5.49k

VCM = 10 V

+15v

-15V

C1

C2

RG

INA

RG

26

INA821

SBOS893A – AUGUST 2018 – REVISED DECEMBER 2018

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Copyright © 2018, Texas Instruments Incorporated

8.3.6 Error Sources

Most modern signal-conditioning systems calibrate errors at room temperature. However, calibration of errors

that result from a change in temperature is normally difficult and costly. Therefore, minimizing these errors is

important by choosing high-precision components such as the INA821 that have improved specifications in

critical areas that impact the precision of the overall system. Figure 64 shows an example application.

Figure 64. Example Application with G = 10 V/V and 1 V Output Voltage

Resistor-adjustable devices (such as the INA821) show the lowest gain error in G = 1 because of the inherently

well-matched drift of the internal resistors of the differential amplifier. At gains greater than 1, (for instance, G =

10 V/V or G = 100 V/V) the gain error becomes a significant error source because of the contribution of the

resistor drift of the 24.7-kΩ feedback resistors in conjunction with the external gain resistor. Except for very high

gain applications, the gain drift is by far the largest error contributor compared to other drift errors, such as offset

drift.

The INA821 offers excellent gain error over temperature for both G > 1 and G = 1 (no external gain resistor).

Table 5 summarizes the major error sources in common INA applications and compares the three cases of G = 1

(no external resistor) and G = 10 (5.49-kΩ external resistor) and G = 100 (499-Ω external resistor). All

example. All calculations refer the error to the input for easy comparison and system evaluation. As can be seen

in Table 5, errors generated by the input stage (such as input offset voltage) are more dominant at higher gain

while the effects of output stage are suppressed because they are divided by the gain when referring them back

to the input. Note that the gain error and gain drift error are much more significant for gains greater than 1

because of the contribution of the resistor drift of the 24.7-kΩ feedback resistors in conjunction with the external

gain resistor. In most applications, static errors (absolute accuracy errors) can readily be removed during

calibration in production, while the drift errors will be the key factors limiting overall system performance.

Table 4. System Specifications for Error Calculation

Quantity

Value

Vout (V)

1

VCM (V)

10

VS(V)

1

Rs+ (Ω)

1000

Rs- (Ω)

999

Rg tolerance (%)

0.01

Rg drift (ppm/°C)

10

Temp range upper limit (°C)

105