AD8222

Rev. A | Page 18 of 24

INPUT PROTECTION

All terminals of the AD8222 are protected against ESD (1 kV,

human body model). In addition, the input structure allows for

dc overload conditions of about 2.5 V beyond the supplies.

Input Voltages Beyond the Rails

For larger input voltages, an external resistor should be used in

series with each input to limit current during overload conditions.

The AD8222 can safely handle a continuous 6 mA current. The

limiting resistor can be computed from

Ω

400

mA

6







SUPPLY

IN

LIMIT

V

V

R

For applications in which the AD8222 encounters extreme over-

load voltages, such as cardiac defibrillators, external series

resistors and low leakage diode clamps, such as the BAV199L,

the FJH1100, or the SP720, should be used.

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:

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

RF INTERFERENCE

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:

)

(2

2

1

C

D

Diff

C

C

R

FilterFreq







C

CM

C

R

FilterFreq







2

1

R

R

AD8222

+15V

+IN

–IN

0.1µF

10µF

10µF

0.1µF

REF

–15V

R1

499Ω

10nF

1nF

1nF

4.02kΩ

4.02kΩ

Figure 48. RFI Suppression

Figure 48 shows an example where the differential filter fre-

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

frequency is approximately 40 kHz.

negative input degrades the CMRR of the AD8222. By using a

mismatch is reduced and performance is improved.

COMMON-MODE INPUT VOLTAGE RANGE

The 3-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.