serial mode (mode = 0), five of the 12 data input/output lines provide serial interface functio ns. these functions are outlined below. mnemonic description sclk serial clock input. this is an externally applied serial clock which is used to load serial data to the control register and to access data from the output register. tfs transmit frame synchronization pulse. active low logic input with serial data expected after the falling edge of this signal. rfs receive frame synchronization pulse. this is an active low logic input with rfs provided externally as a strobe or framing pulse to access serial data from the output register. for applications that require that data be transmitted and received at the same time, rfs and tfs should be connected together. data out serial data output. sixteen bits of serial data are provided with the data format bit and the three address bits of the control register preceding the 12 bits of conversion data. serial data is valid on the falling edge of sclk for sixteen edges after rfs goes low. output conversion data coding is twos comple- ment when the format bit of the control register is 1 and straight binary when the format bit of the control register is 0. data in serial data input. serial data to be loaded to the control register is provided at this input. the first six bits of serial data are loaded to the control register on the first six falling edges of sclk after tfs goes low. serial data on subsequent sclk edges is ignored while tfs remains low. test test pin. when the device is configured for serial mode of operation, two of the pins which had been data inputs become test inputs. to ensure correct operation of the device, both test inputs should be tied to a logic low potential. control register the control register for the ad7891 contains 6 bits of information as described below. these 6 bits can be written to the contr ol register either in a parallel mode write operation or via a serial mode write operation. the default (power-on) condition of al l bits in the control register is 0. six serial clock pulses must be provided to the part in order to write data to the control register. if tfs re- turns high before six serial clock cycles then no data transfer takes place to the control register and the write cycle will ha ve to be restarted to write data to the control register. however, if the swconv bit of the register was previously set to a logic 1 and tfs is brought high before six serial clock cycles, then another conversion will be initiated. b s m 2 a1 a0 av n o c w sy b t s w st a m r o f a2 address input. this input is the most significant address input for multiplexer channel selection. a1 address input. this is the second most significant address input for multiplexer channel selection. a0 address input. least significant address input for multiplexer channel selection. when the address is written to the control register, an internal pulse is initiated to allow for the multiplexer settling time and track/hold acquisi- tion time before the track/hold goes into hold and conversion is initiated. when the internal pulse times out, the track/hold goes into hold and conversion is initiated. the selected channel is given by the formula: a 2 4 + a 1 2 + a 0 + 1 swconv conversion start. writing a 1 to this bit initiates a conversion in a similar manner to the convst input. con- tinuous conversion starts do not take place when there is a 1 in this location. the internal pulse and the conver- sion process are initiated when a 1 is written to this bit. with a 1 in this bit, the hardware conversion start, i.e., the convst input, is disabled. writing a 0 to this bit enables the hardware convst input. swstby standby mode input. writing a 1 to this bit places the device in its standby or power-down mode. writing a 0 to this bit places the device in its normal operating mode. format data format. writing a 0 to this bit sets the conversion data output format to straight (natural) binary. this data format is generally be used for unipolar input ranges. writing a 1 to this bit sets the conversion data output format to twos complement. this output data format is generally used for bipolar input ranges.
ad7891 C10C rev. a converter details the ad7891 is an eight-channel, high speed, 12-bit data acqui- sition system. it provides the user with signal scaling, multi- plexer, track/hold, reference, a/d converter and high speed parallel and serial interface logic functions on a single chip. the signal conditioning on the ad7891-1 allows the part to accept analog input ranges of 5 v or 10 v when operating from a single supply. the input circuitry on the ad7891-2 allows the part to handle input signal ranges of 0 v to +2.5 v, 0 v to +5 v and 2.5 v again while operating from a single +5 v supply. the part requires a +2.5 v reference which can be provided from the parts own internal reference or from an external refer- ence source. conversion is initiated on the ad7891 either by pulsing the convst input or by writing a logic 1 to the swconv bit of the control register. when using the hardware convst input, the on-chip track/hold goes from track to hold mode and the conversion sequence is started on the rising edge of the convst signal. when a software conversion start is initiated, an internal pulse is generated which delays the track/hold acquisition point and the conversion start sequence until the pulse is timed out. this internal pulse is initiated (goes from low to high) whenever a write to the ad7891 control register takes place with a 1 in the swconv bit. it then starts to discharge and the track/hold cannot go into hold and conversion cannot be initiated until the