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296 Keysight 34970A/34972A User’s Guide
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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