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FEATURES Two Fast 14-Bit ADCs Four Input Channels Simultaneous Sampling and Conversion 5.2 s Conversion Time Single Supply Operation Selection of Input Ranges 10 V for AD7863-10 2.5 V for AD7863-3 0 V to 2.5 V for AD7863-2 High Speed Parallel Interface Low Power, 70 mW Typ Power Saving Mode, 105 W Max Overvoltage Protection on Analog Inputs 14-Bit Lead Compatible Upgrade to AD7862 APPLICATIONS AC Motor Control Uninterrupted Power Supplies Data Acquisition Systems Communications GENERAL DESCRIPTION
Simultaneous Sampling Dual 175 kSPS 14-Bit ADC AD7863
FUNCTIONAL BLOCK DIAGRAM
VREF 2k +2.5V REFERENCE VDD
AD7863
VA1 VB1 SIGNAL SCALING MUX SIGNAL SCALING SIGNAL SCALING SIGNAL SCALING MUX TRACK/ HOLD TRACK/ HOLD
14-BIT ADC DB0 OUTPUT LATCH 14-BIT ADC CS RD DB13
VA2 VB2
CONVERSION CONTROL LOGIC
CLOCK
A0
BUSY CONVST
AGND
AGND
DGND
The AD7863 is a high speed, low power, dual 14-bit A/D converter that operates from a single +5 V supply. The part contains two 5.2 s successive approximation ADCs, two track/hold amplifiers, an internal +2.5 V reference and a high speed parallel interface. Four analog inputs are grouped into two channels (A and B) selected by the A0 input. Each channel has two inputs (VA1 and VA2 or VB1 and VB2), which can be sampled and converted simultaneously thus preserving the relative phase information of the signals on both analog inputs. The part accepts an analog input range of 10 V (AD7863-10), 2.5 V (AD7863-3) and 0 V-2.5 V (AD7863-2). Overvoltage protection on the analog inputs for the part allows the input voltage to go to 17 V, 7 V or +7 V respectively, without causing damage. A single conversion start signal (CONVST) simultaneously places both track/holds into hold and initiates conversion on both channels. The BUSY signal indicates the end of conversion and at this time the conversion results for both channels are available to be read. The first read after a conversion accesses the result from VA1 or VB1, while the second read accesses the result from VA2 or VB2, depending on whether the multiplexer select A0 is low or high respectively. Data is read from the part via a 14-bit parallel data bus with standard CS and RD signals. In addition to the traditional dc accuracy specifications such as linearity, gain and offset errors, the part is also specified for dynamic performance parameters including harmonic distortion and signal-to-noise ratio.
The AD7863 is fabricated in Analog Devices' Linear Compatible CMOS (LC2MOS) process, a mixed technology process that combines precision bipolar circuits with low power CMOS logic. It is available in 28-lead SOIC and SSOP.
PRODUCT HIGHLIGHTS
1. The AD7863 features two complete ADC functions allowing simultaneous sampling and conversion of two channels. Each ADC has a two-channel input mux. The conversion result for both channels is available 5.2 s after initiating conversion. 2. The AD7863 operates from a single +5 V supply and consumes 70 mW typ. The automatic power-down mode, where the part goes into power down once conversion is complete and "wakes up" before the next conversion cycle, makes the AD7863 ideal for battery-powered or portable applications. 3. The part offers a high speed parallel interface for easy connection to microprocessors, microcontrollers and digital signal processors. 4. The part is offered in three versions with different analog input ranges. The AD7863-10 offers the standard industrial input range of 10 V; the AD7863-3 offers the common signal processing input range of 2.5 V, while the AD7863-2 can be used in unipolar 0 V-2.5 V applications. 5. The part features very tight aperture delay matching between the two input sample and hold amplifiers.
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. 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 (c) Analog Devices, Inc., 1999
AD7863-SPECIFICATIONS unless otherwise noted.)
Parameter SAMPLE AND HOLD -3 dB Small Signal Bandwidth Aperture Delay2 Aperture Jitter2 Aperture Delay Matching2 DYNAMIC PERFORMANCE3 Signal to (Noise + Distortion) Ratio4 @ +25C TMIN to TMAX Total Harmonic Distortion4 Peak Harmonic or Spurious Noise4 Intermodulation Distortion4 2nd Order Terms 3rd Order Terms Channel-to-Channel Isolation4 DC ACCURACY Resolution Minimum Resolution for Which No Missing Codes are Guaranteed Relative Accuracy4 Differential Nonlinearity4 AD7863-10, AD7863-3 Positive Gain Error4 Positive Gain Error Match4 Negative Gain Error4 Negative Gain Error Match4 Bipolar Zero Error Bipolar Zero Error Match AD7863-2 Positive Gain Error4 Positive Gain Error Match4 Unipolar Offset Error Unipolar Offset Error Match ANALOG INPUTS AD7863-10 Input Voltage Range Input Resistance AD7863-3 Input Voltage Range Input Resistance AD7863-2 Input Voltage Range Input Current REFERENCE INPUT/OUTPUT REF IN Input Voltage Range REF IN Input Current REF OUT Output Voltage REF OUT Error @ +25C REF OUT Error TMIN to TMAX REF OUT Temperature Coefficient LOGIC INPUTS Input High Voltage, VINH Input Low Voltage, VINL Input Current, IIN Input Capacitance, CIN5 A Version1 7 35 50 350 B Version1 7 35 50 350
(VDD = +5 V
5%, AGND = DGND = 0 V, REF = Internal. All specifications TMIN to TMAX
Units MHz typ ns max ps typ ps max
Test Conditions/Comments
fIN = 80.0 kHz, fS = 175 kSPS 78 77 -82 -82 -93 -89 -86 14 14 2.5 +2 to -1 10 10 10 10 10 8 14 16 14 10 78 77 -82 -82 -93 -89 -86 14 14 2 +2 to -1 8 10 8 10 8 6 dB min dB min dB max dB max dB typ dB typ dB typ Bits Bits LSB max LSB max LSB max LSB max LSB max LSB max LSB max LSB max LSB max LSB max LSB max LSB max
Typically -87 dB Typically -90 dB fa = 49 kHz, fb = 50 kHz
fIN = 50 kHz Sine Wave Any Channel
10 9 2.5 3 +2.5 100 2.375/2.625 100 2.5 10 20 25 2.4 0.8 10 10
10 9 2.5 3 +2.5 100 2.375/2.625 100 2.5 10 20 25 2.4 0.8 10 10
Volts k typ Volts k typ Volts nA max 2.5 V 5%
A max V nom mV max mV max ppm/C typ V min V max A max pF max