pulse signal goes low. the conversion clock for the part is internally generated and conversion time for the ad7891 is 1.6 m s from the rising edge of the hardware convst signal. the track/hold acquisition time for the ad7891-1 is 600 ns while the track/hold acquisition time for the ad7891-2 is 400 ns. to obtain optimum performance from the part, the data read operation should not occur during the conversion or during 100 ns prior to the next conversion. this allows the ad7891-1 to operate at throughput rates up to 454.5 ksps and the ad7891-2 at throughput rates up to 500 ksps in the parallel mode and achieve data sheet specifi- cations. in the serial mode, the maximum achievable through- put rate for both the ad7891-1 and the ad7891-2 is 357 ksps (assuming a 20 mhz serial clock). all unused analog inputs should be tied to a voltage within the nominal analog input range to avoid noise pickup. for mini- mum power consumption, the unused analog inputs should be tied to agnd. interface information the ad7891 provides two interface options, a 12-bit parallel interface and a high speed serial interface. the required inter- face mode is selected via the mode pin. the two interface modes are discussed in the following sections. parallel interface mode the parallel interface mode is selected by tying the mode input to a logic high. figure 2 shows a timing diagram illustrating the operational sequence of the ad7891 in parallel mode for a hardware conversion start. the multiplexer address is written to the ad7891 on the rising edge of the wr input. the on-chip track/hold goes into hold mode on the rising edge of convst and conversion is also initiated at this point. when the conversion is complete, the end of conversion line ( eoc ) pulses low to indicate that new data is available in the ad7891s output regis- ter. this eoc line can be used to drive an edge-triggered inter- rupt of a microprocessor. cs and rd going low accesses the 12-bit conversion result. in systems where the part is interfaced to a gate array or asic, this eoc pulse can be applied to the cs and rd inputs to latch data out of the ad7891 and into the gate array or asic. this means that the gate array or asic does not need any conversion status recognition logic and it also elimi- nates the logic required in the gate array or asic to generate the read signal for the ad7891. t 6 t 7 t conv t 1 t 5 t 8 t 1 t 5 t 2 valid data output valid data output t 9 t 10 t 3 t 4 convst (i) eoc (o) cs (o) wr (i) rd (i) db0 - db11 (i/o) note i - input o = output figure 2. parallel mode timing diagram
ad7891 C11C rev. a serial interface mode the serial interface mode is selected by tying the mode input to a logic low. in this case, five of the data/control inputs of the parallel mode assume serial interface functions. the serial interface on the ad7891 is a five-wire interface with read and write capabilities, with data being read from the output register via the data out line and data being written to the control register via the data in line. the part operates in a slave or external clocking mode and requires an externally ap- plied serial clock to the sclk input to access data from the data register or write data to the control register. there are separate framing signals for the read ( rfs ) and write ( tfs ) operations. the serial interface on the ad7891 is designed to allow the part to be interfaced to systems that provide a serial clock that is synchronized to the serial data, such as the 80c51, 87c51, 68hc11 and 68hc05 and most digital signal processors. when using the ad7891 in serial mode, the data lines db11C db10 should be tied to logic low, and the cs , wr and rd inputs should be tied to logic high. pins db4Cdb0 can be tied to either logic high or logic low, but must not be left floating as this condition could cause the ad7891 to draw large amounts of current. read operation figure 3 shows the timing diagram for reading from the ad7891 in serial mode. rfs goes low to access data from the ad7891. the serial clock input does not have to be continuous. the serial data can be accessed in a number of bytes. however, rfs must remain low for the duration of the data transfer operation. six- teen bits of data are transmitted in serial mode with the data format bit first, followed by the three address bits in the control register, followed by the 12-bit conversion result starting with the msb. serial data is clocked out of the device on the rising edge of sclk and is valid on the falling edge of sclk. at the end of the read operation, the data out line is three- stated by a rising edge on either the sclk or rfs inputs, w hich- ever occurs first. write operation figure 4 shows a write operation to the control register of the ad7891. the tfs input goes low to indicate to the part that a serial write is about to occur. the ad7891 control register requires only six bits of data. these are loaded on the first six clock cycles of the serial clock with data on all subsequent clock cycles being ignored. serial data to be written to the ad7891 must be valid on the falling edge of sclk. simplifying the serial interface to minimize the number of interconnect lines to the ad7891 in serial mode, the user can connect the rfs and tfs lines of the ad7891 together and read and write from the part simulta- neously. in this case, new control register data line selecting the input channel and providing a conversion start command should be provided on the data in line, while the part provides the result from the conversion just completed on the data out line. data out (o) sclk (i) rfs (i) t 18a note i = input o = output format a2 a1 a0 db11 db10 db0 3-state t 18 t 16 t 15 t 14 t 12 t 11 t 13 t 17 figure 3. serial mode read operation data in (i) sclk (i) tfs (i) note i = input format a0 a1 a0 t 19 t 22 t 21 t 20 conv stby dont care dont care figure 4. serial mode write operation
