9 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification,

and TI does not warrant its accuracy or completeness. TI’s customers are responsible for

determining suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

9.1 Application Information

9.1.1 Input Bias Current Return Path

The input impedance of the INA823 is extremely high, but a path must be provided for the input bias current of

both inputs. This input bias current is typically 1.2 nA. High input impedance means that this input bias current

changes little with varying input voltage.

For proper operation, input circuitry must provide a path for this input bias current. Figure 9-1 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 INA823, 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

9-1). 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. Furthermore, matched input impedances generally minimize the impact to performance in cases where

runs out of headroom. The input offset current typically remains low; therefore, well-matched input impedances

reduce the differential error voltage that would otherwise arise.

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 note.

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

www.ti.com

INA823

SBOSA75B – JULY 2021 – REVISED NOVEMBER 2021

Copyright © 2021 Texas Instruments Incorporated

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Product Folder Links: INA823