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AD629 Datasheet(PDF) 11 Page - Analog Devices |
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AD629 Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page AD629 Rev. B | Page 11 of 16 ANALOG POWER SUPPLY DIGITAL POWER SUPPLY 0.1µF 0.1µF 0.1µF 0.1µF +IN –IN –VS VIN1 VIN2 VDD VDD OUTPUT AGND GND MICROPROCESSOR DGND +VS AD629 AD7892-2 REF(–) REF(+) 6 7 14 4 1 3 3 2 6 4 1 5 12 +5V GND +5V GND –5V Figure 32. Optimal Grounding Practice for a Bipolar Supply Environment with Separate Analog and Digital Supplies POWER SUPPLY VIN1 VIN2 VDD AGND DGND ADC 0.1µF 0.1µF +IN –IN +VS OUTPUT –VS AD629 REF(–) REF(+) 4 7 3 2 6 1 5 VDD GND MICROPROCESSOR +5V GND 0.1µF Figure 33. Optimal Ground Practice in a Single-Supply Environment If there is only a single power supply available, it must be shared by both digital and analog circuitry. Figure 33 shows how to minimize interference between the digital and analog circuitry. In this example, the ADC’s reference is used to drive Pin REF(+) and Pin REF(–). This means that the reference must be capable of sourcing and sinking a current equal to VCM/200 kΩ. As in the previous case, separate analog and digital ground planes should be used (reasonably thick traces can be used as an alternative to a digital ground plane). These ground planes should connect at the power supply’s ground pin. Separate traces (or power planes) should run from the power supply to the supply pins of the digital and analog circuits. Ideally, each device should have its own power supply trace, but these can be shared by a number of devices, as long as a single trace is not used to route current to both digital and analog circuitry. USING A LARGE SENSE RESISTOR Insertion of a large value shunt resistance across the input pins, Pin 2 and Pin 3, will imbalance the input resistor network, introducing a common-mode error. The magnitude of the error will depend on the common-mode voltage and the magnitude of RSHUNT. Table 3 shows some sample error voltages generated by a common-mode voltage of 200 V dc with shunt resistors from 20 Ω to 2000 Ω. Assuming that the shunt resistor is selected to use the full ±10 V output swing of the AD629, the error voltage becomes quite significant as RSHUNT increases. Table 3. Error Resulting from Large Values of RSHUNT (Uncompensated Circuit) RS (Ω) Error VOUT (V) Error Indicated (mA) 20 0.01 0.5 1000 0.498 0.498 2000 1 0.5 To measure low current or current near zero in a high common- mode environment, an external resistor equal to the shunt resistor value can be added to the low impedance side of the shunt resistor, as shown in Figure 34. REF (–) REF (+) –VS –VS +VS +VS VOUT NC –IN +IN RSHUNT RCOMP ISHUNT 0.1µF 0.1µF NC = NO CONNECT 21.1kΩ 380kΩ 380kΩ 20kΩ 380kΩ AD629 1 2 3 4 8 7 6 5 Figure 34. Compensating for Large Sense Resistors OUTPUT FILTERING A simple 2-pole, low-pass Butterworth filter can be implemented using the OP177 after the AD629 to limit noise at the output, as shown in Figure 35. Table 4 gives recommended component values for various corner frequencies, along with the peak-to- peak output noise for each case. REF (–) REF (+) –VS –VS +VS +VS +VS VOUT NC –IN +IN 0.1µF 0.1µF 0.1µF 0.1µF NC = NO CONNECT 21.1kΩ 380kΩ 380kΩ 20kΩ 380kΩ AD629 1 2 3 4 8 7 6 5 R1 R2 C1 C2 OP177 Figure 35. Filtering of Output Noise Using a 2-Pole Butterworth Filter Table 4. Recommended Values for 2-Pole Butterworth Filter Corner Frequency R1 R2 C1 C2 Output Noise (p-p) No Filter 3.2 mV 50 kHz 2.94 kΩ ± 1% 1.58 kΩ ± 1% 2.2 nF ± 10% 1 nF ± 10% 1 mV 5 kHz 2.94 kΩ ± 1% 1.58 kΩ ± 1% 22 nF ± 10% 10 nF ± 10% 0.32 mV 500 Hz 2.94 kΩ ± 1% 1.58 kΩ ± 1% 220 nF ± 10% 0.1 μF ± 10% 100 μV 50 Hz 2.7 kΩ ± 10% 1.5 kΩ ± 10% 2.2 μF ± 20% 1 μF ± 20% 32 μV |
Similar Part No. - AD629_07 |
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Similar Description - AD629_07 |
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