ad7891 C12C rev. a circuit description reference the ad7891 contains a single reference pin labelled ref out/ ref in, w hich either provides access to the parts own +2.5 v internal reference or to which an external +2.5 v reference can be connected to provide the reference source for the part. the part is specified with a +2.5 v reference voltage. errors in the reference source will result in gain errors in the transfer function of the ad7891 and will add to the specified full scale errors on the part. they will also result in an offset error injected into the attenuator stage. the ad7891 contains an on-chip +2.5 v reference. to use this reference as a reference source for the ad7891, simply connect a 0.1 m f disc ceramic capacitor from the ref out/ref in pin to refgnd. refgnd should be connected to agnd or the analog ground plane. the voltage that appears at the ref out/ ref in pin is internally buffered before being applied to the adc. if this reference is required for use external to the ad7891, it should be buffered as the part has a fet switch in series with the reference, resulting in a source impedance for this output of 2 k w nominal. the tolerance of the internal reference is 10 mv at +25 c with a typical temperature coefficient of 25 ppm/ c and a maximum error over temperature of 20 mv. if the application requires a reference with a tighter tolerance or if the ad7891 needs to be used with a system reference, then an external reference can be connected to the ref out/ref in pin. the external reference will overdrive the internal reference and thus provide the reference source for the adc. the refer- ence input is buffered before being applied to the adc and the maximum input current is 100 m a. suitable reference for the ad7891 include the ad580, the ad680, the ad780 and the ref43 precision +2.5 v references. analog input section the ad7891 is offered as two part types, the ad7891-1 where each input can be configured to have a 10 v or a 5 v input range and the ad7891-2 where each input can be configured to have a 0 v to 2.5 v, 0 v to 5 v and 2.5 v input range. ad7891-1 figure 5 shows the analog input section of the ad7891-1. each input can be configured for 5 v or 10 v operation. for +5 v operation, the v inxa and v inxb inputs are tied together and the input voltage is applied to both. for 10 v operation, the v inxb input is tied to agnd and the input voltage is applied to the v inxa input. the v inxa and v inxb inputs are symmetrical and fully interchangeable. thus for ease of pcb layout on the 10 v range, the input voltage may be applied to the v inxb input while the v inxa input is tied to agnd. 30k v v inxa v inxb agnd to multiplexer ad7891-1 2k v ref out/ref in to adc reference circuitry 7.5k v 30k v 15k v 2.5v reference figure 5. ad7891-1 analog input structure the input resistance for the 5 v range is typically 20 k w . for the 10 v input range the input resistance is typically 34.3 k w . the resistor input stage is followed by the multiplexer and this is followed by the high input impedance stage of the track/hold amplifier. the designed code transitions take place midway between suc- cessive integer lsb values (i.e., 1/2 lsb, 3/2 lsbs, 5/2 lsbs, etc.). lsb size is given by the formula, 1 lsb = fs/4096. thus for the 5 v range, 1 lsb = 10 v/4096 = 2.44 mv. for the 10 v range, 1 lsb = 20 v/4096 = 4.88 mv. output coding is determined by the format bit of the control register. the ideal input/output code transitions are shown in table i. ad7891-2 figure 6 shows the analog input section of the ad7891-2. each input can be configured for input ranges of 0 v to +5 v, 0 v to +2.5 v or 2.5 v. for the 0 v to +5 v input range, the v inxb input is tied to agnd and the input voltage is applied to the v inxa input. for the 0 v to +2.5 v input range, the v inxa and v inxb inputs are tied together and the input voltage is applied to both. for the 2.5 v input range, the v inxb input is tied to +2.5 v and the input voltage is applied to the v inxa input. the +2.5 v source must have a low output impedance. if the inter- nal reference on the ad7891 is used, then it must be buffered before being applied to v inxb . the v inxa and v inxb inputs are symmetrical and fully interchangeable. thus for ease of pcb layout on the 0 v to +5 v range or the 2.5 v range, the input voltage may be applied to the v inxb input while the v inxa input is tied to agnd or +2.5 v. 1.8k v v inxa v inxb agnd to multiplexer ad7891-2 2k v ref out/ref in to adc reference circuitry 1.8k v 2.5v reference figure 6. ad7891-2 analog input structure the input resistance for both the 0 v to +5 v and 2.5 v ranges is typically 3.6 k w . when an input is configured for 0 v to 2.5 v operation, the input is fed into the high impedance stage of the track/hold amplifier via the multiplexer and the two 1.8 k w resistors in parallel. the designed code transitions occur midway between successive integer lsb values (i.e., 1/2 lsb, 3/2 lsbs, 5/2 lsbs etc.). lsb size is given by the formula 1 lsb = fs/4096. thus for the 0 v to +5 v range, 1 lsb = 5 v/4096 = 1.22 mv, for the 0 v to +2.5 v range, 1 lsb = 2.5 v/4096 = 0.61 mv and for the 2.5 v range, 1 lsb = 5 v/4096 = 1.22 mv. output coding is determined by the format bit in the control register. the ideal input/output code transitions for the 2.5 v range are shown in table i. the ideal input/output code transitions for the 0 v to +5 v range and the 0 v to +2.5 v range are shown in table ii.
