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4 Measurement Tutorials
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.
AC filter
200 Hz (Fast)
20 Hz (Medium)
3 Hz (Slow)
200 Hz (Fast)
20 Hz (Medium)
3 Hz (Slow)
DC blocking settling time (1 time constant) = 0.2 seconds.
236
Channel delay
AUTO
AUTO
AUTO
0
0
0
Setting time
0.12 seconds
1 second
7 seconds
0.02 seconds
0.2 seconds
1.5 seconds
Keysight DAQ970A/DAQ973A User's Guide
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