VDD = 5 V 5% VDD = 5 V 5%
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REV. A
AD7863
Parameter LOGIC OUTPUTS Output High Voltage, VOH Output Low Voltage, VOL DB11-DB0 Floating-State Leakage Current Floating-State Capacitance5 Output Coding AD7863-10, AD7863-3 AD7863-2 CONVERSION RATE Conversion Time Mode 1 Operation Mode 2 Operation6 Track/Hold Acquisition Time4, 7 POWER REQUIREMENTS VDD IDD Normal Mode (Mode 1) AD7863-10 AD7863-3 AD7863-2 Power-Down Mode (Mode 2) IDD @ +25C8 Power Dissipation Normal Mode (Mode 1) AD7863-10 AD7863-3 AD7863-2 Power-Down Mode @ +25C A Version1 4.0 0.4 10 10 B Version1 4.0 0.4 10 10 Units V min V max A max pF max Twos Complement Straight (Natural) Binary Test Conditions/Comments ISOURCE = 200 A ISINK = 1.6 mA
5.2 10.0 0.5 +5
5.2 10.0 0.5 +5
s max s max s max V nom
For Both Channels For Both Channels
5% for Specified Performance
17 15 10 20
17 15 10 20
mA max mA max mA max A max mW max mW max mW max W max 40 nA typ. Logic Inputs = 0 V or VDD VDD = 5.25 V, Typically 70 mW VDD = 5.25 V, Typically 70 mW VDD = 5.25 V, Typically 45 mW Typically 210 nW, VDD = 5.25 V
89.25 78.75 52.5 105
89.25 78.75 52.5 105
NOTES 1 Temperature ranges are as follows: A, B Versions: -40C to +85C. 2 Sample tested during initial release. 3 Applies to Mode 1 operation. See section on operating modes. 4 See Terminology. 5 Sample tested @ +25C to ensure compliance. 6 This 10 s includes the "wake-up" time from standby. This "wake-up" time is timed from the rising edge of CONVST, whereas conversion is timed from the falling edge of CONVST, for a narrow CONVST pulsewidth the conversion time is effectively the "wake-up" time plus conversion time, hence 10 s. This can be seen from Figure 6. Note that if the CONVST pulsewidth is greater than 5.2 s, the effective conversion time will increase beyond 10 s. 7 Performance measured through full channel (multiplexer, SHA and ADC). 8 For best dynamic performance of the AD7863, ATE device testing has to be performed with power supply decoupling in place. In the AD7863 power-down mode of operation, the leakage current associated with these decoupling capacitors is greater than that of the AD7863 supply current. Therefore the 40 nA typical figure shown is a characterized and guaranteed by design figure, which reflects the supply current of the AD7863 without decoupling in place. The max figure shown in the Conditions/ Comments column reflects the AD7863 with supply decoupling in place--0.1 F in parallel with a 10 F disc ceramic capacitors on the V DD pin and 2 x 0.1 F disc ceramic capacitors on the V REF pin, in both cases to the AGND plane. Specifications subject to change without notice.
REV. A
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AD7863 TIMING CHARACTERISTICS1, 2
Parameter tCONV tACQ Parallel Interface t1 t2 t3 t4 t5 3 t6 4 t7 t8 A, B Versions 5.2 0.5 0 0 35 45 30 5 30 10 400
(VDD = +5 V 5%, AGND = DGND = 0 V, REF = Internal. All specifications TMIN to TMAX unless otherwise noted.)
Units s max s max ns min ns min ns min ns min ns min ns min ns max ns min ns min Test Conditions/Comments Conversion Time Acquisition Time CS to RD Setup Time CS to RD Hold Time CONVST Pulsewidth Read Pulsewidth Data Access Time after Falling Edge of RD Bus Relinquish Time after Rising Edge of RD Time Between Consecutive Reads Quiet Time
NOTES 1 Sample tested at +25C to ensure compliance. 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 Figure 1. 3 Measured with the load circuit of Figure 2 and defined as the time required for an output to cross 0.8 V or 2.0 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 2. 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.
tACQ t8
CONVST
t3
BUSY
t CONV = 5.2 s
A0
CS
t1 t4
RD
t2
t7
t5
DATA VA1 VA2
t6
VB1 VB2
Figure 1. Timing Diagram
1.6mA
TO OUTPUT PIN 50pF 200 A
Figure 2. Load Circuit for Access Time and Bus Relinquish Time
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AD7863
ABSOLUTE MAXIMUM RATINGS*
(TA = +25C unless otherwise noted)
VDD to AGND . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 V to +7 V VDD to DGND . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 V to +7 V Analog Input Voltage to AGND AD7863-10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 V AD7863-3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 V AD7863-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +7 V Reference Input Voltage to AGND . . . .-0.3 V to VDD + 0.3 V Digital Input Voltage to DGND . . . . . -0.3 V to VDD + 0.3 V Digital Output Voltage to DGND . . . . -0.3 V to VDD + 0.3 V Operating Temperature Range Commercial (A, B Version) . . . . . . . . . . . . -40C to +85C Storage Temperature Range . . . . . . . . . . . -65C to +150C Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . +150C
SOIC Package, Power Dissipation . . . . . . . . . . . . . . . 450 mW JA Thermal Impedance . . . . . . . . . . . . . . . . . . . . 110C/W Lead Temperature, Soldering Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . +215C Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . +220C SSOP Package, Power Dissipation . . . . . . . . . . . . . . . 450 mW JA Thermal Impedance . . . . . . . . . . . . . . . . . . . . 110C/W Lead Temperature, Soldering Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . +215C Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . +220C
*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 listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ORDERING GUIDE
Model AD7863AR-10 AD7863BR-10 AD7863ARS-10 AD7863AR-3 AD7863ARS-3 AD7863BR-3 AD7863AR-2 AD7863ARS-2
Input Ranges 10 V 10 V 10 V 2.5 V 2.5 V 2.5 V 0 V to 2.5 V 0 V to 2.5 V
Relative Accuracy 2.5 LSB 2.0 LSB 2.5 LSB 2.5 LSB 2.5 LSB 2.0 LSB 2.5 LSB 2.5 LSB
Temperature Range -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C
Package Options* R-28 R-28 RS-28 R-28 RS-28 R-28 R-28 RS-28
*R = Small Outline (SOIC), RS = Shrink Small Outline (SSOP).
