47 k

47 k

RG

Microphone,

hydrophone,

and so forth.

-

+

INA188

10 k

RG

Thermocouple

-

+

INA188

RG

Center tap provides

bias current return.

-

+

INA188

23

INA188

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Device Functional Modes (continued)

7.4.3 Input Bias Current Return Path

The input impedance of the INA188 is extremely high—approximately 20 G

Ω. However, a path must be provided

for the input bias current of both inputs. This input bias current is typically 750 pA. High input impedance means

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

Input circuitry must provide a path for this input bias current for proper operation. Figure 52 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 INA188, and the input amplifiers saturate. If the differential source resistance is

low, the bias current return path can be connected to one input (as shown in the thermocouple example in

Figure 52). 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 52. Providing an Input Common-Mode Current Path