AD8228

Rev. 0 | Page 18 of 24

References

The output voltage of the AD8228 is developed with respect to

the potential on the reference terminal. Care should be taken to

tie REF to the appropriate local ground.

Input Bias Current Return Path

The input bias current of the AD8228 must have a return path

to common. When the source, such as a thermocouple, cannot

provide a return current path, one should be created, as shown

in Figure 46.

THERMOCOUPLE

REF

AD8228

CAPACITIVELY COUPLED

REF

C

C

AD8228

TRANSFORMER

REF

AD8228

INCORRECT

CAPACITIVELY COUPLED

REF

C

R

R

C

AD8228

1

THERMOCOUPLE

REF

10MΩ

AD8228

TRANSFORMER

REF

AD8228

CORRECT

INPUT PROTECTION

All terminals of the AD8228 are protected against ESD (1 kV,

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

dc overload conditions of about 3.5 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 AD8228 can safely handle a continuous 6 mA current. The

limiting resistor can be computed from

Ω

600

mA

6

−

−

≥

SUPPLY

IN

LIMIT

V

V

R

For applications where the AD8228 encounters extreme overload

voltages, such as cardiac defibrillators, external series resistors

and low leakage diode clamps such as the BAV199L, the FJH1100s,

or the SP720 should be used.

Large Differential Voltages

When G = 100

When operating at a gain of 100, large differential input voltages

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

This condition occurs when the voltage between +IN and –IN

exceeds 5 V. 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

RADIO FREQUENCY INTERFERENCE (RFI)

RF rectification is often a problem when amplifiers are used in

applications having 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 instru-

mentation amplifier, as shown in Figure 47. The filter limits the

input signal bandwidth, according to the following relationship:

)

2

(

π

2

1

C

D

C

C

R

+

C

RC

π

2

1

R

R

AD8228

+15V

+IN

–IN

0.1µF

10µF

10µF

0.1µF

REF

–15V

10nF

1nF

1nF

4.02kΩ

4.02kΩ

Figure 47. RFI Suppression

at the negative input degrades the CMRR of the AD8228. By using

mismatch is reduced, and performance is improved.