Agilent Technologies 3458A Calibration Manual page 102

The user enters the exact value of the external 10 V DC voltage
standard (for example, "CAL 10"). The following sequence,
performed automatically by the 3458A Multimeter, determines gain
constants for all ranges:
1. Measure the external "10 V" standard on the 10 V range.
2. Create the gain adjustment for the 10 V range using the ratio
of the measured and actual values.
3. Measure accuracy of the internal reference voltage relative to
the external standard, and store the difference as a reference
adjustment. (When subsequently invoked, auto-calibration
uses this stored value to re-determine all gain adjustment
constants).
Gain adjustments are now made for all other DC voltage ranges.
4. Using the input path for the 10 V range, accurately measure
1 V generated internally.
Linearity of the measurement circuits allows a measurement that
accurately reflects the actual l V output. In other words, we transfer
traceable accuracy from the 10 V range to all other ranges.
Figure 3.
On the 1000 V range, nonlinear
self-heating errors of the 100:1
resistive attenuator are
noticeable.
102 Appendix B Electronic Calibration of the 3458A (Product Note 3458A-3)
The lower ranges use amplifiers to condition the input for the
10 V full-scale analog-to-digital converter. Each amplifier used
requires a gain constant, G , to adjust normal readings. The following
A
process determines these constants.
5. In the 1 V range, measure the same 1 V previously measured
with the 10 V range.
6. Calculate a 1 V range gain adjustment so that the two
measurements agree. Note that neither the precise value nor
the long-term stability of the internal 1 volt source are
important. The internal 1 volt source need only be stable for
the time it takes to measure it twice.
7. Using the adjusted 1 V range, accurately measure 0.1 V
generated internally.
8. Measure the same 0.1 V using the 100 mV range.
9. Calculate a 100 mV range gain adjustment so that the two
measurements agree.
Normal 100 V and 1000 V range measurements use a
100:1 resistor network to attenuate the input. To correct errors
introduced by this network, we apply zero volts to the input. Then, we
apply 10 V and measure the actual value. Finally, we measure 0.1 V,
with the zero error removed, and compute the gain adjustment
constant.
Input voltages greater than 100 V (1000 V range) create a self-heating
error in the resistor network, as shown in Figure 3. This easily
identified error is simply specified as part of the instrument's
published error.
Additional measurements result in constants to compensate for
switching transients and leakage currents.