5 V

OUT

INA351A

IN+

IN–

A_IN+

A_IN–

A_OUT

100 k

1 µF

5 V

100 k

INA output with

calibrated offset

R1

R2

C2

Figure 9-2. INA351A in Servo-Loop Calibration Configuration

9.1.2 Input Bias Current Return Path

The input impedance of the INA351A is extremely high, but a path must be provided for the input bias current of

both inputs. This input bias current is typically a few pico amps but at high temperature this can be a few nano

amps. High input impedance means that the input bias current changes little with varying input voltage.

For proper operation, input circuitry must provide a path for this input bias current. Figure 9-3 shows various

provisions for an input bias current path. Without a bias current path, the inputs float to a potential that exceeds

the common-mode range of the INA351A, and the input amplifiers saturate. If the differential source resistance

is low, the bias current return path connects to one input (as shown in the thermocouple example in Figure

9-3). With a higher source impedance, use two equal resistors to provide a balanced input, with the possible

advantages of a lower input offset voltage as a result of bias current, and better high-frequency common-mode

rejection.

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INA351A

SBOSAG2 – JUNE 2023

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