Keysight 3458A - AC Flatness and Gain Adjustments

Electronic Calibration of the 3458A (Product Note 3458A-3) B
Keysight 3458A Calibration Manual 123
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 W to 545.2 kW.
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” 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).
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 Keysight
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.
Figure 4.
The AC input paths are first adjusted for flatness. Later, in the
normal calibration process, gain adjustments are made.
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