Editing Voltmeter (section)

== Digital voltmeter ==
[[Image:Voltmeter.jpg|thumb|right|200px|Two digital voltmeters. Note the 40 microvolt difference between the two [[measurement]]s, an offset of 34 parts per million.]]

A '''digital voltmeter''' (DVM) measures an unknown input voltage by converting the voltage to a digital value and then displays the voltage in numeric form. DVMs are usually designed around a special type of [[analog-to-digital converter]] called an [[Integrating ADC|integrating converter]].

DVM measurement accuracy is affected by many factors, including temperature, input impedance, and DVM power supply voltage variations. Less expensive DVMs often have input resistance on the order of 10 MΩ. Precision DVMs can have input resistances of 1 GΩ or higher for the lower voltage ranges (e.g. less than 20 V). To ensure that a DVM's accuracy is within the manufacturer's specified tolerances, it must be periodically calibrated against a [[Voltage reference|voltage standard]] such as the [[Weston cell]].

The first digital voltmeter was invented and produced by [[Andrew Kay]] of Non-Linear Systems (and later founder of [[Kaypro]]) in 1954.<ref name="Andrew Kay Obituary by John Markoff ">{{cite web | url=https://www.nytimes.com/2014/09/06/business/andrew-kay-pioneer-in-computing-dies-at-95.html | title=Andrew Kay, Pioneer in Computing, Dies at 95 | publisher=New York Times | work=Obituary | date=5 Sep 2014 | access-date=7 September 2014 | author=Markoff, John}}</ref>

Simple AC voltmeters use a rectifier connected to a DC measurement circuit, which responds to the average value of the waveform. The meter can be calibrated to display the [[root mean square]] value of the waveform, assuming a fixed relation between the average value of the rectified waveform and the RMS value. If the waveform departs significantly from the sinewave assumed in the calibration, the meter will be inaccurate, though for simple wave shapes the reading can be corrected by multiplying by a constant factor. Early "true RMS" circuits used a thermal converter that responded only to the RMS value of the waveform. Modern instruments calculate the RMS value by electronically calculating the square of the input value, taking the average, and then calculating the square root of the value. This allows accurate RMS measurements for a variety of waveforms.<ref>{{Cite web |date=2021-06-21 |title=What is RMS Voltage |url=https://www.electrical4u.com/rms-or-root-mean-square-value-of-ac-signal/ |access-date=2023-04-23 |website=Electrical4U |language=en-US}}</ref>