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 AD7863 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
REV. A
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AD7863
PIN FUNCTION DESCRIPTIONS
Pin 1-6 7 8 9-15 16 17 18 19
Mnemonic DB12-DB7 DGND CONVST DB6-DB0 AGND VB2 VA2 VREF
Description Data Bit 12 to Data Bit 7. Three-state TTL outputs. Digital Ground. Ground reference for digital circuitry. Convert Start Input. Logic Input. A high to low transition on this input puts both track/holds into their hold mode and starts conversion on both channels. Data Bit 6 to Data Bit 0. Three-state TTL outputs. Analog Ground. Ground reference for Mux, track/hold, reference and DAC circuitry. Input Number 2 of Channel B. Analog Input voltage ranges of 10 V (AD7863-10), 2.5 V (AD7863-3) and 0 V-2.5 V (AD7863-2). Input Number 2 of Channel A. Analog Input voltage ranges of 10 V (AD7863-10), 2.5 V (AD7863-3) and 0 V-2.5 V (AD7863-2). Reference Input/Output. This pin is connected to the internal reference through a series resistor and is the output reference source for the analog-to-digital converter. The nominal reference voltage is 2.5 V and this appears at the pin. Multiplexer Select. This input is used in conjunction with CONVST to determine on which pair of channels the conversion is to be performed. If A0 is low when the conversion is initiated then channels VA1, VA2 will be selected. If A0 is high when the conversion is initiated, channels VB1, VB2 will be selected. Chip Select Input. Active low logic input. The device is selected when this input is active. Read Input. Active low logic input. This input is used in conjunction with CS low to enable the data outputs and read a conversion result from the AD7863. Busy Output. The busy output is triggered high by the falling edge of CONVST and remains high until conversion is completed. Analog and Digital Positive Supply Voltage, +5.0 V 5%. Input Number 1 of Channel A. Analog Input voltage ranges of 10 V (AD7863-10), 2.5 V (AD7863-3) and 0 V-2.5 V (AD7863-2). Input Number 1 of Channel B. Analog Input voltage ranges of 10 V (AD7863-10), 2.5 V (AD7863-3) and 0 V-2.5 V (AD7863-2). Analog Ground. Ground reference for Mux, track/hold, reference and DAC circuitry. Data Bit 13 (MSB). Three-state TTL output. Output coding is twos complement for the AD7863-10 and AD7863-3. Output coding is straight (natural) binary for the AD7863-2.
20
A0
21 22 23 24 25 26 27 28
CS RD BUSY VDD VA1 VB1 AGND DB13
PIN CONFIGURATION
DB12 1 DB11 2 DB10 3 DB9 4 DB8 5 DB7 6 DGND 7
28 DB13 27 AGND 26 VB1 25 VA1 24 VDD
AD7863
23 BUSY
TOP VIEW 22 RD (Not to Scale) 21 CS CONVST 8 DB6 9 DB5 10 DB4 11 DB3 12 DB2 13 DB1 14
20 A0 19 VREF 18 VA2 17 VB2 16 AGND 15 DB0
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REV. A
AD7863
TERMINOLOGY Signal to (Noise + Distortion) Ratio Channel-to-Channel Isolation
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 (fS/2), excluding dc. The ratio is dependent upon the number of quantization levels in the digitization 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) = (6.02N + 1.76) dB Thus for a 14-bit converter, this is 86.04 dB.
Total Harmonic Distortion
Channel-to-Channel isolation is a measure of the level of crosstalk between channels. It is measured by applying a fullscale 50 kHz sine wave signal to all nonselected channels and determining how much that signal is attenuated in the selected channel. The figure given is the worst case across all 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 Gain Error (AD7863-10, 10 V, AD7863-3, 2.5 V)
Total harmonic distortion (THD) is the ratio of the rms sum of harmonics to the fundamental. For the AD7863 it is defined as:
THD (dB) = 20 log V22 +V32 +V42 +V52
V1 where V1 is the rms amplitude of the fundamental and V2, V3, V4 and V5 are the rms amplitudes of the second through the fifth harmonics.
Peak Harmonic or Spurious Noise
This is the deviation of the last code transition (01 . . . 110 to 01 . . . 111) from the ideal 4 x VREF - 1 LSB (AD7863-10 10 V range) or VREF - 1 LSB (AD7863-3, 2.5 V range), after the Bipolar Offset Error has been adjusted out.
Positive Gain Error (AD7863-2, 0 V to 2.5 V)
This is the deviation of the last code transition (11 . . . 110 to 11 . . . 111) from the ideal VREF - 1 LSB, after the unipolar offset error has been adjusted out.
Bipolar Zero Error (AD7863-10, 10 V, AD7863-3, 2.5 V)
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 fS/2 and excluding dc) to the rms value of the fundamental. Normally, the value of this specification is determined 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
This is the deviation of the midscale transition (all 0s to all 1s) from the ideal 0 V (AGND).
Unipolar Offset Error (AD7863-2, 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 LSB.
Negative Gain Error (AD7863-10, 10 V, AD7863-3, 2.5 V)
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 - fb), while the third order terms include (2fa + fb), (2fa - fb), (fa + 2fb) and (fa - 2fb). The AD7863 is tested using two input frequencies. In this case, the second and third order terms are of different 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 separately. 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.
This is the deviation of the first code transition (10 . . . 000 to 10 . . . 001) from the ideal -4 x VREF + 1 LSB (AD7863-10 10 V range) or -VREF + 1 LSB (AD7863-3, 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 VAX/BX input of the AD7863. 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 VAX/BX before starting another conversion, to ensure that the part operates to specification.
REV. A
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AD7863
CONVERTER DETAILS
The AD7863 is a high speed, low power, dual 14-bit A/D converter that operates from a single +5 V supply. The part contains two 5.2 s successive approximation ADCs, two track/ hold amplifiers, an internal +2.5 V reference and a high speed parallel interface. Four analog inputs are grouped into two channels (A and B) selected by the A0 input. Each channel has two inputs (VA1 and VA2 or VB1 and VB2) which can be sampled and converted simultaneously thus preserving the relative phase information of the signals on both analog inputs. The part accepts an analog input range of 10 V (AD7863-10), 2.5 V (AD7863-3) and 0 V-2.5 V (AD7863-2). Overvoltage protection on the analog inputs for the part allows the input voltage to go to 17 V, 7 V or +7 V respectively, without causing damage. The AD7863 has two operating modes, the high sampling mode and the auto sleep mode where the part automatically goes into sleep after the end of conversion. These modes are discussed in more detail in the Timing and Control section. Conversion is initiated on the AD7863 by pulsing the CONVST input. On the falling edge of CONVST, both on-chip track/ holds are simultaneously placed into hold and the conversion sequence is started on both channels. The conversion clock for the part is generated internally using a laser-trimmed clock oscillator circuit. The BUSY signal indicates the end of conversion and at this time the conversion results for both channels are available to be read. The first read after a conversion accesses the result from VA1 or VB1, while the second read accesses the result from VA2 or VB2 depending on whether the multiplexer select A0 is low or high respectively before the conversion is initiated. Data is read from the part via a 14-bit parallel data bus with standard CS and RD signals. Conversion time for the AD7863 is 5.2 s in the high sampling mode (10 s for the auto sleep mode), and the track/hold acquisition time is 0.5 s. To obtain optimum performance from the part, the read operation should not occur during the conversion or during the 400 ns prior to the next conversion. This allows the part to operate at throughput rates up to 175 kHz and achieve data sheet specifications.
