Digital Electronics: Principles, Devices and Applications

666 Digital Electronics16.6.1 Advantages of Using a Digital MultimeterThe digital multimeter has the advantages of offering unambiguous display with no allowance for anyhuman error, improved accuracy (±01% as against ±3% in analogue meters) and improved resolution(+01% as against 1% in analogue meters). Other advantages include easy incorporation of featuressuch as autoranging, automatic polarity and diode/transistor test and so on. The cost advantage thatused to exist in favour of analogue meters has narrowed down to a small amount with advances inIC 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 noisyenvironment.16.6.2 Inside the Digital MeterFigure 16.8 shows the schematic arrangement of a typical digital meter. The signal scalar at the inputis basically an attenuator/amplifier block and is partly used for range selection function. In autorangingmeters, the input signal level is sensed on application of the input signal, and the signal scalar gain isselected 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 latterbeing the preferred one because of its higher accuracy, insensitivity to changes in integrator parametersand low cost. All the building blocks depicted in Fig. 16.8, except for the display, are available on asingle chip. ICL 7106/7107 is an example.16.6.3 Significance of the Half-DigitDigital multimeters (DMMs) invariably have a display that has an additional half-digit. We have 3 1 / 2 -,4 1 / 2 - and 5 1 / 2 -digit digital multimeters rather than 3-, 4- and 5-digit multimeters. While the usuallyso-called full digits can display all digits from 0 to 9, a half-digit can display either a ‘0’ or a ‘1’. TheDisplayDecoder/DriverSignalScalarSignalConditionerADCCounterControlFigure 16.8Block schematic of a digital meter.