NULL Reading
The DMM allows a separate null setting to be saved for the temperature function. When making null meas-
urements, each reading is the difference between a stored null value and the input signal. One application
of NULL is to increase accuracy of two-wire resistance measurements by first nulling the closed–circuit
test lead resistance.
Autozero On/Off
Enabling the autozero feature (ON) provides greater accuracy; however, the additional measurement (of
zero) reduces the reading speed.
Making High–Speed AC Measurements
The multimeter's AC voltage and AC current functions implement three low–frequency filters. These filters
allow you to trade–off minimum measured frequency for faster reading speed. The FAST filter settles in
0.025 seconds, and is useful for frequencies above 200 Hz. The MEDIUM filter settles in 0.625 seconds for
voltage and 0.25 seconds for current, and is useful for measurements above 20 Hz. The SLOW filter settles
in 2.5 seconds for voltage and 1.66 seconds for current, and is useful for frequencies above 3 Hz.
With a few precautions, you can perform AC measurements at speeds up to 500 readings per second. Use
manual ranging to eliminate autoranging delays. By setting the trigger delay to 0, the FAST, MEDIUM, and
SLOW filters allow up to 500, 150, and 50 readings per second, but with reduced accuracy because the fil-
ter may not fully settle. If the sample–to–sample levels are similar, little settling time is required for each
new reading. Under this specialized condition, the MEDIUM filter provides reduced accuracy results at 20
readings per second, and the FAST filter provides reduced accuracy results at 200 readings per second.
AC Fil-
ter
Filter Band-
width
Settling Time
(s)
Full Settling (rdg-
s/s)
Partial Set-
tling
Maximum rdg-
s/s
ACV
ACI
ACV
ACI
ACV/ACI
ACV/ACI
Slow
3 Hz
2.5
1.67
0.4
0.6
2
50
Medium
20 Hz
0.63
0.25
1.6
4
20
150
Fast
200 Hz
0.025
0.025
40
40
200
500
In applications where sample–to–sample levels vary widely, but the DC offset level does not change, the
MEDIUM filter settles at 2 to 4 readings per second (depending upon the lowest frequency component in
the waveform) as shown in the following table:
Lowest Frequency Component 20 Hz 50 Hz 100 Hz 200 Hz
AC Current (allowable rate in rdgs/s) 4
4
4
4
AC Voltage (allowable rate in rdgs/s) 2
3
4
4
For AC voltage, additional settling time may be required when the DC level varies from sample to sample.
The default sample delays allow for a DC level change of 3% of range for all filters. If the DC level change
exceeds these levels, additional settling time is required. The multimeter's DC blocking circuitry has a set-
tling time constant of 0.2 seconds. This settling time only affects measurement accuracy when DC offset
levels vary from sample to sample. If maximum measurement speed is desired, you may want to add an
Keysight True
volt
Series Operating and Service Guide
173
Measurement Tutorial
