Data Sheet

AD8428

Rev. 0 | Page 15 of 20

INPUT BIAS CURRENT RETURN PATH

The input bias current of the AD8428 must have a return path

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

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

in Figure 41.

THERMOCOUPLE

REF

AD8428

CAPACITIVELY COUPLED

REF

C

C

AD8428

TRANSFORMER

REF

AD8428

INCORRECT

CAPACITIVELY COUPLED

REF

C

R

R

C

AD8428

1

THERMOCOUPLE

REF

10MΩ

AD8428

TRANSFORMER

REF

AD8428

CORRECT

Figure 41. Creating an Input Bias Current Return Path

INPUT PROTECTION

Do not allow the inputs of the AD8428 to exceed the ratings

stated in the Absolute Maximum Ratings section. If these ratings

cannot be adhered to, add protection circuitry in front of the

AD8428 to limit the maximum current into the inputs (see the

IMAX section).

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.

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 each input can be computed using

the following equation:

MAX

SUPPLY

IN

PROTECT

I

V

V

R

−

≥

Noise sensitive applications may require a lower protection

resistance. Low leakage diode clamps, such as the BAV199, can

be used at the inputs to shunt current away from the AD8428

inputs and, therefore, allow smaller protection resistor values.

To ensure that current flows primarily through the external

protection diodes, place a small value resistor, such as a 33 Ω

resistor, between the diodes and the AD8428.

SIMPLE METHOD

LOW NOISE METHOD

AD8428

I

+

–

+

–

AD8428

33Ω

33Ω

I

+

–

+

–

Figure 42. 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 using the following equation:

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

−

−

×

≥

G

MAX

DIFF

PROTECT

R

I

V

R

V

1

2

1

Noise sensitive applications may require a lower protection

resistance. Low leakage diode clamps, such as the BAV199,

can be used across the AD8428 inputs to shunt current away

from the inputs and, therefore, allow smaller protection resistor

values.

AD8428

I

+

–

Figure 43. Protection for Large Differential Voltages