Editing Multimeter (section)

=== Sensitivity and input impedance ===
When used for measuring voltage, the input impedance of the multimeter must be very high compared to the impedance of the circuit being measured; otherwise circuit operation may be affected and the reading will be inaccurate. Meters with electronic amplifiers (all digital multimeters and some analog meters) have a fixed input impedance that is high enough not to disturb most circuits. This is often either one or ten [[megohm]]s; the [[standardization]] of the input resistance allows the use of external high-resistance [[Test probe|probes]] which form a [[voltage divider]] with the input resistance to extend voltage range up to tens of thousands of volts.  High-end multimeters generally provide an input impedance greater than 10&nbsp;GΩ for ranges less than or equal to 10&nbsp;V.  Some high-end multimeters provide >10&nbsp;Gigaohms of impedance to ranges greater than 10&nbsp;V.<ref name="Keithley Instruments"/> Most analog multimeters of the moving-pointer type are [[Buffer amplifier|unbuffered]], and draw current from the circuit under test to deflect the meter pointer. The [[Electrical impedance|impedance]] of the meter varies depending on the basic sensitivity of the meter movement and the range which is selected. For example, a meter with a typical 20,000&nbsp;Ω/V sensitivity will have an input resistance of 2&nbsp;MΩ on the 100&nbsp;V range (100&nbsp;V × 20,000&nbsp;Ω/V = 2,000,000&nbsp;Ω). On every range, at full-scale voltage of the range, the full current required to deflect the meter movement is taken from the circuit under test. Lower sensitivity meter movements are acceptable for testing in circuits where source impedances are low compared to the meter impedance, for example, power circuits; these meters are more rugged mechanically. Some measurements in signal circuits require higher sensitivity movements so as not to load the circuit under test with the meter impedance.<ref>{{cite book|publisher=[[McGraw-Hill]]/TAB Electronics|title=How to Test Almost Everything Electronic|first=Delton|last=Horn|pages=4–6|year=1993|isbn=0-8306-4127-0}}</ref><ref name=":0">{{Cite book|url=https://books.google.com/books?id=Us6vnQEACAAJ|title=Electrical circuits|last=Siskind|first=Charles S.|date=1956|language=en}}</ref>

Sensitivity should not be confused with [[Sensor resolution|resolution]] of a meter, which is defined as the lowest signal change (voltage, current, resistance and so on) that can change the observed reading.<ref name=":0" />

For general-purpose digital multimeters, the lowest voltage range is typically several hundred millivolts AC or DC, but the lowest current range may be several hundred microamperes, although instruments with greater current sensitivity are available. Multimeters designed for (mains) "electrical" use instead of general [[electronics engineering]] use will typically forego the microamps current ranges. Measurement of low resistance requires lead resistance (measured by touching the test probes together) to be subtracted for best accuracy. This can be done with the "delta", "zero", or "null" feature of many digital multimeters. Contact pressure to the device under test and cleanliness of the surfaces can affect measurements of very low resistances. Some meters offer a four wire test where two probes supply the source voltage and the others take measurement. Using a very high impedance allows for very low voltage drop in the probes and resistance of the source probes is ignored resulting in very accurate results. The upper end of multimeter measurement ranges varies considerably; measurements over perhaps 600&nbsp;volts, 10&nbsp;amperes, or 100&nbsp;[[Ohm|megohms]] may require a specialized test instrument.