A current reading below the meter across the shunt

Tags: Measurement Current sensing

Symptom

A pulsed current, averaged from the samples of a sense amplifier and an ADC, reads lower than a DMM on its dc voltage range reads across the same shunt. The gap grows with the load, or as the current’s pulses become narrower and taller for the same average.

Why

The shunt’s voltage follows the current at every instant (sense voltage). A DMM’s integrating converter accumulates a charge proportional to the average of its input over its integration time [Kester & Bryant, ADC Architectures VIII], and it passes dc and low-frequency signals while it attenuates noise at specific frequencies [Keithley Instruments 2014]. Across the shunt it therefore reads the dc component of the voltage, its average, provided that it integrates over many switching periods, as a slow reading rate does, and that its input does not overload on the pulses’ peaks. For a train of pulses that average is the pulses’ height times their duty ratio (pulse-train harmonics).

The sense chain reads only up to its limits: the amplifier’s largest output and the ADC’s full scale (sense chain). A chain sized for the average current can clip on the pulses’ peaks, and the average of clipped samples is lower than the true average: the chain loses the part of every pulse above its limit.

How to confirm

Look at the amplifier’s output with an oscilloscope: flat tops at the same voltage in every pulse are clipping. Then pass a steady dc current, within the chain’s range, and compare the chain’s reading with the DMM’s: if they agree there, the chain is calibrated, and the gap with the pulsed current comes from its limits or from its sampling (sampling that misses the pulses). Repeat the DMM reading on its next range up: the same reading shows that the meter itself is not the one clipping.

Fix

Size the chain’s full scale for the peak of the current, not its average: lower the gain until the highest peak fits below both limits. The sense chain tool gives the full-scale current and flags a reading that can clip.

References

• [Kester & Bryant, ADC Architectures VIII]: an integrating converter’s charge is proportional to its input’s average over the integration time.
• [Keithley Instruments 2014]: an integrating converter passes dc and attenuates noise at specific frequencies.
• Sense chain: the amplifier’s and the ADC’s limits.
• Fourier series of switched waveforms: the dc component of a pulse train.