Choosing a Digital Multimeter

Paul Lantz, VP Engineering , Signametrics Corp.

How you choose a Digital Multimeter (DMM) is mainly driven by your application. If you are doing field service work you probably need a portable handheld DMM.  If you are working in a calibration lab you probably need a bench-system DMM.  If you are doing lab-automation or production testing you have more choices to make.  If your application requires very high accuracy and low throughput you may prefer a bench-system DMM with an IEEE-488 or RS-232 control interface.  If your application requires high throughput, has limited space or has a tight budget, you want a DMM that plugs into a computer bus.  This article deals with how to choose a plug-in DMM for an automated test application.

Choosing a computer bus
The PXI and PCI buses are the most popular computer buses for integrating instrumentation systems, today. The VXIbus is still used mainly for military applications, but it tends to be expensive and rather specialized. 

PXI can integrate from 6 to 18 instruments into a single chassis, providing high density for medium sized systems.  However, PXI requires either a PXI compatible computer or a bridge from the PCI bus of a PC, raising its cost.    If you only require 2 or 3 instruments, you may be able to plug them into the PCI motherboard of your computer.  This is a very cost-effective way to integrate a small system.  Most manufacturers offer both PXI and PCI products.

DMM selection criteria
Manufacturers typically highlight their highest accuracy and their highest speed.  Unfortunately you don’t get both at the same time.  Sometimes it is difficult to understand what accuracy goes with the speed you want.  The user needs to make sure that the DMM has a mode that is fast enough and accurate enough for the application. Some manufacturers specify accuracy with background zeroing turned on and reading rates with it turned off.  How fast does it go with background zeroing turned on?  How accurate is it with it turned off?  Sometimes you have to ask questions.

Does your workload include more than the standard measurements of Voltage, Current, Resistance and Frequency?   Perhaps you need to:

• Measure capacitance or inductance?

• Measure temperature with a Thermocouple or RTD (Resistive Temperature Detector)?

• Provide a programmable audio signal source or an accurate DC current or voltage source?

The Signametrics SMX2044, for example, can measure and source these parameters and some more.  That ability might save you from adding another instrument to your system.

Some DMMs have measurement ranges that go up to 1V, 10 V and 100 V while others go to 3 V, 30 V and 300 V.  All these DMMs are described as having 6-1/2 digits resolution, but the higher range limits are more convenient to work with in some applications.

Measurements below 3 Volts frequently involve sensitive circuits which have high output resistance.  To measure them accurately, you want a DMM with at least 1000 MegOhms input impedance.  Better DMMs automatically provide this high input impedance.  There are DMMs which let the user select the impedance.  But we know of one case where the DMM defaults back to 10 MegOhms input impedance, potentially altering the measurement accuracy by orders of magnitude.  We have seen cases where a DMM emits pulses from its measurement terminals, potentially altering your measurement results.

Are you testing low value resistances such as the contact resistance of a connector, a socket or a relay?  Or perhaps the resistances of a wire harness?  In these applications you care about the resistance floor of the DMM, and the resolution of the lowest range. 10 microOhm resolution can be very helpful here.  The ability to resolve small differences is usually more important than high accuracy.

What software will you use to implement your test system?  System integrators tend to use Visual Basic or C++ because they are easy to use, they are widely understood and what you see is what you get.  There are other software alternatives available but they are not something every engineer learns in school.

If you are measuring current, make sure the DMM has current shunts suitable for your measurements.  Some DMMs require a voltage drop of 0.1 Volt when measuring 1 Ampere, while others require a voltage drop approaching 1 Volt.  If you are measuring the load on a power supply, a 1 Volt drop will alter the behavior of the circuit under test.

Conclusion
PXI DMMs and PCI DMMs provide fast and accurate solutions for many automated test applications.  They are affordable, reliable and compact.  But they are not all the same and you will have to make a choice based on your application.  To find the best fit, you may have to talk with support people as well as sales people.  Don’t hesitate to ask questions and critically evaluate the answers you receive.

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