Choosing Between a Digital Multimeter (DMM) and an Ohmmeter, Ammeter, or Voltmeter

Digital multimeters (DMMs) are one of the handiest of tools for any electrician or engineer, putting a host of measurement capabilities in one hand. A multimeter and its probes can measure voltage (V), current (I), and resistance (R) while operating with battery power wherever and whenever needed, over fairly wide measurement ranges depending on the model. Many excellent instruments are currently available from a number of highly respected manufacturers. When sorting through a group of different DMMs, the instrument of choice should provide sufficient accuracy and resolution, and it should be rugged enough to withstand the rigors of the workplace, which may include occasional mishandling. A DMM is truly portable and versatile, but is it always the best choice for a measurement, or does it make more sense to use an instrument more focused on a particular measurement, such as an ohmmeter, ammeter, or voltmeter?

Of course, a DMM is a “jack of all trades” when it comes to basic electrical measurements and, considering its broad capabilities, the measurement ranges, resolutions, and accuracies of a battery-powered, handheld instrument like the Fluke 87-V DMMs are quite impressive. But are they good enough for all applications when measuring such electrical parameters? Some users may prefer a benchtop instrument with all the functionality of a DMM and more, including an IEEE-488 interface for programming as part of an automatic-test-equipment (ATE) system. The North Atlantic 2250 Phase Angle Voltmeter provides the capabilities of a DMM and more, in a somewhat larger enclosure. It can measure voltage, input and output impedance, performs harmonic analysis, and measure signal amplitude and phase. It is essentially a digital multimeter with high sensitivity and resolution that is “built for the benchtop” as well as for automated test setups.

Whereas a standard DMM measures an existing voltage, some applications may require that a known voltage be supplied, and the rack-mount Keithley 2657A high-power source measure unit (SMU) is a versatile electrical test instrument that combines a high-precision, high-voltage, low-current source with a DMM, allowing testers to generate a known test signal and then measure changes to the test voltage or current through a circuit or device of interest. As a source, it can generate continuous or pulsed voltage or current waveforms. It is a DMM, but it also includes a precision power supply, a current source, an 18-b electronic load, a trigger controller, and an arbitrary waveform generator (AWG) for generating a wide range of test signals. If a DMM can be considered multiple instruments in a box, the Keithley 2657A is almost equivalent to a complete test laboratory in a somewhat larger box.

These are but a few of the multiple-function instruments available for electronic testing. For some applications, when exceedingly small voltages, or currents, or impedances must be measured, an instrument designed for such purposes is a better choice than a standard DMM. As an example, the Keysight 34420A NanoVolt/Micro-Ohm meter, provides pico-volt measurement sensitivity for extremely low-level measurements. It includes a 7-1/2-digital display to provide high resolution for those low levels, and outstanding low-noise performance for detecting the lowest possible voltage, current, and resistance values. While not in the lightweight, hand-held package of the Fluke DMM, this benchtop instrument is also portable, with a built-in carrying handle.

Specifiers can match the needs of their measurement applications with the performance capabilities of many additional different units by visiting the Axiom Test Equipment website at www.axiomtest.com or email Axiom Test Equipment’s sales department at sales@axiomtest.com or by calling 760-806-6600.