AD8224ACPZ-R7 datasheet(25/29 Pages) AD

25 / 29 page

AD8224

Data Sheet

Rev. D | Page 24 of 28

For applications where the AD8224 encounters extreme

overload voltages, as in cardiac defibrillators, external series

resistors and low leakage diode clamps, such as BAV199Ls,

FJH1100s, or SP720s, should be used.

TRANSFORMER

+VS

REF

–VS

AD8224

INCORRECT

TRANSFORMER

+VS

REF

–VS

AD8224

CORRECT

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8224

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8224

fHIGH-PASS = 2πRC

Figure 60. Creating an IBIAS Path

RF INTERFERENCE

RF rectification is often a problem in applications where there are

large RF signals. The problem appears as a small dc offset voltage.

The AD8224 by its nature has a 5 pF gate capacitance (CG) at its

inputs. Matched series resistors form a natural low-pass filter that

reduces rectification at high frequency (see Figure 61).

AD8224

VOUT

–VS

REF

–VS

+IN

–IN

+15V

–15V

0.1µF

10µF

0.1µF

10µF

Figure 61. RFI Filtering Without External Capacitors

The relationship between external, matched series resistors and

the internal gate capacitance is expressed as

DIFF

FilterFreq





FilterFreq





To eliminate high frequency common-mode signals while using

smaller source resistors, a low-pass RC network can be placed at

the input of the instrumentation amplifier (see Figure 62). The

filter limits the input signal bandwidth according to the

following relationship:

)

(

DIFF

FilterFreq







)

(

FilterFreq







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.

AD8224

+15V

+IN

–IN

0.1µF

10µF

0.1µF

REF

VOUT

–15V

10nF

1nF

4.02kΩ

Figure 62. 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 25 through Figure 28 show the

allowable common-mode input voltage ranges for various

output voltages, supply voltages, and gains.

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