AD629

Rev. B | Page 11 of 16

ANALOG POWER

SUPPLY

DIGITAL

POWER SUPPLY

0.1µF

0.1µF

0.1µF

0.1µF

+IN

–IN

OUTPUT

AGND

GND

MICROPROCESSOR

DGND

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

AGND DGND

ADC

0.1µF

0.1µF

+IN

–IN

OUTPUT

AD629

REF(–) REF(+)

4

7

3

2

6

1

5

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

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

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

(Uncompensated Circuit)

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 (+)

NC

–IN

+IN

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 (+)

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