User`s guide

283
Chapter 7 Tutorial
Measurement Fundamentals
4
7
Making High-Speed AC Measurements The internal DMM’s AC
voltage and AC current functions implement three low-frequency filters.
These filters allow you to trade-off low frequency accuracy for faster
scanning speed. The fast filter settles in 0.12 seconds and is useful for
measurements above 200 Hz. The medium filter settles in 1 second and
is useful for measurements above 20 Hz. The slow filter settles in 7
seconds and is useful for measurements above 3 Hz.
With a few precautions, you can perform AC measurements at speeds up
to 100 readings per second (use manual ranging to eliminate
autoranging delays). By setting the preprogrammed channel settling
delays to zero, each filter will allow up to 100 channels per second.
However, the measurement might not be very accurate since the filter is
not fully settled. In scanning applications where sample-to-sample levels
vary widely, the medium filter (20 Hz) will settle at 1 reading per second,
and the fast filter (200 Hz) will settle at 10 readings per second.
If the sample-to-sample levels are similar, little settling time is required
for each new reading. Under this specialized condition, the medium filter
(20 Hz) will provide reduced accuracy results at 5 readings per second,
and the fast filter (200 Hz) will provide reduced accuracy results at 50
readings per second. Additional settling time may be required when the
DC level varies from sample to sample.
The internal DMM’s DC blocking circuitry has a settling 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 in a scanning system, you may want to add an external
DC blocking circuit to those channels with significant DC voltages
present. This circuit can be as simple as a resistor and a capacitor.
DC blocking settling time (1 time constant) = 0.2 seconds.
AC Filter Channel Delay Settling Time
200 Hz (Fast)
20 Hz (Medium)
3 Hz (Slow)
200 Hz (Fast)
20 Hz (Medium)
3 Hz (Slow)
AUTO
AUTO
AUTO
0
0
0
0.12 seconds
1 second
7 seconds
0.02 seconds
0.2 seconds
1.5 seconds
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