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Making High-Speed AC Measurements
The multimeter’s ac voltage and ac current functions implement three
different low-frequency filters. These filters allow you to trade-off low
frequency accuracy for faster reading speed. The fast filter settles in
0.1 seconds, and is useful for frequencies 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 frequencies above 3 Hz.
With a few precautions, you can perform ac measurements at speeds up
to 50 readings per second. Use manual ranging to eliminate autoranging
delays. By setting the preprogrammed settling (trigger) delays to 0,
each filter will allow up to 50 readings per second. However, the
measurement might not be very accurate since the filter is not fully
settled. In applications where sample-to-sample levels vary widely,
the medium filter will settle at 1 reading per second, and the fast filter
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 will provide reduced accuracy results at 5 readings per second,
and the fast filter 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 multimeter’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.
Chapter 7 Measurement Tutorial
Making High-Speed AC Measurements
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