AD620 datasheet(17/20 Pages) AD

17 / 20 page

AD620

Rev. G | Page 17 of 20

499

Ω

AD620

–

VOUT

+IN

–IN

REF

–15V

0.1

µ F1 µ F

+15V

0.1

µ F1 µ F

Figure 43. Circuit to Attenuate RF Interference

COMMON-MODE REJECTION

Instrumentation amplifiers, such as the AD620, offer high

CMR, which is a measure of the change in output voltage when

both inputs are changed by equal amounts. These specifications

are usually given for a full-range input voltage change and a

specified source imbalance.

For optimal CMR, the reference terminal should be tied to a

low impedance point, and differences in capacitance and

resistance should be kept to a minimum between the two

inputs. In many applications, shielded cables are used to

minimize noise; for best CMR over frequency, the shield

should be properly driven. Figure 44 and Figure 45 show active

data guards that are configured to improve ac common-mode

rejections by “bootstrapping” the capacitances of input cable

shields, thus minimizing the capacitance mismatch between the

inputs.

REFERENCE

VOUT

AD620

100

Ω

100

Ω

– INPUT

+ INPUT

AD648

–VS

+VS

–VS

Figure 44. Differential Shield Driver

100

Ω

– INPUT

+ INPUT

REFERENCE

VOUT

AD620

–VS

+VS

AD548

Figure 45. Common-Mode Shield Driver

GROUNDING

Since the AD620 output voltage is developed with respect to the

potential on the reference terminal, it can solve many

grounding problems by simply tying the REF pin to the

appropriate “local ground.”

To isolate low level analog signals from a noisy digital

environment, many data-acquisition components have separate

analog and digital ground pins (Figure 46). It would be

convenient to use a single ground line; however, current

through ground wires and PC runs of the circuit card can cause

hundreds of millivolts of error. Therefore, separate ground

returns should be provided to minimize the current flow from

the sensitive points to the system ground. These ground returns

must be tied together at some point, usually best at the ADC

package shown in Figure 46.

DIGITAL P.S.

+5V

ANALOG P.S.

+15V C –15V

AD574A

DIGITAL

DATA

OUTPUT

µF

AD620

0.1

µ F

AD585

S/H

ADC

0.1

µ F

µF 1µF

Figure 46. Basic Grounding Practice

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