Track/Hold Section
The track/hold amplifiers acquire input signals to 14-bit accuracy in less than 500 ns. The operation of the track/holds are essentially transparent to the user. The two track/hold amplifiers sample their respective input channels simultaneously, on the falling edge of CONVST. The aperture time for the track/ holds (i.e., the delay time between the external CONVST signal and the track/hold actually going into hold) is well matched across the two track/holds on one device and also well matched from device to device. This allows the relative phase information between different input channels to be accurately preserved. It also allows multiple AD7863s to simultaneously sample more than two channels. At the end of conversion, the part returns to its tracking mode. The acquisition time of the track/hold amplifiers begins at this point.
Reference Section
The AD7863 contains a single reference pin, labeled VREF, which either provides access to the part's own +2.5 V 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 AD7863's transfer function and will add to the specified full-scale errors on the part. On the AD7863-10 and AD7863-3, it will also result in an offset error injected in the attenuator stage. The AD7863 contains an on-chip +2.5 V reference. To use this reference as the reference source for the AD7863, simply connect two 0.1 F disc ceramic capacitors from the VREF pin to AGND. The voltage that appears at this pin is internally buffered before being applied to the ADC. If this reference is required for use external to the AD7863, it should be buffered as the part has a FET switch in series with the reference output resulting in a source impedance for this output of 5.5 k nominal. The tolerance on the internal reference is 10 mV at 25C with a typical temperature coefficient of 25 ppm/C and a maximum error over temperature of 25 mV. If the application requires a reference with a tighter tolerance or the AD7863 needs to be used with a system reference, the user has the option of connecting an external reference to this VREF pin. The external reference will effectively overdrive the internal reference and thus provide the reference source for the ADC. The reference input is buffered before being applied to the ADC with a maximum input current of 100 A. A suitable reference source for the AD7863 is the AD780 precision +2.5 V reference.
The track/hold amplifiers on the AD7863 allow the ADCs to accurately convert an input sine wave of full-scale amplitude to 14-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 175 kHz (i.e., the track/hold can handle input frequencies in excess of 87.5 kHz).
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AD7863
CIRCUIT DESCRIPTION Analog Input Section
The AD7863 is offered as three part types: the AD7863-10, which handles a 10 V input voltage range, the AD7863-3, which handles input voltage range 2.5 V and the AD7863-2, which handles a 0 V to +2.5 V input voltage range.
2k VREF
+2.5V REFERENCE
AD7863-10/AD7863-3
TO ADC REFERENCE CIRCUITRY MUX R3 TRACK/ HOLD TO INTERNAL COMPARATOR
The analog input section for the AD7863-2 contains no biasing resistors and the VAX/BX pin drives the input directly to the multiplexer and track/hold amplifier circuitry. The analog input range is 0 V to +2.5 V into a high impedance stage with an input current of less than 100 nA. This input is benign, with no dynamic charging currents. Once again, the designed code transitions occur on successive integer LSB values. Output coding is straight (natural) binary with 1 LSB = FS/16384 = 2.5 V/16384 = 0.15 mV. Table II shows the ideal input/output transfer function for the AD7863-2.
Table II. Ideal Input/Output Code Table for the AD7863-2
R2 VAX AGND R1
Analog Input1 +FSR - 1 LSB2 +FSR - 2 LSB +FSR - 3 LSB GND + 3 LSB GND + 2 LSB GND + 1 LSB
Digital Output Code Transition 111 . . . 110 to 111 . . . 111 111 . . . 101 to 111 . . . 110 111 . . . 100 to 111 . . . 101 000 . . . 010 to 000 . . . 011 000 . . . 001 to 000 . . . 010 000 . . . 000 to 000 . . . 001
Figure 3. AD7863-10/-3 Analog Input Structure
Figure 3 shows the analog input section for the AD7863-10 and AD7863-3. The analog input range of the AD7863-10 is 10 V into an input resistance of typically 9 k. The analog input range of the AD7863-3 is 2.5 V into an input resistance of typically 3 k. This input is benign, with no dynamic charging currents as the resistor stage is followed by a high input impedance stage of the track/hold amplifier. For the AD7863-10, R1 = 8 k, R2 = 2 k and R3 = 2 k. For the AD7863-3, R1 = R2 = 2 k and R3 is open circuit. For the AD7863-10 and AD7863-3, the designed code transitions occur on successive integer LSB values (i.e., 1 LSB, 2 LSBs, 3 LSBs . . .). Output coding is twos complement binary with 1 LSB = FS/16384. The ideal input/output transfer function for the AD7863-10 and AD7863-3 is shown in Table I.
Table I. Ideal Input/Output Code Table for the AD7863-10/-3
NOTES 1 FSR is Full-Scale Range and is 2.5 V for AD7863-2 with V REF = +2.5 V. 2 1 LSB = FSR/16384 and is 0.15 mV for AD7863-2 with V REF = +2.5 V.
OFFSET AND FULL-SCALE ADJUSTMENT
In most Digital Signal Processing (DSP) applications, offset and full-scale errors have little or no effect on system performance. Offset error can always be eliminated in the analog domain by ac coupling. Full-scale error effect is linear and does not cause problems as long as the input signal is within the full dynamic range of the ADC. Invariably, some applications will require that the input signal span the full analog input dynamic range. In such applications, offset and full-scale error will have to be adjusted to zero. Figure 4 shows a typical circuit that can be used to adjust the offset and full-scale errors on the AD7863 (VA1 on the AD786310 version is shown for example purposes only). Where adjustment is required, offset error must be adjusted before full-scale error. This is achieved by trimming the offset of the op amp driving the analog input of the AD7863 while the input voltage is 1/2 LSB below analog ground. The trim procedure is as follows: apply a voltage of -0.61 mV (-1/2 LSB) at V1 in Figure 4 and adjust the op amp offset voltage until the ADC output code flickers between 11 1111 1111 1111 and 00 0000 0000 0000.
