AD8224ACPZ-R7 datasheet(22/28 Pages) AD

22 / 28 page

AD8224

Rev. 0 | Page 22 of 28

Mismatched CC capacitors result in mismatched low-pass filters.

The imbalance causes the AD8224 to treat what would have

been a common-mode signal as a differential signal. To reduce

the effect of mismatched external CC capacitors, select a value of

CD greater than 10 times CC. This sets the differential filter

frequency lower than the common-mode frequency.

The relationship between external, matched series resistors and the

internal gate capacitance is expressed as follows:

DIFF

FilterFreq

AD8224

+15V

+IN

–IN

0.1µF

10µF

0.1µF

REF

VOUT

–15V

10nF

1nF

4.02kΩ

AD8224

VOUT

–VS

REF

–VS

+IN

–IN

+15V

–15V

0.1µF

10µF

0.1µF

10µF

Figure 57. RFI Suppression

COMMON-MODE INPUT VOLTAGE RANGE

The 3-op amp architecture of the AD8224 applies gain and then

removes the common-mode voltage. Therefore, internal nodes

in the AD8224 experience a combination of both the gained

signal and the common-mode signal. This combined signal can

be limited by the voltage supplies even when the individual input

and output signals are not. Figure 21 through Figure 24 show the

allowable common-mode input voltage ranges for various

output voltages, supply voltages, and gains.

Figure 56. RFI Filtering Without External Capacitors

To eliminate high frequency common-mode signals while using

smaller source resistors, a low-pass R-C network can be placed

at the input of the instrumentation amplifier (see Figure 57).

The filter limits the input signal bandwidth according to the

following relationship:

)

(

DIFF

FilterFreq

)

(

FilterFreq

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