How Digital Voltmeters Work
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Integrator
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The integrator stabilizes the voltage as a first step in measuring it. It takes a brief time sample, integrates it and outputs a proportional voltage. Integrating a time sample makes the voltmeter more immune to noise in the signal; spurious changes in the signal are ignored.
Analog/Digital Converter
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The analog/digital converter, or ADC, is the heart of the voltmeter. It is a clocked circuit that takes an input voltage sample and outputs a number representing the voltage value. Since it's clocked, you get a steady stream of numbers that change when the input voltage changes. The higher the clock rate, the more often the measurement is updated. With voltmeters that read-out on a display, speed is not so much a critical issue. With a digital voltmeter feeding a computerized control system, fast response may be important. As with any voltmeter, the electronics need to be precise in all the analog components to make accurate measurements. Precision also depends on the bit resolution of the ADC: to drive a four-segment display, 16 bits of resolution is adequate. More decimal digits need more internal bits of precision.
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Zeroing and Scaling
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A digital voltmeter needs to know where zero is in order to give a true reading. This is done with a precision zero reference voltage provided either internal to the ADC chip or externally. Zero is defined as the midpoint between the voltmeter's positive and negative power supply voltages.
For a general-purpose voltmeter, scaling is important. Scale can be switched automatically by detecting overflow in the display. In 2009, most digital multimeters have automatic scaling, though the least expensive might use a manual range switch.
Display
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Usually, four or more digits of accuracy, a minus sign, and a floating decimal point are provided. Other indicators for battery status and range may also be included. A well-designed integrator and ADC will keep the display from jittering between values; otherwise usability suffers. For example, if the voltage is 5.21 volts, without some stabilizing, the display might waver between 5.21 and 5.22.
The digit display needs driver circuitry to handle the drive current. The same ADC chip frequently also has the display drivers also. Connecting the chip directly to the display simplifies the design and adds reliability.
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