INA821 datasheet(25/52 Pages) TI

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8.3.2 Offset Voltage

Low offset voltage is one of the key parameters for an instrumentation amplifier (INA). In a current-feedback INA,

this error source is classified in three stages: input, output, and intermediate. The input-stage dc offset (VOSI) is

mainly caused by the mismatch of the input transistors Q1 and Q2, (see Figure 8-1). The output-stage dc offset

(VOSO) is caused partially by the mismatch of the output amplifier A3. In the INA851, A3 is a fully-differential

amplifier and gained up by the noise gain of the circuit (1 + R5 / R3). An additional intermediate stage offset

contribution error adds to VOSO that is caused by the mismatch of the current mirrors in the front end (through R1

and R2).

Unlike typical instrumentation amplifiers that incorporate a difference amplifier (A3) with a fixed output gain,

the INA851 has two different output gain stages that subsequently contribute differently to VOSO; see Gaussian

distributions for G = GOUT = 1 V/V in Figure 7-3 and for G = GOUT = 0.2 V/V in Figure 7-4.

The following equation calculates the total offset voltage error referred to the input:

VOS= VOSI2+ VOSOGOUT=1 or GOUT=0.2

GIN

(3)

8.3.3 Input Common-Mode Range

The linear input voltage range of the INA851 input circuitry extends within 2.5 V (maximum) of both power

supplies, and maintains excellent common-mode rejection throughout this range. The valid input common-mode

range is a function of the input common-mode voltage, input differential voltage, gain, and output common-mode

voltage.

The common-mode range is best calculated using the INA851 Input-Output Range Design Calculator .

The common-mode range for the most common operating conditions are shown in Figure 8-3 to Figure 8-9.

Output Differential Voltage (V)

-1

-0.8

-0.6

-0.4

-0.2

0.2

0.4

0.6

0.8

-2

-1.6

-1.2

-0.8

-0.4

0.4

0.8

1.2

1.6

G = 0.2 V/V

G = 0.5 V/V

VS± = ±4 V

Figure 8-3. Input Common-Mode Voltage vs Output

Voltage, Low Gains

Output Differential Voltage (V)

-6

-5

-4

-3

-2

-1

-15

-12

-9

-6

-3

G = 0.2 V/V

G = 0.5 V/V

VS± = 15 V

Figure 8-4. Input Common-Mode Voltage vs Output

Voltage, Low Gains

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INA851

SBOS999A – MARCH 2022 – REVISED OCTOBER 2022

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