Electronic Calibration of the 3458A (Product Note 3458A-3) B
Keysight 3458A Calibration Manual 121
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.
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.
The user enters the exact value of the external
10 V DC voltage standard (for example, “CAL
10"). The following sequence, performed
automatically by the 3458A multimeter,
determines gain constants for all ranges:
1 Measure the external “10 V” standard on the
10 V range.
2 Create the gain adjustment for the 10 V range
using the ratio of the measured and actual
values.
3 Measure accuracy of the internal reference
voltage 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 all gain adjustment constants).
To Next Page
Loading...