ad7891 C13C rev. a table i. ideal code transition table for the ad7891-1, 6 10 v and 6 5 v ranges and the ad7891-2, 6 2.5 v range digital output code transition 1 analog input input voltage twos complement straight binary +fsr 2 /2 C 3/2 lsbs 3 (9.99268 v, 4.99634 v or 2.49817 v) 4 011...110 to 011...111 111...110 to 111...111 +fsr/2 C 5/2 lsbs (9.98779 v, 4.99390 v or 2.49695 v) 011...101 to 011...110 111...101 to 111...110 +fsr/2 C 7/2 lsbs (9.99145 v, 4.99146 v or 2.49573 v) 011...100 to 011...101 111...100 to 111...101 agnd + 3/2 lsbs (7.3242 mv, 3.6621 mv or 1.8310 mv) 000...001 to 000...010 100...001 to 100...010 agnd + 1/2 lsb (2.4414 mv, 1.2207 mv or 0.6103 mv) 000...000 to 000...001 100...000 to 100...001 agnd C 1/2 lsb (C2.4414 mv, C1.2207 mv or C0.6103 mv) 111...111 to 000...000 011...111 to 100...000 agnd C 3/2 lsbs (C7.3242 mv, C3.6621 mv or C1.8310 mv) 111...110 to 111...111 011...110 to 011...111 Cfsr/2 + 5/2 lsbs (C9.98779 v, C4.99390 v or C2.49695 v) 100...010 to 100...011 000...010 to 000...011 Cfsr/2 + 3/2 lsbs (C9.99268 v, C4.99634 v or C2.49817 v) 100...001 to 100...010 000...001 to 000...010 Cfsr/2 + 1/2 lsb (C9.99756 v, C4.99878 v or C2.49939 v) 100...000 to 100...001 000...000 to 000...001 notes 1 output code format is determined by the format bit in the control register 2 fsr is full-scale range and is 20 v for the 10 v range, 10 v for the 5 v range and 5 v for the 2.5 v range, with refin = +2.5 v . 3 1 lsb = fsr/4096 = 4.88 mv ( 10 v range), 2.44 mv ( 5 v range) and 1.22 mv ( 2.5 v range), with ref in = +2.5 v. 4 10 v range, 5 v range or 2.5 v range. table ii. ideal code transition table for the ad7891-2, 0 v to +5 v and 0 v to +2.5 v ranges digital output code transition 1 analog input input voltage twos complement straight binary +fsr 2 C 3/2 lsbs 3 (4.99817 v or 2.49908 v) 4 011...110 to 011...111 111...110 to 111...111 +fsr C 5/2 lsbs (4.99695 v or 2.49847 v) 011...101 to 011...110 111...101 to 111...110 +fsr C 7/2 lsbs (4.99573 v or 2.49786 v) 011...100 to 011...101 111...100 to 111...101 agnd + 5/2 lsbs (3.0518 mv or 1.52588 mv) 100...010 to 000...011 000...010 to 000...011 agnd + 3/2 lsbs (1.83105 mv or 0.9155 mv) 100...001 to 000...010 000...001 to 000...010 agnd + 1/2 lsb (0.6103 mv or 0.3052 mv) 100...000 to 000...001 000...000 to 000...001 notes 1 output code format is determined by the format bit in the control register 2 fsr is full-scale range and is 5 v for the 0 to 5 v range and 2.5 v for the 0 to 2.5 v range with ref in = +2.5 v. 3 1 lsb = fs/4096 = 1.22 mv (0 to 5 v range) or 610 m v (0 to 2.5 v range), with ref in = 2.5 v. 4 0 v to +5 v range or 0 v to + 2.5 v range. transfer function of the ad7891-1 and ad7891-2 the transfer function of the ad7891-1 and ad7891-2 can be expressed as follows: input voltage = (m refin d/4096) + (n refin) d is the output data from the ad7891 and is in the range 0 to 4095 for straight binary encoding and from C2048 to 2047 for twos complement encoding. values for m depend upon the input voltage range. values for n depend upon the input volt- age range and the output data format. these values are given in table iii. refin is the reference voltage applied to the ad7891. table iii. transfer function m and n values range output data format m n ad7891-1 10 v straight binary 8 C4 10 v twos complement 8 0 5 v straight binary 4 C2 5 v twos complement 4 0 ad7891-2 0 v to +5 v straight binary 2 0 0 v to +5 v twos complement 2 1 0 v to +2.5 v straight binary 1 0 0 v to +2.5 v twos complement 1 0.5 2.5 v straight binary 2 C1 2.5 v twos complement 2 0
ad7891 C14C rev. a track/hold amplifier section the track/hold amplifier on the ad7891 allows the adc to accurately convert an input sine wave of full-scale amplitude to 12-bit accuracy. the input bandwidth of the track/hold is greater than the nyquist rate of the adc even when the adc is operated at its maximum throughput rate of 454 khz (ad7891- 1) or 500 khz (ad7891-2). in other words, the track/hold amplifier can handle input frequencies in excess of 227 khz (ad7891-1) or 250 khz (ad7891-2). the track/hold amplifier acquires an input signal in 600 ns (ad7891-1) or 400 ns (ad7891-2). the operation of the track/ hold is essentially transparent to the user. the track/hold ampli- fier goes from its tracking mode to its hold mode on the rising edge of convst . the aperture time for the track/hold (i.e., the delay between the external convst signal and the track/ hold actually going into hold) is typically 15 ns. at the end of conversion, the part returns to its tracking mode. the track/hold starts acquiring the next signal at this point. standby operation the ad7891 can be put