INPUT RANGE = V1 R1 10k R2 500 R3 10k R5 10k AGND R4 10k 10V
Analog Inputl +FSR/2 - 1 LSB2 +FSR/2 - 2 LSBs +FSR/2 - 3 LSBs GND + 1 LSB GND GND - 1 LSB -FSR/2 + 3 LSBs -FSR/2 + 2 LSBs -FSR/2 + 1 LSB
Digital Output Code Transition 011 . . . 110 to 011 . . . 111 011 . . . 101 to 011 . . . 110 011 . . . 100 to 011 . . . 101 000 . . . 000 to 000 . . . 001 111 . . . 111 to 000 . . . 000 111 . . . 110 to 111 . . . 111 100 . . . 010 to 100 . . . 011 100 . . . 001 to 100 . . . 010 100 . . . 000 to 100 . . . 001
NOTES 1 FSR is full-scale range = 20 V (AD7863-10) and = 5 V (AD7863-3) with REF IN = +2.5 V. 2 1 LSB = FSR/16384 = 1.22 mV (AD7863-10) and 0.3 mV (AD7863-3) with REF IN = +2.5 V.
VA1
AD7863*
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 4. Full-Scale Adjust Circuit
REV. A
-9-
AD7863
tACQ t8
CONVST
t3
BUSY
t CONV = 5.2 s
A0
CS
t1 t4
RD
t2
t7
t5
DATA VA1 VA2
t6
VB1 VB2
Figure 5a. Mode 1 Timing Operation Diagram for High Sampling Performance
Gain error can be adjusted at either the first code transition (ADC negative full scale) or the last code transition (ADC positive full scale). The trim procedures for both cases are as follows:
Positive Full-Scale Adjust (-10 Version)
Apply a voltage of +9.9927 V (FS/2 - 1 LSBs) at V1. Adjust R2 until the ADC output code flickers between 01 1111 1111 1110 and 01 1111 1111 1111.
Negative Full-Scale Adjust (-10 Version)
Apply a voltage of -9.9976 V (-FS + 1 LSB) at V1 and adjust R2 until the ADC output code flickers between 10 0000 0000 0000 and 10 0000 0000 0001. An alternative scheme for adjusting full-scale error in systems that use an external reference is to adjust the voltage at the VREF pin until the full-scale error for any of the channels is adjusted out. The good full-scale matching of the channels will ensure small full-scale errors on the other channels.
TIMING AND CONTROL
with standard CS and RD signal, i.e., the read operation consists of a negative going pulse on the CS pin combined with two negative going pulses on the RD pin (while the CS is low), accessing the two 14-bit results. Once the read operation has taken place, a further 400 ns should be allowed before the next falling edge of CONVST to optimize the settling of the track/ hold amplifier before the next conversion is initiated. The achievable throughput rate for the part is 5.2 s (conversion time) plus 100 ns (read time) plus 0.4 s (quiet time). This results in a minimum throughput time of 5.7 s (equivalent to a throughput rate of 175 kHz).
Read Options
Apart from the Read Operation described above and displayed in Figure 5a, other CS and RD combinations can result in different channels/inputs being read in different combinations. Suitable combinations are shown in Figures 5b through 5d.
CS
Figure 5a shows the timing and control sequence required to obtain optimum performance (Mode 1) from the AD7863. In the sequence shown, a conversion is initiated on the falling edge of CONVST. This places both track/holds into hold simultaneously and new data from this conversion is available in the output register of the AD7863 5.2 s later. The BUSY signal indicates the end of conversion and at this time the conversion results for both channels are available to be read. A second conversion is then initiated. If the multiplexer select A0 is low, the first and second read pulses after the first conversion accesses the result from Channel A (VA1 and VA2 respectively). The third and fourth read pulses, after the second conversion and A0 high, accesses the result from Channel B (VB1 and VB2 respectively). A0's state can be changed any time after the CONVST goes high, i.e., track/holds into hold and 500 ns prior to the next falling edge of CONVST. Note that A0 should not be changed during conversion if the nonselected channels have negative voltages applied to them which are outside the input range of the AD7863, as this will affect the conversion in progress. Data is read from the part via a 14-bit parallel data bus -10-
RD
DATA
VA1
VA2
Figure 5b. Read Option A (A0 Is Low)
CS
RD
DATA
VA1
VA2
VA1
Figure 5c. Read Option B (A0 Is Low)
REV. A
AD7863
A0
Mode 2 Operation (Power-Down, Auto-Sleep After Conversion)
CS
RD
DATA
VA1
VA2
The timing diagram in Figure 6 is for optimum performance in operating Mode 2 where the part automatically goes into sleep mode once BUSY goes low after conversion and "wakes up" before the next conversion takes place. This is achieved by keeping CONVST low at the end of the second conversion, whereas it was high at the end of the second conversion for Mode 1 operation. The operation shown in Figure 6 shows how to access data from both Channels A and B, followed by the Auto Sleep mode. One can also set up the timing to access data from Channel A only or Channel B only (see Read Options section) and then go into Auto-Sleep mode. The rising edge of CONVST "wakes up" the part. This wake-up time is 4.8 s when using an external reference and 5 ms when using the internal reference, at which point the track/hold amplifiers go into their hold mode provided the CONVST has gone low. The conversion takes 5.2 s after this giving a total of 10 s (external reference, 5.005 ms for internal reference) from the rising edge of CONVST to the conversion being complete, which is indicated by the BUSY going low. Note that since the wake-up time from the rising edge of CONVST is 4.8 s, if the CONVST pulsewidth is greater than 5.2 s the conversion will take more than the 10 s (4.8 s wake-up time +5.2 s conversion time) shown in Figure 6 from the rising edge of CONVST. This is because the track/hold amplifiers go into their hold mode on the falling edge of CONVST and the conversion will not be complete for a further 5.2 s. In this case, the BUSY will be the best indicator of when the conversion is complete. Even though the part is in sleep mode, data can still be read from the part. The read operation is identical to that in Mode 1 operation and must also be complete at least 400 ns before the falling edge of the next CONVST to allow the track/hold amplifiers to have enough time to settle. This mode is very useful when the part is converting at a slow rate as the power consumption will be significantly reduced from that of Mode 1 operation.
