Agilent Technologies 3458A Calibration Manual page 101

Measurements using a Josephson junction standard confirm linearity
of the analog-to-digital converter design. These measurements reveal
integral linearity below 0.1 parts per million and differential linearity
of 0.02 parts per million. This performance, incidentally, is
comparable to a Kelvin-Varley divider.
The only errors not removed in the 3458A Multimeter calibration are
drifts of the internal voltage reference and the internal resistance
standard. The internal reference voltage has an average drift during its
first 90 days of less than 2 parts per million. As shown in Figure 1, the
three sigma points are less than 4 parts per million. For DC volt
transfer measurements, the 3458A Multimeter's short-term stability is
within 0.1 parts per million of reading.
The internal reference resistor has a specified drift of 5 parts per
million per year and a temperature coefficient of 1 part per million per
Celsius degree.
Auto-calibration adjusts for time and temperature drifts in the rest of
the circuitry, relative to these internal references.
Offset Adjustments
To remove offset errors of internal circuits, the multimeter internally
connects a short in place of the input terminals and measures the
offset. Normal measurements of signals on the input terminals
subtract this offset to give the correct reading. The only offset errors
not removed by this approach are thermocouple offsets along the path
from the input terminals to the point that is switched to the internal
short.
These errors require a four-wire short on both the front and rear input
terminals (switch selected). With these external inputs, one
command, CAL 0, executes zero offset measurements that result in
additional offset calibration constants to correct subsequent readings.
Figure 1.
This plot shows stabillity with
time of the reference voltage
standard used in the 3458
Multimeter.
Appendix B Electronic Calibration of the 3458A (Product Note 3458A-3)
Other multimeters use this approach to removing offset errors. The
3458A Multimeter simply makes improvements by using more stable
components, again minimizing time and environmental errors.
DC Gain Adjustments
Gain adjustments of all five DC voltage ranges (100 mV full- scale to
1000 V full-scale) require only one external DC voltage standard. The
DC voltage input path, shown in Figure 2, requires three adjustments,
potentially. The product, represents the calibration gain used on any
given range.
Internal tolerance limits for each gain adjustment are factory set. A
gain value outside the associated tolerance indicates a malfunctioning
instrument. Therefore, as the gain adjustments are being computed,
the instrument checks the value of each gain adjustment and flags
errors accordingly.
Figure 2.
The DC input path to the analog-to-digital converter is
either through an amplifier only or through an
additional resistive attenuator, depending on the
range used.
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