AD623

–13–

REV. C

are common mode (the same on both in amp inputs) and are

not applied differentially. This second low pass network, R1+R2

and C3, has a –3 dB frequency equal to: 1/(2

π (R1+R2) (C3)).

Using a C3 value of 0.047

µF as shown, the –3 dB signal BW of

this circuit is approximately 400 Hz. The typical dc offset shift

over frequency will be less than 1.5

µV and the circuit’s RF

signal rejection will be better than 71 dB. The 3 dB signal band-

width of this circuit may be increased to 900 Hz by reducing

resistors R1 and R2 to 2.2 k

Ω. The performance is similar to

that using 4 k

Ω resistors, except that the circuitry preceding the

in amp must drive a lower impedance load.

The circuit of Figure 43 should be built using a PC board with a

ground plane on both sides. All component leads should be as

short as possible. Resistors R1 and R2 can be common 1% metal

film units but capacitors C1 and C2 need to be

±5% tolerance

devices to avoid degrading the circuit’s common-mode rejection.

Either the traditional 5% silver mica units or Panasonic

±2%

PPS film capacitors are recommended.

–IN

LOCATE C1–C3 AS CLOSE

TO THE INPUT PINS AS POSSIBLE

0.01 F

REFERENCE

AD623

0.33 F

0.01 F

0.33 F

C3

0.047 F

C2

1000pF

5%

C1

1000pF

5%

R2

4.02k

1%

R1

4.02k

1%

+IN

Figure 43. Circuit to Attenuate RF Interference

In many applications shielded cables are used to minimize noise;

for best CMR over frequency the shield should be properly

driven. Figure 44 shows an active guard drive that is configured

to improve ac common-mode rejection by “bootstrapping” the

capacitances of input cable shields, thus minimizing the capaci-

tance mismatch between the inputs.

2

–INPUT

+INPUT

100

AD623

AD8031

REFERENCE

2

Figure 44. Common-Mode Shield Driver

GROUNDING

Since the AD623 output voltage is developed with respect to the

potential on the reference terminal, many grounding problems

can be solved by simply by tying the REF pin to the appropri-

ate “local ground.” The REF pin should, however, be tied to a

low impedance point for optimal CMR.

The use of ground planes is recommended to minimize the

impedance of ground returns (and hence the size of dc errors).

In order to isolate low level analog signals from a noisy digital

environment, many data-acquisition components have separate

analog and digital ground returns (Figure 45). All ground pins

from mixed signal components such as analog-to-digital converters

should be returned through the “high quality” analog ground

DIGITAL POWER SUPPLY

0.1 F

AGND DGND

AD7892-2

ADC

12

AGND

PROCESSOR

0.1 F

0.1 F

0.1 F

AD623

ANALOG POWER SUPPLY

+5V

–5V

GND

GND

+5V

Figure 45. Optimal Grounding Practice for a Bipolar Supply Environment with Separate Analog and Digital Supplies

AGND DGND

AD7892-2

ADC

12

DGND

PROCESSOR

0.1 F

AD623

0.1 F

0.1 F

POWER SUPPLY

+5V

GND

Figure 46. Optimal Ground Practice in a Single Supply Environment