DMM digits depend on performance
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 6/1/2005
Makers of handheld digital multimeters (DMMs) specify a meter's resolution based on its display. A 3.5-digit meter, for example, produces counts from 0000 to ±1999. It has three full digits (0–9) and a half digit (0–1). A 3.75-digit DMM counts from 0000 to ±3999, where the three-quarter digit counts from 0 to 3.
Card-based DMMs, such as the 6.5-digit and 7.5-digit PXI and PCI models from National Instruments and Signametrics, don't base their digits on display counts because they lack displays. Instead, they base their digits on the DMM's performance range and noise. To calculate the number of digits, the companies use an equation similar to the following:
Digits = log10 (ADC counts)
The table shows the relationship among analog-to-digital converter (ADC) bits, ADC counts, and DMM digits (rounded to two decimal places). If a DMM had a noiseless, perfectly linear, 1-million count ADC, it would have exactly 6 digits of resolution. ADCs, however, have counts based on powers of two. Thus, a 7.5-digit DMM needs 25 effective bits.
Unfortunately, all measurement systems suffer from noise and nonlinearities, which reduces their effective number of bits (ENOB) and thus their effective number of digits (ENOD). Ken Reindel, director of measurement technology at National Instruments, uses this equation to characterize his company's card-based DMMs:
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Zwie Amitai, director of operations at Signametrics, points out that the actual bits of a DMM's ADC must be greater than shown in the table to compensate for noise and nonlinearities. "Our SM2064 PXI DMM has 24,000,000 effective counts (up and down), which corresponds to log10 (24,000,000), or 7.38 digits." To get the 24 million effective counts, the DMM needs an ADC with better than 25 bits.
| ADC Bits | ADC Counts | Digits |
| 18 | 262,144 | 5.42 |
| 19 | 524,288 | 5.72 |
| 20 | 1,048,576 | 6.02 |
| 21 | 2,097,152 | 6.32 |
| 22 | 4,194,304 | 6.62 |
| 23 | 8,388,608 | 6.92 |
| 24 | 16,777,216 | 7.22 |
| 25 | 33,554,432 | 7.53 |
| 26 | 67,108,864 | 7.83 |
