Agilent Technologies 3458A Calibration Manual page 100

Saving Calibration Time and Money
T
he 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.
100 Appendix B Electronic Calibration of the 3458A (Product Note 3458A-3)
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 and offset correction
for each function and range. These correction constants were then
stored in non-volatile memory and used to correct the errors of the
internal circuitry. Calibration was improved because covers were
removed during calibration and the multimeter's internal circuits
required no adjustable components.
The 3458A goes beyond these techniques by conveniently correcting
errors due to time or environmental variations. Adjustments primarily
consist of offset and gain constants, although all other errors are
considered. A patent pending technique prevents the loss of
calibration constants in non- volatile memory.
The analog-to-digital converter's linearity and transfer accuracy are
fundamentally important to the calibration technique used in the
3458A Multimeter. The linearity of the analog-to-digital converter
gives the instrument the ability to measure the ratio of two DC
voltages at state-of- the-art accuracies. In other words, this converter
maintains its accuracy over the entire measurement range, without
any internal adjustments. The speed of the analog-to-digital converter
allows an internal DC to AC transfer of accuracy, again
state-of-the-art.
The analog-to-digital converter achieves this performance using a
patented technique known as "multislope integration." This technique
uses charge balancing, where the charge from the input signal is
cancelled by charge injected from reference signals. Multi-slope
integration also allows the integration aperture to be changed so that
measurement resolution can be traded for measurement speed.