into power save or standby mode by use of the standby pin or the swstby bit of the control register. normal operation of the ad7891 takes place when the standby input is at a logic one and the swstby bit is at a logic zero. when the standby pin is brought low or a one is written to the swstby bit, then the part goes into its standby mode of operation, which reduces its power consumption to typically 75 m w. the ad7891 is returned to normal operation when the standby input is at a logic 1 and the swstby bit is a logic zero. the wake-up time of the ad7891 is normally determined by the amount of time required to charge the 0.1 m f capacitor between the ref out/ref in pin and refgnd. if the inter- nal reference is being used as the reference source, then this capacitor is charged via a nominal 2 k w resistor. assuming 10 time constants to charge the capacitor to 12-bit accuracy, this implies a wake-up time of 2 ms. if an external reference is used, then this will have to be taken into account when working out how long it will take to charge the capacitor. if the external reference has remained at 2.5 v during the time the ad7891 was in standby mode, then the capacitor will already be charged when the part is taken out of standby mode. thus the wake-up time is now the time required for the internal circuitry of the ad7891 to settle to 12-bit accu- racy. this typically takes 5 m s. if the external reference was also put into standby then the wake-up time of the reference, com- bined with the amount of time taken to recharge the reference capacitor from the external reference, determines how much time must elapse before conversions can begin again. microprocessor interfacing ad7891 to 8x51 serial interface a serial interface between the ad7891 and the 8x51 micro- controller is shown in figure 7. txd of the 8x51 drives sclk of the ad7891 while rxd transmits data to and receives data from the part. the serial clock speed of the 8x51 is slow com- pared to the maximum serial clock speed of the ad7891, so maximum throughput of the ad7891 is not achieved with this interface. 8x51* data out ad7891* *additional pins omitted for clarity sclk data in rfs tfs p3.4 p3.3 txd rxd figure 7. ad7891 to 8x51 interface the 8x51 provides the lsb of its sbuf register as the first bit in the serial data stream. the ad7891 expects the msb of the 6-bit write first. therefore, the data in the sbuf register must be arranged correctly so that this is taken into account. when data is to be transmitted to the part, p3.3 is taken low. the 8xc51 transmits its data in 8-bit bytes with only 8 falling clock edges occurring in the transmit cycle. one 8-bit transfer is needed to write data to the control register of the ad7891. after the data has been transferred, the p3.3 line is taken high to complete the transmission. when reading data from the ad7891, p3.4 of the 8x51 is taken low. two 8-bit serial reads are performed by the 8x51 and p3.4 is taken high to complete the transfer. again, the 8x51 expects the lsb first, while the ad7891 transmits msb first, so this must be taken into account in the 8x51 software. no provision has been made in the given interface to determine when a conversion has ended. if the conversions are initiated by software, then the 8x51 can wait a predetermined amount of time before reading back valid data. alternately the falling edge of the eoc signal can be used to initiate an interrupt service routine which reads the conversion result from part to part. ad7891 to 68hc11 serial interface figure 8 shows a serial interface between the ad7891 and the 68hc11 microcontroller. sck of the 68hc11 drives sclk of the ad7891, the mosi output drives data in of the ad7891 and the miso input receives data from data out of the ad7891. ports pc6 and pc7 of the 68hc11 drive the tfs and rfs lines of the ad7891 respectively. for correct operation of this interface, the 68hc11 should be configured such that its cpol bit is a 1 and its cpha bit is a 0. when data is to be transferred to the ad7891, pc7 is taken low. when data is to be received from the ad7891, pc6 is taken low. the 68hc11 transmits and receives its serial data in 8-bit bytes, msb first. the ad7891 transmits and receives data msb first also. eight falling clock edges occur in a read or write cycle from the 68hc11. a single 8-bit write with pc7 low is required to write to the control register. when data has been written, pc7 is taken high. when reading from the ad7891, pc6 is left low after the first eight bits have been read. a second byte of data is then transmitted serially from the ad7891. when this transfer is complete, the pc6 line is taken high.
