INA821 datasheet(28/51 Pages) TI2

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9.1.2 Input Bias Current Return Path

The input impedance of the INA819 is extremely high—approximately 100 GΩ. However, a path must be

provided for the input bias current of both inputs. This input bias current is typically 150 pA. 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 9-4 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 INA819, 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 9-4).

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.

TI Device

47 kW

TI Device

10 kW

Microphone,

Hydrophone,

and So Forth

Thermocouple

TI Device

Center tap provides

bias current return.

Figure 9-4. Providing an Input Common-Mode Current Path

INA819

SBOS959D – DECEMBER 2018 – REVISED APRIL 2022

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INA821 Datasheet(PDF) 28 Page - Texas Instruments