AD8429

Rev. 0 | Page 17 of 20

INPUT BIAS CURRENT RETURN PATH

The input bias current of the AD8429 must have a return path to

ground. When using a floating source without a current return

path, such as a thermocouple, create a current return path, as

shown in Figure 50.

THERMOCOUPLE

REF

AD8429

CAPACITIVELY COUPLED

REF

C

C

AD8429

TRANSFORMER

REF

AD8429

INCORRECT

CAPACITIVELY COUPLED

REF

C

R

R

C

AD8429

1

THERMOCOUPLE

REF

10MΩ

AD8429

TRANSFORMER

REF

AD8429

CORRECT

Figure 50. Creating an Input Bias Current Return Path

INPUT PROTECTION

Do not allow the inputs of the AD8429 to exceed the ratings

stated in the Absolute Maximum Ratings section of this data

sheet. If this cannot be done, protection circuitry can be added

in front of the AD8429 to limit the current into the inputs to a

Input Voltages Beyond the Rails

If voltages beyond the rails are expected, use an external resistor

in series with each input to limit current during overload condi-

tions. The limiting resistor at the input can be computed from

MAX

SUPPLY

IN

PROTECT

I

V

V

R

|

|

−

≥

Noise sensitive applications may require a lower protection resis-

tance. Low leakage diode clamps, such as the BAV199, can be used

at the inputs to shunt current away from the AD8429 inputs,

thereby allowing smaller protection resistor values. To ensure

current flows primarily through the external protection diodes,

place a small value resistor, such as a 33 Ω, between the diodes and

the AD8429.

SIMPLE METHOD

LOW NOISE METHOD

AD8429

I

+

–

+

–

AD8429

33Ω

33Ω

I

+

–

+

–

Figure 51. Protection for Voltages Beyond the Rails

Large Differential Input Voltage at High Gain

If large differential voltages at high gain are expected, use an

external resistor in series with each input to limit current during

overload conditions. The limiting resistor at each input can be

computed by using the following equation:

G

MAX

DIFF

PROTECT

R

I

V

R

−

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

−

≥

V

1

2

1

Noise sensitive applications may require a lower protection resis-

tance. Low leakage diode clamps, such as the BAV199, can be used

across the inputs to shunt current away from the AD8429 inputs

and, therefore, allow smaller protection resistor values.

AD8429

I

+

–

AD8429

I

+

–

SIMPLE METHOD

LOW NOISE METHOD

Figure 52. Protection for Large Differential Voltages

on time and temperature. At room temperature, the device can

withstand a current of 10 mA for at least one day. This time is

cumulative over the life of the device.

RADIO FREQUENCY INTERFERENCE (RFI)

RF rectification is often a problem when amplifiers are used in

applications that have 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 53.