ad7891 C15C rev. a as in the 8x51 circuit above, the way that the 68hc11 is in- formed that a conversion is completed is not shown in the dia- gram. the eoc line can be used to inform the 68hc11 that a conversion is complete by using it as an interrupt signal. the interrupt service routine reads in the result of the conversion. if a software conversion start is used, the 68hc11 can wait for 2.0 m s (ad7891-2) or 2.2 m s (ad7891-1) before reading from the ad7891. 68hc11* data out ad7891* *additional pins omitted for clarity sclk data in rfs tfs pc7 pc6 sck mosi moso figure 8. ad7891 to 68hc11 interface ad7891 to adsp-21xx serial interface an interface between the ad7891 and the adsp-21xx is shown in figure 9. in the interface shown either sport0 or sport1 can be used to transfer data to the ad7891. when reading from the part, the sport must be set up with a serial word length of 16 bits. when writing to the ad7891, a serial word length of 6 bits or more can be used. other setups for the serial interface on the adsp-21xx internal sclk, alternate framing mode and active low framing signal. normally the eoc line from the ad7891 would be connected to the irq2 line of the adsp- 21xx to interrupt the dsp at the end of a conversion (not shown in diagram). adsp-21xx* data out ad7891* *additional pins omitted for clarity sclk data in rfs tfs rfs tfs sclk dt dr figure 9. ad7891 to adsp-2101 serial interface ad7891 to dsp5600x serial interface figure 10 shows a serial interface between the ad7891 and the dsp5600x series of dsps. when reading from the ad7891, the dsp5600x should be set up for 16-bit data transfers, msb first, normal mode synchronous operation, internally generated word frame sync and gated clock. when writing to the ad7891, 8-bit or 16-bit data transfers can be used. the frame sync signal from the dsp5600x must be inverted before being applied to the rfs and tfs inputs of the ad7891 as shown in figure 10. to monitor the conversion time of the ad7891, a scheme such as outlined in previous interfaces with eoc can be used. this can be implemented by connecting the eoc line directly to the irqa input of the dsp5600x. dsp56000/ dsp56002* data out ad7891* *additional pins omitted for clarity sclk data in rfs tfs fst (sc2) sck std srd figure 10. ad7891 to dsp5600x serial interface ad7891 to tms320xxx serial interface the ad7891 can be interfaced to the serial port of tms320xxx dsps as shown in figure 11. external timing generation cir- cuitry is necessary to generate the serial clock and syncs neces- sary for the interface. tms32020/ tms320c25/ tms320c5x/ tms320c3x* data out ad7891* *additional pins omitted for clarity sclk data in rfs tfs fsx clkx dx dr clkr fsr timing generation circuitry figure 11. ad7891 to tmsxxx serial interface
ad7891 C16C rev. a parallel interfacing the parallel port on the ad7891 allows the device to be inter- faced to microprocessors or dsp processors as a memory mapped or i/o mapped device. the cs and rd inputs are common to all memory peripheral interfacing. typical interfaces to different processors are shown in figures 12 to 15. in all the interfaces shown, an external timer controls the convst input of the ad7891 and the eoc output interrupts the host dsp. ad7891 to adsp-21xx figure 12 shows the ad7891 interfaced to the adsp-21xx series of dsps as a memory mapped device. a single wait state may be necessary to interface the ad7891 to the adsp-21xx depending on the clock speed of the dsp. this wait state can be programmed via the data memory waitstate control register of the adsp-21xx (please see adsp-2100 family users manual for details). the following instruction reads data from the ad7891: mr = dm(adc) where adc is the address of the ad7891. data bus address bus db11Cdb0 ad7891* *additional pins omitted for clarity cs wr irq2 d23Cd8 eoc rd wr rd addr decode en dms adsp-21xx* a13Ca0 figure 12. ad7891 to adsp-21xx parallel interface ad7891 to tms32020, tms320c25 and tms320c5x parallel interfaces between the ad7891 and the tms32020, tms320c25 and tms320c5x family of dsps are shown in figure 13. the memory mapped address chosen for the ad7891 should be chosen to fall in the i/o memory space of the dsps. tms320c25 only data bus address bus db11Cdb0 ad7891* *additional pins omitted for clarity cs wr int x d23Cd0 eoc rd msc addr decode en is a15Ca0 tms32020/ tms320c25/ tms320c50* ready r/ w strb figure 13. ad7891 to tms32020/c25/c5x parallel interface the parallel interface on the ad7891 is fast enough to interface to the tms32020 with no extra wait states. if high speed glue logic such as 74as devices are used to drive the wr and rd lines when interfacing to the tms320c25, then again no wait states are necessary. however, if slower logic is used, data ac- cesses may be slowed sufficiently when reading from and writing to the part to require the insertion of one wait state. in such a case, this wait state can be generated using the single or gate to combine the cs and msc signals to drive the ready line of the tms320c25, as shown in figure 13. extra wait states will be necessary when using the tms320c5x at their fastest clock speeds. wait states