4.8 s*/5ms** WAKE-UP TIME
Figure 5d. Read Option C
OPERATING MODES Mode 1 Operation (Normal Power, High Sampling Performance)
The timing diagram in Figure 5a is for optimum performance in operating Mode 1 where the falling edge of CONVST starts conversion and puts the track/hold amplifiers into their hold mode. This falling edge of CONVST also causes the BUSY signal to go high to indicate that a conversion is taking place. The BUSY signal goes low when the conversion is complete, which is 5.2 s max after the falling edge of CONVST and new data from this conversion is available in the output latch of the AD7863. A read operation accesses this data. If the multiplexer select A0 is low, the first and second read pulses after the first conversion accesses the result from Channel A (VA1 and VA2 respectively). The third and fourth read pulses, after the second conversion and A0 high, access the result from Channel B (VB1 and VB2, respectively). Data is read from the part via a 14-bit parallel data bus with standard CS and RD signals. This data read operation consists of a negative going pulse on the CS pin combined with two negative going pulses on the RD pin (while the CS is low), accessing the two 14-bit results. For the fastest throughput rate the read operation will take 100 ns. The read operation must be complete at least 400 ns before the falling edge of the next CONVST and this gives a total time of 5.7 s for the full throughput time (equivalent to 175 kHz). This mode of operation should be used for high sampling applications.
tACQ t8
CONVST
t3
BUSY
t3
t CONV = 5.2 s
A0
t CONV = 5.2 s
CS
RD
DATA
VA1
VA2
VB1
VB2
* WHEN USING AN EXTERNAL REFERENCE, WAKE-UP TIME = 4.8 s ** WHEN USING AN INTERNAL REFERENCE, WAKE-UP TIME = 5ms
Figure 6. Mode 2 Timing Diagram Where Automatic Sleep Function Is Initiated
REV. A
-11-
AD7863
AD7863 DYNAMIC SPECIFICATIONS
0 -10 -20 -30 -40 -50 -60
dB
The AD7863 is specified and tested for dynamic performance specifications as well as traditional dc specifications such as Integral and Differential Nonlinearity. These ac specifications are required for the signal processing applications such as phased array sonar, adaptive filters and spectrum analysis. These applications require information on the ADC's effect on the spectral content of the input signal. Hence, the parameters for which the AD7863 is specified include SNR, harmonic distortion, intermodulation distortion and peak harmonics. These terms are discussed in more detail in the following sections.
Signal-to-Noise Ratio (SNR)
FSAMPLE = 175kHz FIN = 10kHz SNR = +80.72dB THD = -92.96dB
-70 -80 -90
-100 -110 -120 -130 -140 -150
SNR is the measured signal to noise ratio at the output of the ADC. The signal is the rms magnitude of the fundamental. Noise is the rms sum of all the nonfundamental signals up to half the sampling frequency (fS/2), excluding dc. SNR is dependent upon the number of quantization levels used in the digitization process; the more levels, the smaller the quantization noise. The theoretical signal to noise ratio for a sine wave input is given by SNR = (6.02N + 1.76) dB where N is the number of bits. Thus for an ideal 14-bit converter, SNR = 86.04 dB. Figure 7 shows a histogram plot for 8,192 conversions of a dc input using the AD7863 with 5 V supply. The analog input was set at the center of a code transition. It can be seen that the codes appear mainly in the one output bin, indicating very good noise performance from the ADC.
8000 7000 6000
10
20
30 40 50 60 FREQUENCY - kHz
70
80
90
Figure 8. AD7863 FFT Plot
Effective Number of Bits
(1)
The formula given in Equation 1 relates the SNR to the number of bits. Rewriting the formula, as in Equation 2, it is possible to obtain a measure of performance expressed in effective number of bits (N).
N= SNR -1.76 6.02
(2)
The effective number of bits for a device can be calculated directly from its measured SNR. Figure 9 shows a typical plot of effective number of bits versus frequency for an AD7863-2 with a sampling frequency of 175 kHz. The effective number of bits typically falls between 13.11 and 11.05 corresponding to SNR figures of +80.68 dB and +68.28 dB.
14.0 13.5 13.0 12.5
ENOB
COUNTS
5000 4000 3000 2000 1000 0 746 747 748 749 750 751 CODE 752 753 754 755
12.0 11.5 11.0 10.5
Figure 7. Histogram of 8,192 Conversions of a DC Input
The output spectrum from the ADC is evaluated by applying a sine wave signal of very low distortion to the VAX/BX input, which is sampled at a 175 kHz sampling rate. A Fast Fourier Transform (FFT) plot is generated from which the SNR data can be obtained. Figure 8 shows a typical 8,192 point FFT plot of the AD7863 with an input signal of 10 kHz and a sampling frequency of 175 kHz. The SNR obtained from this graph is -80.72 dB. It should be noted that the harmonics are taken into account when calculating the SNR.
10.0
0
200
400 600 FREQUENCY - kHz
800
1000
Figure 9. Effective Numbers of Bits vs. Frequency
-12-
REV. A
AD7863
Total Harmonic Distortion (THD)
1
Total Harmonic Distortion (THD) is the ratio of the rms sum of harmonics to the rms value of the fundamental. For the AD7863, THD is defined as
THD (dB) = 20 log V22 +V32 +V42 +V52 V1
DNL ERROR - LSB
0.5
0
where V1 is the rms amplitude of the fundamental and V2, V3, V4 and V5 are the rms amplitudes of the second through the sixth harmonic. The THD is also derived from the FFT plot of the ADC output spectrum.
Intermodulation Distortion
-0.5
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 - fb) while the third order terms include (2fa + fb), (2fa - fb), (fa + 2fb) and (fa - 2fb). In this case, the second and third order terms are of different 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 separately. 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. In this case, the input consists of two equal amplitude, low distortion sine waves. Figure 10 shows a typical IMD plot for the AD7863.
0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 -130 -140 -150 INPUT FREQUENCIES F1 = 50.13kHz F2 = 49.13kHz FSAMPLE = 175kHz IMD: 2ND ORDER TERM -98.21dB 3RD ORDER TERM -93.91dB
-1
0
2048
4096
6144
8192 10240 12288 ADC CODE
14336 16383
Figure 11. DC DNL Plot
1
0.5
INL ERROR - LSB
0
-0.5
-1 0 2048 4096 6144 8192 10240 ADC CODE 12288 14336 16383
Figure 12. DC INL Plot
Power Considerations
In the automatic power-down mode then the part may be operated at a sample rate that is considerably less than 175 kHz. In this case, the power consumption will be reduced and will depend on the sample rate. Figure 13 shows a graph of the power consumption versus sampling rates from 1 Hz to 100 kHz in the automatic power-down mode. The conditions are 5 V supply 25C.
50 45 40
dB
0
10
20
30 40 50 60 FREQUENCY - kHz
70
80
90
35 POWER - mW 30 25 20 15 10 5 0 0 10 20 30 40 50 60 70 FREQUENCY - kHz 80 90 100
Figure 10. IMD Plot
Peak Harmonic or Spurious Noise
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 fS/2 and excluding dc) to the rms value of the fundamental. Normally, the value of this specification will be determined by the largest harmonic in the spectrum, but for parts where the harmonics are buried in the noise floor the peak will be a noise peak.
