AD8222BCPZ-R7 datasheet(18/24 Pages) AD

18 / 24 page

AD8222

Rev. 0 | Page 18 of 24

AD8222

+15V

+IN

–IN

0.1µF

10µF

0.1µF

REF

VOUT

–15V

499Ω

10nF

1nF

4.02kΩ

Differential Input Voltages at High Gains

When operating at high gain, large differential input voltages

can cause more than 6 mA of current to flow into the inputs.

This condition occurs when the differential voltage exceeds

the following critical voltage

VCRITICAL = (400 + RG) × (6 mA)

This is true for differential voltages of either polarity.

The maximum allowed differential voltage can be increased by

adding an input protection resistor in series with each input.

The value of each protection resistor should be

RPROTECT = (VDIFF_MAX − VCRITICAL)/6 mA

Figure 48. RFI Suppression

RF INTERFERENCE

Figure 48 shows an example where the differential filter

frequency is approximately 2 kHz, and the common-mode filter

frequency is approximately 40 kHz.

RF rectification is often a problem when amplifiers are used in

applications where there are strong RF signals. The disturbance

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

can be filtered with a low-pass, RC network placed at the input

of the instrumentation amplifier, as shown in Figure 48. The

filter limits the input signal bandwidth according to the

following relationship.

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 degrades the CMRR of the AD8222. By

using a value of CD 10× larger than the value of CC, the effect of

the mismatch is reduced and performance is improved.

)

Diff

FilterFreq

COMMON-MODE INPUT VOLTAGE RANGE

The three op amp architecture of the AD8222 applies gain and

then removes the common-mode voltage. Therefore, internal

nodes in the AD8222 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 7 and Figure 8

show the allowable common-mode input voltage ranges for

various output voltages, supply voltages, and gains.

FilterFreq

where CD ≥ 10CC.

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