9.1.5 Input Bias Current Return Path

The input impedance of the INA851 is very high at approximately 1 GΩ. However, a path must be provided for

the input bias current of both inputs. This input bias current is typically 5 nA. High input impedance means that

this input bias current changes very little with varying input voltage.

For proper operation, input circuitry must provide a path for input bias current. Figure 9-1 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 INA851, and the input amplifiers saturate. If the differential source resistance is

low, the bias current return path can connect to one input (as shown in the thermocouple example in Figure 9-1).

With a higher source impedance, using two equal resistors provides a balanced input with possible advantages

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

For more details about why a valid input bias current return path is necessary, see the Importance of Input Bias

Current Return Paths in Instrumentation Amplifier Applications application report.

TI Device

47 kW

47 kW

TI Device

10 kW

Microphone,

Hydrophone,

and So Forth

Thermocouple

TI Device

Center tap provides

bias current return.

Copyright © 2017, Texas Instruments Incorporated

Figure 9-1. Providing an Input Common-Mode Current Path

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INA851

SBOS999 – MARCH 2022

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