DC Linearity Plot
Figures 11 and 12 show typical DNL and INL plots for the AD7863. REV. A
Figure 13. Power vs. Sample Rate in Auto Power-Down Mode
-13-
AD7863
MICROPROCESSOR INTERFACING
The AD7863 high speed bus timing allows direct interfacing to DSP processors as well as modern 16-bit microprocessors. Suitable microprocessor interfaces are shown in Figures 14 through 18.
AD7863-ADSP-2100 Interface
PA2 ADDRESS BUS PA0 ADDR DECODE MEN INT DEN EN
OPTIONAL
TMS32010
CS A0 BUSY
CONVST
Figure 14 shows an interface between the AD7863 and the ADSP-2100. The CONVST signal can be supplied from the ADSP-2100 or from an external source. The AD7863 BUSY line provides an interrupt to the ADSP-2100 when conversion is completed on both channels. The two conversion results can then be read from the AD7863 using two successive reads to the same memory address. The following instruction reads one of the two results: MR0 = DM (ADC) where MR0 is the ADSP-2100 MR0 register and ADC is the AD7863 address.
DMA13 ADDRESS BUS DMA0 OPTIONAL
AD7863*
RD DB13 DB0
D15 DATA BUS D0 *ADDITIONAL PINS OMITTED FOR CLARITY
Figure 15. AD7863-TMS32010 Interface
AD7863-TMS320C25 Interface
ADSP-2100 (ADSP-2101/ ADSP-2102) DMS
IRQn DMRD (RD)
ADDR DECODE EN
CS A0 BUSY RD DB13 DB0
CONVST
AD7863*
Figure 16 shows an interface between the AD7863 and the TMS320C25. As with the two previous interfaces, conversion can be initiated from the TMS320C25 or from an external source, and the processor is interrupted when the conversion sequence is completed. The TMS320C25 does not have a separate RD output to drive the AD7863 RD input directly. This has to be generated from the processor STRB and R/W outputs with the addition of some logic gates. The RD signal is ORgated with the MSC signal to provide the one WAIT state required in the read cycle for correct interface timing. Conversion results are read from the AD7863 using the following instruction: IN D, ADC where D is Data Memory address and ADC is the AD7863 address.
OPTIONAL ADDRESS BUS A0 ADDR DECODE IS INTn STRB R/ W READY MSC DB13 DB0 DMD15 DATA BUS DMD0 *ADDITIONAL PINS OMITTED FOR CLARITY EN CONVST
DMD15 DATA BUS DMD0 *ADDITIONAL PINS OMITTED FOR CLARITY
Figure 14. AD7863-ADSP-2100 Interface
AD7863-ADSP-2101/ADSP-2102 Interface
A15
The interface outlined in Figure 14 also forms the basis for an interface between the AD7863 and the ADSP-2101/ADSP-2102. The READ line of the ADSP-2101/ADSP-2102 is labeled RD. In this interface, the RD pulsewidth of the processor can be programmed using the Data Memory Wait State Control Register. The instruction used to read one of the two results is as outlined for the ADSP-2100.
AD7863-TMS32010 Interface
TMS320C25
CS A0 BUSY
AD7863*
RD
An interface between the AD7863 and the TMS32010 is shown in Figure 15. Once again the CONVST signal can be supplied from the TMS32010 or from an external source, and the TMS32010 is interrupted when both conversions have been completed. The following instruction is used to read the conversion results from the AD7863: IN D, ADC where D is Data Memory address and ADC is the AD7863 address.
Figure 16. AD7863-TMS320C25 Interface
Some applications may require that the conversion is initiated by the microprocessor rather than an external timer. One option is to decode the AD7863 CONVST from the address bus so that a write operation starts a conversion. Data is read at the end of the conversion sequence as before. Figure 18 shows an example of initiating conversion using this method. Note that for all interfaces, it is preferred that a read operation not be attempted during conversion.
-14-
REV. A
AD7863
AD7863-MC68000 Interface Vector Motor Control
An interface between the AD7863 and the MC68000 is shown in Figure 17. As before, conversion can be supplied from the MC68000 or from an external source. The AD7863 BUSY line can be used to interrupt the processor or, alternatively, software delays can ensure that conversion has been completed before a read to the AD7863 is attempted. Because of the nature of its interrupts, the MC68000 requires additional logic (not shown in Figure 18) to allow it to be interrupted correctly. For further information on MC68000 interrupts, consult the MC68000 users manual. The MC68000 AS and R/W outputs are used to generate a separate RD input signal for the AD7863. CS is used to drive the 68000 DTACK input to allow the processor to execute a normal read operation to the AD7863. The conversion results are read using the following 68000 instruction: MOVE.W ADC, D0 where D0 is the 68000 D0 register and ADC is the AD7863 address.
A15 ADDRESS BUS A0 CONVST OPTIONAL
The current drawn by a motor can be split into two components: one produces torque and the other produces magnetic flux. For optimal performance of the motor, these two components should be controlled independently. In conventional methods of controlling a three-phase motor, the current (or voltage) supplied to the motor and the frequency of the drive are the basic control variables. However, both the torque and flux are functions of current (or voltage) and frequency. This coupling effect can reduce the performance of the motor because, for example, if the torque is increased by increasing the frequency, the flux tends to decrease. Vector control of an ac motor involves controlling phase in addition to drive and current frequency. Controlling the phase of the motor requires feedback information on the position of the rotor relative to the rotating magnetic field in the motor. Using this information, a vector controller mathematically transforms the three phase drive currents into separate torque and flux components. The AD7863 is ideally suited for use in vector motor control applications. A block diagram of a vector motor control application using the AD7863 is shown in Figure 19. The position of the field is derived by determining the current in each phase of the motor. Only two phase currents need to be measured because the third can be calculated if two phases are known. VA1 and VA2 of the AD7863 are used to digitize this information. Simultaneous sampling is critical to maintain the relative phase information between the two channels. A current sensing isolation amplifier, transformer or Hall effect sensor is used between the motor and the AD7863. Rotor information is obtained by measuring the voltage from two of the inputs to the motor. VB1 and VB2 of the AD7863 are used to obtain this information. Once again the relative phase of the two channels is important. A DSP microprocessor is used to perform the mathematical transformations and control loop calculations on the information fed back by the AD7863.
