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DETERMINING MEASUREMENT ACCURACY

by Hristo Stefanov

Start ý Defining the Terms ý A Little Theory ý Sources of Error ý Error Budget ý Error Budget Example ý Offset and Gain Errors ý Sources and PDF

OFFSET AND GAIN ERRORS

Offset errors in the amplifier circuit are caused mainly by op-amp input offset voltage and input currents. All absolute offset errors listed in Table 1 are referred to the input (RTI).

Factors causing gain errors in the amplifier circuit come from the resistorsý value deviation and the finite open-loop gain of the op-amp.

The gain of the circuit (assuming infinite open-loop gain) is:

To estimate the influence of resistor deviation on the gain, you have to differentiate the above equation with respect to R₁ and R₂:

Letýs use these relations in the error budget to calculate the error caused by the resistorsý tolerance and temperature coefficient.

To estimate the error caused by the finite open-loop gain (A_VO) of the op-amp, I will include in the error budget an article that is the voltage between the two op-amp inputs. This voltage is:

Using these assumptions, Iýve completed the error budget table (see Table 2). Assigned are the deviation of factors that affect some sources of errorýtime (12 months), power supply voltage (± 1 V), and temperature drift (± 25° C):

The next logical step after calculating the errors is to analyze them and see how they can be reduced. From the above results, you can see that an analyzed circuit is not quite accurate as a result of the large gain error, 22325 ppm of full scale. The main reason for this is the tolerance of the resistors R₁ and R₂; it causes 19,800-ppm gain error. To avoid this significant source of error, you can implement trimming either R₁ or R₂. This approach can reduce the overall error to 2716.85 ppm! Furthermore, you can reduce gain error by usage of resistors with matched temperature coefficients so that they track.

The most significant factor causing offset error is the input offset voltage of the op-amp. Fortunately, it can be canceled by using the offset-nulling circuit given by the manufacturer of the op-amp.

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