8.1.2 Input Bias Current Return Path

The input impedance of the INA630 is extremely high (approximately 100GΩ). However, a path must be

provided for the input bias current of both inputs. This input bias current is typically 150pA. 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 8-2 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 INA630 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 8-2).

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

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 8-2. Providing an Input Common-Mode Current Path

INA630

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