8.2.2.3 Input Bias Current Return

The input impedance of the INAx126 is extremely high at approximately 109 Ω. However, a path must be

provided for the input bias current of both inputs. This input bias current is typically –10 nA (current flows out

of the input pins). 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 8-3 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, and the input amplifiers will saturate.

If the differential source resistance is low, the bias current return path can be connected to one input (see

the thermocouple example in Figure 8-3). With higher source impedance, using two equal resistors provides a

balanced input with the advantages of lower input offset voltage due to bias current and better high-frequency

common-mode rejection.

47kΩ

47kΩ

10kΩ

Microphone,

Hydrophone

etc.

Thermocouple

Center-tap provides

bias current return.

INA126

INA126

INA126

Figure 8-3. Providing an Input Common-Mode Current Path

8.2.2.4 Input Common-Mode Range

The input common-mode range of the INAx126 is shown in Section 6.7. The common-mode range is limited

on the negative side by the output voltage swing of A2, an internal circuit node that cannot be measured on an

external pin. The output voltage of A2 can be expressed as shown in Equation 2:

VO2 = 1.25 V– IN – (V+ IN – V– IN) (10 kΩ/RG)

(2)

where

• Voltages referred to Ref, pin 5

The internal op amp A2 is identical to A1, with an output swing typically limited to 0.7 V from the supply rails.

When the input common-mode range is exceeded (A2 output is saturated), A1 can still be in linear operation and

respond to changes in the noninverting input voltage. The output voltage, however, will be invalid.

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INA126, INA2126

SBOS062C – SEPTEMBER 2000 – REVISED JANUARY 2022

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