Complete Guide to Selection Skills for Digital Multimeters
① Functions
Modern digital multimeters can measure AC/DC voltage, AC/DC current, resistance and frequency, among five basic functions. They also feature recording, data hold, relative mode, tolerance comparison, diode test, dBm/dBv measurement, IEEE‑488 interface or RS‑232 interface and other functions. Select the appropriate functions according to actual application requirements.
② Range and Span
Digital multimeters provide a wide selection of ranges, and the basic range delivers the highest accuracy. Many digital multimeters support auto‑/manual‑ranging for convenient, safe and fast measurement. A large number of digital multimeters also possess over‑range capability. When the measured value exceeds the current range but is still below the maximum display value, range switching is unnecessary, which improves accuracy and resolution.
③ Accuracy
The maximum permissible error of a digital multimeter depends on both its accuracy class and precision.
④ Input Resistance and Zero Current
Excessively low input resistance and high zero current of digital multimeters will introduce measurement errors. The key lies in the limit values permitted by the measuring instrument, i.e. the internal resistance of the signal source. For signal sources with high impedance, select instruments with high input impedance and low zero current to keep such errors negligible.
⑤ Series‑Mode Rejection Ratio and Common‑Mode Rejection Ratio
When various interferences such as electric fields, magnetic fields and high‑frequency noise exist, or during long‑distance measurement, interfering signals may be coupled in and cause inaccurate readings. Therefore, select instruments with high series‑mode and common‑mode rejection ratios according to operating conditions. Especially for high‑precision measurement, choose digital multimeters equipped with guard terminal G for effective suppression of common‑mode interference.
⑥ Display Format
Digital multimeters are not limited to numeric readouts. They can display graphs, texts and symbols. Some new‑generation instruments can simultaneously show multiple parameters of one signal to facilitate on‑site observation, operation and management.
⑦ Operating Voltage
Benchtop digital multimeters are generally powered by 220 V mains supply. Some new‑type digital multimeters feature wide‑range power supply from 96 V to 264 V. Hand‑held digital multimeters can be powered by AC mains, rechargeable batteries or external batteries.
⑧ Response Time, Measurement Speed and Frequency Response
Shorter response time is preferred. Some meters have long response time and readings will stabilize only after a certain period. Measurement speed matters when interfaced with system test equipment and must match the whole system. Frequency response shall be reasonably selected according to measured signals.
⑨ True‑RMS Measurement
AC voltage measurement includes average‑value conversion, peak‑value conversion and true‑RMS conversion. Average‑value conversion and peak‑value conversion produce inaccurate results under heavy waveform distortion, while true‑RMS conversion is immune to waveform shapes and delivers more reliable measurement results.
⑩ Resistance Connection Method
Resistance measurement adopts either four‑wire or two‑wire connection. Four‑wire resistance measurement shall be adopted for low‑resistance and high‑precision measurement.
