HP WaterSkis E1312A, WaterSkis E1412A DC Current Measurement Errors, True RMS AC Measurements

DC Current Measurement Errors

When you connect the multimeter in series with a test circuit to measure current, a measurement error is introduced. The error is caused by the multimeter’s series burden voltage. A voltage is developed across the wiring resistance and current shunt resistance of the multimeter as shown below.

True RMS AC Measurements

True RMS responding multimeters, like the HP E1312A and HP E1412A, measure the “heating” potential of an applied signal. Unlike an “average responding” measurement, a true RMS measurement can be used to determine the power dissipated in a resistance, even by non-sinusoidal signals. The power is proportional to the square of the measured true RMS voltage, independent of waveshape. An average responding ac multimeter is calibrated to read the same as a true RMS meter for sinewave inputs only. For other waveform shapes, an average responding meter will exhibit substantial errors as shown below.

The multimeter's ac voltage and ac current functions measure the ac-coupled true RMS value. This is in contrast to the ac+dc true RMS value shown above. Only the “heating value” of the ac components of the input waveform are measured (dc is rejected). For non-offset sinewaves, triangle waves, and square waves, the ac and ac+dc values are equal since these waveforms do not contain a dc offset. Non-symmetrical waveforms, such as pulse trains, contain dc voltages which are rejected by ac-coupled true RMS measurements.

An ac-coupled true RMS measurement is desirable in situations where you are measuring small ac signals in the presence of large dc offsets such as when measuring ac ripple present on dc power supplies. There are situations, however, where you might want to know the ac+dc true RMS value. You can

32 HP E1312A/E1412A Multimeter Application Information Chapter 2