HP WaterSkis E1312A manual AC Current Measurement Errors, Frequency and Period Measurement Errors

AC Current Measurement Errors

Burden voltage errors, which apply to dc current, also apply to ac current measurements. However, the burden voltage for ac current is larger due to the multimeter’s series inductance and your measurement connections. The burden voltage increases as the input frequency increases. Some circuits may oscillate when performing current measurements due to the multimeter’s series inductance and your measurement connections.

Making High-Speed AC Voltage or Current Measurements

The multimeter’s ac voltage and ac current functions implement three different low-frequency filters. These filters allow you to trade low frequency accuracy for faster reading speed. The fast filter settles in

0.1seconds, and is useful for frequencies above 200Hz. The medium filter settles in 1 second, and is useful for measurements above 20Hz. The slow filter settles in 7 seconds, and is useful for frequencies above 3Hz.

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 (20Hz) will settle adequately at almost 1 reading per second, and the fast filter (200Hz) will settle adequately at almost 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.

DC Blocking Circuitry The multimeter’s dc blocking circuitry has a settling time constant of

0.2seconds. This time constant 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.

Frequency and Period Measurement Errors

The multimeter uses a reciprocal counting technique to measure frequency and period. This method generates constant measurement resolution for any input frequency. The multimeter’s ac voltage measurement section performs input signal conditioning. All frequency counters are susceptible to errors when measuring low-voltage, low-frequency signals. The effects of both internal noise and external noise pickup are critical when measuring “slow” signals. The error is inversely proportional to frequency. Measurement errors will also occur if you attempt to measure the frequency (or period) of an input following a dc offset voltage change. You must allow the multimeter's input dc blocking capacitor to fully settle before making frequency measurements.

36 HP E1312A/E1412A Multimeter Application Information Chapter 2