TI Device

47 kW

47 kW

TI Device

10 kW

Microphone,

Hydrophone,

and So Forth

Thermocouple

TI Device

Center tap provides

bias current return.

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28

INA821

SBOS893D – AUGUST 2018 – REVISED JUNE 2020

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Application Information (continued)

9.1.2 Input Bias Current Return Path

The input impedance of the INA821 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 little with varying input voltage.

For proper operation, Input circuitry must provide a path for this input bias current. Figure 68 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 INA821 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 68).

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.

Figure 68. Providing an Input Common-Mode Current Path