The digital multimeter has the advantages of offering unambiguous display with no allowance for any human error, improved accuracy (±0.1% as against ±3% in analogue meters) and improved resolution (+0.1% as against 1% in analogue meters). Other advantages include easy incorporation of features such as autoranging, automatic polarity and diode/transistor test and so on. The cost advantage that used to exist in favour of analogue meters has narrowed down to a small amount with advances in IC technology. Digital multimeters are fast replacing analogue meters even for routine measurements. However, analogue meters are relatively immune to noise and are preferred in an electrically noisy environment.

Figure 16.8 shows the schematic arrangement of a typical digital meter. The signal scalar at the input is basically an attenuator/amplifier block and is partly used for range selection function. In autoranging meters, the input signal level is sensed on application of the input signal, and the signal scalar gain is selected accordingly. The signal conditioner generates a DC voltage proportional to the input signal. The ADC employed is usually the integrating-type ADC, single slope or dual slope, with the latter being the preferred one because of its higher accuracy, insensitivity to changes in integrator parameters and low cost. All the building blocks depicted in Fig. 16.8, except for the display, are available on a single chip. ICL7106/7107 is an example.

Digital multimeters (DMMs) invariably have a display that has an additional half-digit. We have 3½, 4½, and 5½ digit digital multimeters rather than 3-, 4- and 5-digit multimeters. While the usually so-called full digits can display all digits from 0 to 9, a half-digit can display either a ‘0’ or a ‘1’. The addition of a half-digit in the MSB position of the display preserves the resolution of the multimeter up to a higher range. For instance, a three-digit multimeter has a resolution of 0.1 V up to 99.9 V. A 3½-digit meter with practically no additional hardware would give you a resolution of 0.1 V up to 199.9 V. This increase in resolution range comes with the addition of one additional seven-segment display and no change in hardware complexity. The display resolution is also sometimes expressed in terms of counts. The 3½-digit DMM has a 2000-count resolution. DMMs with a 4000-count resolution, referred to as 4½-digit meters, are also commercially available. These meters will also have four seven-segment displays but have some additional hardware.

Digital multimeters are made in a large variety of sizes, shapes and performance specifications, ranging from pen-type 3½-digit DMMs to 7½-digit high-resolution benchtop versions. Handheld versions are available, typically up to 4½-digit resolution. The majority of them have a built-in diode test, transistor test and continuity check features. Some of them even offer L-C measurement and frequency measurement without any significant change in price. Figure 16.9 shows a photograph of one such multimeter (the Fluke 115 multimeter). It has a 6000 count display and an in-built continuity check, diode test, frequency measurement, capacitance measurement, etc., in addition to conventional functions. Figure 16.10 shows a photograph of a high-end benchtop version of a digital multimeter (Fluke 8845A).

Example 16.6: The specification sheet of a certain 3½-digit digital multimeter lists its display to be a 4000 count display. Determine the resolution offered by the multimeter for the following measurements: (a) the maximum DC voltage that can be measured with a resolution of 0.1 V; (b) the maximum resistance value that can be measured with a resolution of 1Ω; (c) the maximum DC current that can be measured with a resolution of 10µA.