can be programmed via the iowsr and cwsr registers (please see tms 320c5x user guide for details). data is read from the adc using the following instruction: in d,adc where d is the memory location where the data is to be stored and adc is the i/o address of the ad7891. ad7891 to tms320c30 figure 14 shows a parallel interface between the ad7891 and the tms320c3x family of dsps. the ad7891 is interfaced to the expansion bus of the tms320c3x. a single wait state is required in this interface. this can be programmed using the wtcnt bits of the expansion bus control register (see tms320c3x users guide for details). data from the ad7891 can be read using the following instruction: ldi *arn,rx where arn is an auxiliary register containing the lower 16 bits of the address of the ad7891 in the tms320c3x memory space and rx is the register into which the adc data is loaded. expansion data bus address bus db11Cdb0 ad7891* *additional pins omitted for clarity cs wr int x xd23Cxd0 eoc rd addr decode xa15Cxa0 xr/ w iostrb tms320c30* figure 14. ad7891 to tms320c30 parallel interface
ad7891 C17C rev. a ad7891 to dsp5600x figure 15 shows a parallel interface between the ad7891 and the dsp5600x series of dsps. the ad7891 should be mapped into the top 64 locations of y data memory. if extra wait states are needed in this interface, they can be programmed using the port a bus control register (please see dsp5600x users manual for details). data can be read from the ad7891 using the following instruction: moveo y:adc,x0 where adc is the address in the dsp5600x address space to which the ad7891 has been mapped. data bus address bus db11Cdb0 ad7891* *additional pins omitted for clarity cs wr irq d23Cd0 eoc rd wr rd addr decode ds a15Ca0 x/ y dsp56000/ dsp56002* figure 15. ad7891 to dsp5600x parallel interface power supply bypassing and grounding in any circuit where accuracy is important, careful consideration of the power supply and ground return layout helps to ensure the specified performance. the printed circuit board on which the ad7891 is mounted should be designed such that the ana- log 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. digital and analog ground planes should be joined at only one place. if the ad7891 is the only device requiring an agnd to dgnd con- nection then the ground planes should be connected at the agnd and dgnd pins of the ad7891. if the ad7891 is in a system where multiple devices require an agnd to dgnd connection, the connection should still be made at one point only, a star ground point which should be established as close as possible to the ad7891. digital lines running under the device should be avoided as these will couple noise onto the die. the analog ground plane should be allowed to run under the ad7891 to avoid noise coupling. the power supply lines of the ad7891 should use as large a trace as possible 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 parts of the board and 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 not always possible with a double sided board. in this technique, the component side of the board is dedicated to ground plane while signal traces are placed on the solder side. the ad7891 should have ample supply bypassing located as close to the package as possible, ideally right up against the device. one of the v dd pins (pin 10 of the pqfp package, pin 4 on the plcc package) drives mainly the analog circuitry on the chip. this pin should be decoupled to the analog ground plane with a 10 m f tantalum bead capacitor in parallel with a 0.1 m f capacitor. the other v dd pin (pin 19 on the pqfp package, pin 13 on the plcc package) drives mainly digital circuitry on the chip. this pin should be decoupled to the digital ground plane with a 0.1 m f capacitor. the 0.1 m 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. figure 16 shows the recommended decoupling scheme. v dd (pin 10, pqfp pin 4, plcc) dgnd ad7891 agnd agnd v dd (pin 19, pqfp pin 13, plcc) 10 m f 0.1 m f 0.1 m f figure 16. recommended decoupling scheme for the ad7891
ad7891 C18C rev. a ad7891 performance linearity the linearity of the ad7891 is primarily determined by the on- chip 12-bit d/a converter. this is a segmented dac which is laser trimmed for 12-bit integral linearity and differential linear- ity. typical inl for the ad7891 is 0.25 lsb while typical dnl is 0.5 lsb. noise in an a/d converter, noise exhibits itself as code uncertainty in dc applications and as the noise floor (in an fft for example) in ac applications. in a sampling a/d such as the ad7891, all information about the analog input appears in the baseband from dc to half the sampling frequency. the input bandwidth of the track/hold amplifier exceeds the nyquist bandwidth and, therefore, an antialiasing filter should be used to remove un- wanted signals above f s /2 in the input signal in applications where such signals exist. figure 17 shows a histogram plot for 16384 conversions of a dc input signal using the ad7891-1. the analog input was set at the center of a code transition in the following way. an initial dc input level was selected and a number of conversions were made. the