Digital Voltmeters and Multimeters

Digital measurement

A digital voltmeter conditions an input, converts it with an ADC and displays the result. A multimeter adds ranges or circuits for current, resistance and other quantities. Display digits or counts describe resolution; accuracy also depends on reference, gain, offset, linearity, noise and operating conditions. Voltage measurements use a high-input-impedance parallel connection. Current measurement generally inserts a shunt in series and introduces burden voltage. Using a current-input connection across a voltage source can create a short circuit. Select terminals, range and ratings according to the instrument and circuit procedure.

Accuracy specifications

A common DC specification is ±(a% of reading + b counts). One count is the least significant display increment on the selected range. This stated error limit is not automatically a standard uncertainty or confidence interval. True-RMS capability, AC coupling, bandwidth and crest-factor limits matter for nonsinusoidal AC. An average-responding meter calibrated for sine waves can misread other waveforms.

Worked example

A meter reads 12.00 V on a range with 0.01 V resolution. Its applicable specification is ±(0.5% of reading + 2 counts). Reading term = 0.005 × 12.00 = 0.06 V. Count term = 2 × 0.01 = 0.02 V. Total stated limit = ±0.08 V, so the indication corresponds to 12.00 V ±0.08 V under those specified conditions. Reporting ±0.06 V would omit the count term.

Loading example

A 10 MΩ voltmeter connected to a source with 1 MΩ Thevenin resistance indicates 10/(10 + 1) = 90.91% of the unloaded source voltage in the simple DC model. High input impedance is useful but not infinite.

Quick check

1. Are more digits a guarantee of greater accuracy? No.
2. What is burden voltage? The voltage drop introduced by the current-measurement path.
3. Does every AC meter measure arbitrary-waveform RMS accurately? No.