Features of Standard and Dual-Impedance Digital Multimeters
Selecting and Using a Digital Multimeter: Measurement Guidelines
A standard digital multimeter consists of the basic components shown in the diagram. The dual-slope integrating A/D converter is the "heart" of the digital multimeter, performing analog-to-digital conversion. Peripheral circuits mainly include function converters, function and range selection switches, LCD or LED displays, as well as buzzer oscillation circuits, driver circuits, continuity test circuits, low-voltage indicator circuits, and driver circuits for decimal points and symbols (such as polarity signs).
The A/D converter is the core of the digital multimeter. This design uses the 7106 large-scale integrated circuit. The 7106 features internal XOR gate outputs, can drive LCD displays, and consumes very little power. Its main characteristics include single-supply operation over a wide voltage range, use of a 9V battery for miniaturization, high input impedance, and automatic zero adjustment and polarity switching via internal analog switches. Its disadvantage is relatively slow A/D conversion speed, but it fully satisfies general electrical measurement requirements.
Basic Knowledge About Impedance
Today, almost all digital multimeters sold for industrial, electrical, and electronic system measurement have very high input impedance, typically greater than 1 MΩ. Simply put, when a DMM measures a circuit, it has almost no effect on circuit performance. This is ideal for most measurement needs, especially for sensitive electronic or control circuits.
Older troubleshooting tools, such as analog multimeters and solenoid testers, usually have low input impedance, around 10 kΩ or lower. Although such tools are not affected by stray voltages, they are only suitable for measuring power circuits or other applications where low input impedance does not degrade or alter circuit behavior.
A Model for Combining Two Input Impedances
With a dual-impedance meter, technicians can reliably troubleshoot sensitive electronic or control circuits, including those affected by stray voltages, and more accurately determine whether real voltage exists in a circuit.
For standard electrical measurements, high-impedance meters are generally preferred unless stray voltages are present.
