AD8221 datasheet(16/20 Pages) AD

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AD8221

Rev. A | Page 16 of 20

RF INTERFERENCE

RF rectification is often a problem when amplifiers are used in

applications where there are strong RF signals. The disturbance

may appear as a small dc offset voltage. High frequency signals

can be filtered with a low-pass R-C network placed at the input

of the instrumentation amplifier, as shown in Figure 49. The

filter limits the input signal bandwidth according to the

following relationship:

CD affects the difference signal and CC affects the common-

mode signal. Values of R and CC should be chosen to minimize

RFI. Mismatch between the R × CC at the positive input and the

R × CC at negative input will degrade the AD8221’s CMRR. By

using a value of CD one magnitude larger than CC, the effect of

the mismatch is reduced, and hence, performance is improved.

PRECISION STRAIN GAGE

)

(

Diff

FilterFreq

The AD8221’s low offset and high CMRR over frequency make

it an excellent candidate for bridge measurements. As shown in

Figure 50, the bridge can be directly connected to the inputs of

the amplifier.

FilterFreq

+5V

+2.5V

µF

0.1

µF

AD8221

+IN

–IN

350

Ω

350

Ω

350

Ω

350

Ω

–

where CD ≥ 10CC.

AD8221

+15V

+IN

–IN

0.1

µF

0.1

µF

REF

VOUT

–15V

499

Ω

10nF

1nF

4.02k

Ω

4.02k

Ω

Figure 50. Precision Strain Gage

Figure 49. RFI Suppression

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