Agilent Technologies 3458A Calibration Manual page 103

Resistance and DC Current Adjustments
Calibration of all resistance (nine ranges from 10 9 to 1 G9) and DC
current ranges (eight ranges from 100 nA to 1 A) requires only one
external resistance standard. Resistance is measured by applying a
known current through the unknown resistance and measuring the
voltage across it. Current is measured by applying the unknown
current through a known shunt resistor and measuring the voltage
across it. The process explained previously has already corrected
errors in the DC voltage input path. Measuring the actual values of the
current sources and shunt resistors results in the additional
information needed to adjust resistance and current measurements.
Both current and resistance are calibrated concurrently. For resistance
measurements, a current source provides 500 nA to
10 mA, depending on the measurement range, Current measurements
use shunt resistor values that vary from 0.1 9 to 545.2 k9.
The user enters the exact value of the external 10 kilohm standard (for
example, "CAL 10E3"). The following sequence, performed
automatically by the 3458A Multimeter, determines adjustment
constants for all ranges of resistance and DC current:
1. Make a four-wire offset-compensated measurement of the
external "10 kW" standard using the 10 kW range.
2. Use the ratio of the measured and actual values as the 10 kW
range calibration constant (current source adjustment for the
10 kW range).
Figure 4.
The AC input paths are first
adjusted for flatness. Later, in
the normal calibration process,
gain adjustments are made.
Appendix B Electronic Calibration of the 3458A (Product Note 3458A-3)
3. Measure the internal reference resistor 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 adjustment constants.)
4. Use the calibrated internal reference resistor to adjust current
source values used for other resistance ranges.
5. Use calibrated current sources to adjust shunt resistor values
used for DC current measurements.
Leakage currents for resistance measurements and offsets produced
with shunt resistors for current measurements are additional sources
of error. Adjustment of these errors is simply a matter of measuring
and storing the results as adjustment constants.
AC Flatness and Gain Adjustments
Routine calibration of AC voltage and current functions requires no
external AC standards. To accurately measure AC signals, the internal
circuits must have constant gain versus frequency of the input signal.
An Agilent Technologies patented technique electronically adjusts the
entire AC section, shown in Figure 4. This technique first adjusts
frequency response flatness of the AC attenuator network, then
adjusts gains of the RMS converter and track-and-hold amplifier.
Similar to the adjustment of an oscilloscope probe, proper adjustment
of the AC attenuator network produces a maximally flat response to a
step input voltage, as shown in Figure 5. A circuit that responds to a
step input with an undistorted step output has constant gain versus
frequency of the input signal, which can be shown using Fourier
transform theory.
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