Electronic Calibration of the 3458A (Product Note 3458A-3) B
Keysight 3458A Calibration Manual 119
Saving Calibration Time and
Money
The increasing accuracy required of today's
instrumentation tends to increase complexity and
cost of maintaining calibration of these
instruments. In an effort to reduce the cost and
complexity of calibration, the 3458A multimeter
reduces the number of external reference
standards required for calibration. All functions
and ranges require only one external DC voltage
standard and only one external resistance
standard.
Many of the external reference standards
traditionally maintained and used by metrology
laboratories for calibration (for example, resistive
networks and DC-to-AC transfer devices) are
being replaced with internal circuitry and
algorithms that can achieve comparable results.
With the 3458A multimeter, all adjustments are
electronic - there are no potentiometers in this
instrument.
For many applications, you can substantially
increase the time between calibrations, saving
costs. For example, the standard 3458A
multimeter is as accurate at the end of a year as
most multimeters are at the end of a day.
In systems, rack temperatures are typically more
than 40°C and have wide temperature variations.
Auto-calibration of the 3458A multimeter
improves measurement accuracy under these
circumstances.
The end result is that the 3458A multimeter
measures DC and AC with unmatched accuracy,
precision, and speed, while avoiding the usual
high cost of maintaining such an instrument.
The basis for auto-calibration
Only three external inputs are needed as the
basis for all normal adjustments:
– Four-wire short
– 10 V DC voltage standard
– 10 kW resistance standard
Normal calibration, described below, provides
traceability of all functions, ranges, and internal
reference standards to the two external
standards. An additional auto-calibration process
adjusts the 3458A multimeter using internal
reference standards that are traceable to the
external standards via the normal calibration
process. Thus invoking auto-calibration at any
time produces exemplary accuracy over a long
time frame and over widely varying operating
temperatures.
Multimeter designers and users have always had
to cope with how to reduce offset and gain error
introduced into measurements by internal circuits
of the multimeter. These errors constantly
change because component characteristics vary
with time, temperature, humidity, and other
environmental conditions. Early multimeters
reduced internal errors by adjusting the value of
key components. The use of adjustable
components had two major drawbacks. First,
making adjustments often required removing the
multimeter's covers. Unfortunately, removing the
covers changed the temperature within the
multimeter. Second, adjustable components
were often a major contributor to drift that
caused inaccuracies.
With the emergence of non-volatile memory,
multimeters were designed with few or no
adjustable components. Instead,
microprocessors were used to calculate a gain
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