resulting histogram was noted and the applied level was adjusted so that only two codes were generated with an equal number of occurrences. this indicated that the transition point between the two codes had been found. the voltage level at which this occurred was recorded. the other edge of one of these two codes was then found in a similar manner. the dc level for the center of code could then be calculated as the aver- age of the two transition levels. the ad7891-1 inputs were configured for 5 v input range and the data was read from the part in parallel mode, after conversion. similar results have been found with the ad7891-1 on the 10 v range and on all input ranges of the ad7891-2. the same performance is achieved in serial mode, again with the data read from the ad7891-1 after conversion. all the codes, except for 3, appear in one output bin, indicating excellent noise performance from the adc. output code 18000 16000 0 2148 2149 number of occurrences 2150 8000 6000 4000 2000 12000 10000 14000 16381 codes 1 code 2 codes figure 17. typical histogram plot (ad7891-1) dynamic performance the ad7891 contains an on-chip track/hold amplifier, allowing the part to sample input signals of up to 250 khz on any of its input channels. many of the ad7891s applications will simply require it to sequence through low frequency input signals across its eight channels. there may be some applications, how- ever, for which the dynamic performance of the converter on signals of up to 250 khz input frequency is of interest. it is recommended for these wider bandwidth signals that hardware conversion start method of sampling is used. these applications require information on the spectral content of the input signal. signal to (noise + distortion), total harmonic distortion, peak harmonic or spurious tone and intermodulation distortion are all specified. figure 18 shows a typical fft plot of a 10 khz, 10 v input after being digitized by the ad7891-1 operating at 500 khz, with the input connected for 10 v opera- tion. the signal to (noise + distortion) ratio is 72.2 db and the total harmonic distortion is C87 db. figure 19 shows a typical fft plot of a 100 khz, 0 v to +5 v input after being digitized by the ad7891-2 operating at 500 khz, with the input connected for 0 v to +5 v operation. the signal to (noise + distortion) ratio is 71.17 db and the total harmonic distortion is C82.3 db. it should be noted that reading from the part during conversion does have a significant impact on dynamic performance. there- fore, for sampling applications, it is recommended not to read during conversion. 0 C30 C150 db C60 C90 C120 f s /2 2048 point fft snr = 72.2db figure 18. typical ad7891-1 fft plot 0 C30 C150 db C60 C90 C120 f s /2 2048 point fft snr = 71.17db figure 19. typical ad7891-2 fft plot
ad7891 C19C rev. a effective number of bits the formula for signal to (noise + distortion) ratio (see termi- nology) is related to the resolution, or number of bits, of the converter. rewriting the formula, below, gives a measure of performance expressed in effective number of bits (enob): enob = (snr - 1.76)/6.02 where snr is the signal to (noise + distortion) ratio. the effective number of bits for a device can be calculated from its measured snr. figure 20 shows a typical plot of effective number of bits versus frequency for the ad7891-1 and the ad7891-2 from dc to 200 khz. the sampling frequency is 500 khz. the ad7891-1 inputs were configured for 10 v operation. the ad7891-2 inputs were configured for 0 to +5 v operation. the ad7891-1 plot only goes to 100 khz as a 10 v sine wave of sufficient quality was unavailable at higher frequencies. figure 20 shows that the ad7891-1 converts an input sine wave of 100 khz to an effective number of bits of of 11 which equates to a signal-to-(noise + distortion) level of 68.02 dbs. the ad7891-2 converts an input sine wave of 200 khz to an effec- tive number of bits of 11.07 which equates to a signal-to-(noise + distortion) level of 68.4 dbs. frequency C khz 12.0 11.9 11.1 40 80 120 11.5 11.4 11.3 11.2 11.7 11.6 11.8 20 0 60 100 140 160 180 200 11.0 effective number of bits ad7891-2 enob ad7891-1 enob figure 20. effective number of bits vs. frequency
ad7891 C20C rev. a c2069aC1C8/98 printed in u.s.a. outline dimensions dimension shown in inches and (mm). 44-lead plcc (p-44a) 6 pin 1 identifier 7 40 39 17 18 29 28 top view (pins down) 0.695 (17.65) 0.685 (17.40) sq 0.656 (16.66) 0.650 (16.51) sq 0.048 (1.21) 0.042 (1.07) 0.048 (1.21) 0.042 (1.07) 0.020 (0.50) r 0.021 (0.53) 0.013 (0.33) 0.050 (1.27) bsc 0.63 (16.00) 0.59 (14.99) 0.032 (0.81) 0.026 (0.66) 0.180 (4.57) 0.165 (4.19) 0.040 (1.01) 0.025 (0.64) 0.025 (0.63) 0.015 (0.38) 0.110 (2.79) 0.085 (2.16) 0.056 (1.42) 0.042 (1.07) 44-terminal pqfp (s-44) 0.398 (10.1) 0.390 (9.9) 0.083 (2.1) 0.077 (1.95) 0.040 (1.02) 0.032 (0.82) 0.040 (1.02) 0.032 (0.82) seating plane 0.096 (2.45) max 0.037 (0.95) 0.026 (0.65) 8 8 0.8 8 0.557 (14.15) 0.537 (13.65) 0.033 (0.85) 0.029 (0.75) 0.016 (0.4) 0.012 (0.3) top view (pins down) 1 33 34 44 11 12 23 22 0.397 (10.1) 0.390 (9.9)

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