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52120A
Users Manual
A40B Shunt Resistance Correction
Table 5-2 is a list of the nominal resistance values of the A40B shunts.
The shunts can have a maximum of 300 ppm resistance error, which shows on the
calibration certificate. To get the necessary accuracies, you must correct for this error. If
the certificate shows the deviation from nominal value, this value is the shunt error. The
Correction necessary is 1 – error.
For example, if the deviation from nominal (error) is +100 ppm, the correction is given
by:
If the certificate gives the actual measured value, then divide the nominal value by the
measured value to get the correction.
For example, a 20A shunt has a nominal value of 0.04 Ω. If the measured value is
0.0400040 Ω, the correction is given by:
System Characterization for AC Measurements
To keep measurement uncertainty of ac measurements to a minimum, you must
characterize the 8508A against the 5720A at each A40B output voltage and frequency.
You must use the 8508A Rear Input option so you do not have to change connections in
the procedure. This procedure limits measurement uncertainty of the reference system to
the linearity and short-term stability of the 5720A and 8508A. The uncertainty is summed
together by root-sum-of-squares (RSS) with the ac accuracy of the A40B shunt. It is not
necessary to calculate values manually because the 8508A Math Functions are used in the
procedure below.
This verification measurement gives the transconductance of the Product. Amplitude
errors in the voltage applied to the input of the Product (measured by the 8508A) also
show as amplitude errors at the output. They are in proportion to the input errors. As this
error is seen in the 5720A and the Product outputs, the amplitude error (ppm output) part
of the 5720A specification can be ignored. The remaining measurement errors are the
linearity error of the 5720A and the A40B accuracy.
For this document, the linearity of the 5720A is shown by the "+μV" specification value.
This is actually pessimistic because other accuracy contributions are included in this part
of the 5720A specification. The uncertainties shown in the measurements table also
include a typical measurement noise contribution of 10 ppm for dc and 20 ppm for ac
measurements and where the input to the A40B shunt is not full scale, the A40B power
coefficient correction.
5-6
Table 5-2. Nominal Resistance for A40B Shunts
Shunt
2 A
20 A
100 A
Correction = 1 – deviation = 1 –
Correction =
100
= 1 – 0.0001 = 0.999900
6
10
0.04
= 0.999900
0.040004
Nominal Resistance
0.4 Ω
0.04 Ω
0.008 Ω
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