DSP MICROPROCESSOR TORQUE & FLUX CONTROL LOOP CALCULATIONS & TWO TO THREE PHASE INFORMATION TORQUE SETPOINT DAC DAC DAC DRIVE CIRCUITRY
MC68000
ADDR DECODE EN
A0 CS
DTACK AS R/ W DB13 DB0 D15 DATA BUS D0 RD
AD7863*
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 17. AD7863-MC68000 Interface
AD7863-80C196 Interface
Figure 18 shows an interface between the AD7863 and the 80C196 microprocessor. Here, the microprocessor initiates conversion. This is achieved by gating the 80C196 WR signal with a decoded address output (different from the AD7863 CS address). The AD7863 BUSY line is used to interrupt the microprocessor when the conversion sequence is completed.
A15 ADDRESS BUS A1 ADDR DECODE CS A0
IC IB IA VB VA 3 PHASE MOTOR
FLUX SETPOINT TRANSFORMATION TO TORQUE & FLUX CURRENT COMPONENTS
ISOLATION AMPLIFIERS VA1 VA2
80C196
EN
AD7863*
VB1 VB2
AD7863*
BUSY RD DB13 DB0 D15 DATA BUS D0 *ADDITIONAL PINS OMITTED FOR CLARITY
WR RD
*ADDITIONAL PINS OMITTED FOR CLARITY
VOLTAGE ATTENUATORS
Figure 19. Vector Motor Control Using the AD7863
Figure 18. AD7863-80C196 Interface
REV. A
-15-
AD7863
MULTIPLE AD7863S
Figure 20 shows a system where a number of AD7863s can be configured to handle multiple input channels. This type of configuration is common in applications such as sonar, radar, etc. The AD7863 is specified with typical limits on aperture delay. This means that the user knows the difference in the sampling instant between all channels. This allows the user to maintain relative phase information between the different channels.
VA1 VB1 VA2 VB2 VREF
APPLICATIONS HINTS PC Board Layout Considerations
The AD7863 is optimally designed for lowest noise performance, both radiated and conducted noise. To complement the excellent noise performance of the AD7863 it is imperative that great care be given to the PC board layout. Figure 21 shows a recommended connection diagram for the AD7863.
Ground Planes
RD
RD
AD7863 (1)
CS
The AD7863 and associated analog circuitry should have a separate ground plane referred to as the analog ground plane (AGND). This analog ground plane should encompass all AD7863 ground pins (including the DGND pin), voltage reference circuitry, power supply bypass circuitry, the analog input traces and any associated input/buffer amplifiers. The regular PCB ground plane (referred to as the DGND for this discussion) area should encompass all digital signal traces, excluding the ground pins, leading up to the AD7863.
Power Planes
VA1 VB1 VA2 VB2 VREF
RD
AD7863 (2)
CS ADDRESS DECODE ADDRESS
VREF VA1 VB1 VA2 VB2
RD
AD7863 (n)
CS
The PC board layout should have two distinct power planes, one for analog circuitry and one for digital circuitry. The analog power plane should encompass the AD7863 (VDD) and all associated analog circuitry. This power plane should be connected to the regular PCB power plane (VCC) at a single point, if necessary through a ferrite bead, as illustrated in Figure 21. This bead (part numbers for reference: Fair-Rite 274300111 or Murata BL01/02/03) should be located within three inches of the AD7863. The PCB power plane (VCC) should provide power to all digital logic on the PC board, and the analog power plane (VDD) should provide power to all AD7863 power pins, voltage reference circuitry and any input amplifiers, if needed. A suitable low noise amplifier for the AD7863 is the AD797, one for each input. Ensure that the +VS and the -VS supplies to each amplifier are individually decoupled to AGND. The PCB power (VCC) and ground (DGND) should not overlay portions of the analog power plane (VDD). Keeping the VCC power and the DGND planes from overlaying the VDD will contribute to a reduction in plane-to-plane noise coupling.
Figure 20. Multiple AD7863s in Multichannel System
A common read signal from the microprocessor drives the RD input of all AD7863s. Each AD7863 is designated a unique address selected by the address decoder. The reference output of AD7863 number 1 is used to drive the reference input of all other AD7863s in the circuit shown in Figure 20. One VREF can be used to provide the reference to several other AD7863s. Alternatively, an external or system reference can be used to drive all VREF inputs. A common reference ensures good full-scale tracking between all channels.
-16-
REV. A
AD7863
Supply Decoupling
Noise on the analog power plane (VDD) can be further reduced by use of multiple decoupling capacitors (Figure 21). Optimum performance is achieved by the use of disc ceramic capacitors. The VDD and reference pins (whether using an
external or an internal reference) should be individually decoupled to the analog ground plane (AGND). This should be done by placing the capacitors as close as possible to the AD7863 pins with the capacitor leads as short as possible, thus minimizing lead inductance.
L (FERRITE BEAD)
VIN TEMP 0.1 F VOUT +15V 0.1 F 0.1 F +VS VA1 0.1 F AGND DGND AGND VA1 VDD VREF
0.1 F
10 F
47 F
ANALOG SUPPLY +5V
AD780
AD7863
VB1 VB1
VA2
VA2
VB2 ANALOG SUPPLY -15V
VB2 -VS
4
AD797s
0.1 F
Figure 21. Typical Connections Diagram Including the Relevant Decoupling
REV. A
-17-
AD7863
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
28-Lead Wide Body (SOIC) (R-28)
0.7125 (18.10) 0.6969 (17.70)
28 15
1
14
PIN 1
0.1043 (2.65) 0.0926 (2.35)
0.4193 (10.65) 0.3937 (10.00)
0.2992 (7.60) 0.2914 (7.40)
0.0291 (0.74) x 45 0.0098 (0.25)
0.0118 (0.30) 0.0040 (0.10)
0.0500 (1.27) BSC
8 0.0500 (1.27) 0.0192 (0.49) 0 0.0157 (0.40) SEATING 0.0125 (0.32) 0.0138 (0.35) PLANE 0.0091 (0.23)
28-Lead Shrink Small Outline (SSOP) (RS-28)
0.407 (10.34) 0.397 (10.08)
28
15
0.311 (7.9) 0.301 (7.64)
1
14
0.078 (1.98) PIN 1 0.068 (1.73)
0.07 (1.79) 0.066 (1.67)
0.212 (5.38) 0.205 (5.21)
0.008 (0.203) 0.0256 (0.65) 0.002 (0.050) BSC
8 0.015 (0.38) SEATING 0.009 (0.229) 0 0.010 (0.25) PLANE 0.005 (0.127)
0.03 (0.762) 0.022 (0.558)
-18-
REV. A
PRINTED IN U.S.A.
C3290a-0-5/99

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