9.1.2 Input Bias Current Return Path

The input impedance of the INA849 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 6 nA. High input impedance

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

Input circuitry must provide a path for this input bias current for proper operation. 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 INA849, 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). 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.

Thermocouple

10 NŸ

Microphone,

Hydrophone,

and more

+

±

+

±

+

±

+

±

Transformer

R1

R2

AC Coupling

R3

R4

R5

T1

L1

C2

C1

INA849

INA849

INA849

INA849

+VS

í VS

REF

REF

REF

REF

+VS

í VS

+VS

í VS

+VS

í VS

NOTE: Center tap in the transformer provides bias current return.

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

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INA849

SBOS945B – NOVEMBER 2020 – REVISED APRIL 2021

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