Tip 10: Making High-Speed AC Measurements

A multimeter's ac voltage and ac current functions should implement different low-frequency filters. These filters allow you to trade-off low frequency accuracy for faster reading speed. For example the 34401A has three filters, each has a dramatically different settling time. 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. It is common to have different dc offsets in a scanning system. 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.