Are you buying more DMM digits than you really need?
Jerry Janesch- January 4, 2013In one form or another, digital multimeters (DMMs) have been included in virtually every electrical engineer’s standard toolset for decades. In addition to a growing array of feature and function options, modern bench/system DMMs are available with maximum resolutions ranging from 4‑1/2 digits to 8‑1/2 digits. (Although lower‑resolution DMMs are available, these are typically handheld units that aren’t suitable for precision test and measurement.) In fact, resolution is likely to be one of the first criteria considered in the DMM selection process. The ongoing pressure to control instrumentation budgets makes it very difficult to justify paying for more DMM resolution than the application actually demands.
It may be useful to define exactly what resolution means in this context. Resolution is the smallest portion of a signal that can be measured or displayed, for example, one digit out of 20,000 on a 4‑1/2‑digit display. The resolution of the display is the ratio of the smallest count to the maximum count 1/20,000 or 0.005% for a 4‑1/2‑digit display. A 6‑1/2‑digit DMM provides a resolution of 1/2,000,000 or 0.00005%. On the 100mV range, that amounts to 100nV.
In contrast, sensitivity is the smallest change of the measured signal that can be detected. It depends on both resolution and the lowest measurement range of the instrument. For example, the sensitivity of a 5‑1/2‑digit DMM on the 200mV range is 1μV. (On a 200mV display reading 200,000, the last zero is the one‑microvolt digit.)
Accuracy is the degree of conformity of a measurement to a standard or true value. A DMM’s accuracy is usually specified as a percent of reading plus a percent of range (or a number of counts of the least significant digit). It may also be specified in parts per million (ppm), where 1ppm is the equivalent of 0.0001%.
So, now that we understand the differences between resolution, sensitivity, and accuracy, let’s consider how many digits of resolution various applications typically require. For applications in which portability is more critical than high resolution, such as field service work, 4‑1/2‑digit handheld DMMs are typically sufficient. There are some benchtop 4‑1/2‑digit designs, but they are most widely used in student labs and some service benches.
DMMs with 5‑1/2‑digit maximum resolution are suitable for a much wider range of test and measurement tasks, including portable and benchtop applications that demand higher resolution. Increasingly, the latest models are optimized for benchtop uses such as R&D, service, and university teaching labs, and include capabilities rarely available before in 5‑1/2‑digit DMMs, such as temperature, capacitance, and four‑wire resistance measurements and broader measurement ranges. Many also offer the throughput, interfaces, and triggering capabilities necessary for use in automated or PC-controlled production test. As a result, a growing number of 5‑1/2‑digit units are being integrated into automated test systems for low‑cost electronic devices and components. For example, Keithley’s 5‑1/2‑digit Model 2110 DMM (Figure 1) delivers up to 200 readings/second via its standard USB remote interface. For production and monitoring applications in which throughput is critical, it can read up to 50,000 readings/second at 4‑1/2 digits. In the past, manufacturers often had to buy more expensive 6‑1/2‑digit DMMs, but newer 5-1/2-digit DMMs now offer greater accuracy, higher speeds, and more advanced feature sets to become legitimate solutions for these applications.
Figure 1: A 5 1/2 digit DMM from Keithley Instruments.
At 6‑1/2 digits and above, DMM prices tend to be significantly higher, depending on the features and capabilities they provide in addition to the standard DC/AC voltage and current and 2‑wire resistance measurement capabilities all DMMs offer. Like 5‑1/2‑digit DMMs, many 6‑1/2‑digit DMMs are suitable for both general‑purpose benchtop and production test applications. Most support both front panel control and remote operation and multiple interface options are increasingly available to allow for greater flexibility when integrating them into complex test systems.
High‑performance 7‑1/2‑digit and 8-1/2-digit DMMs offer the extended range of resolution needed for applications such as research and design or production testing of precision components such as high reliability sensors or transducers, high-bit A/D and D/A converters, low PPM resistors, references, connectors, switches, and relays. In addition, these higher performance multimeters often serve as system references in high-value ATE production testers. Metrology and calibration labs are another common application area, especially for 8‑1/2-digit DMMs, where high resolution DMMs are often used as calibration instruments for other, less accurate instruments.
Have you ever felt like you’ve bought more DMM resolution than you really needed? Tell us about it in the comments.
Guide to Understanding Electrical Test and Measurement.
About the Author
Jerry Janesch is a senior market development manager at Keithley Instruments, Inc., headquartered in Cleveland, Ohio, which is part of the Tektronix test and measurement portfolio. He earned a bachelor’s degree in electrical engineering from Fenn College of Engineering and a master’s of business administration from John Carroll University. He has been with Keithley since 2000.
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