What Characteristics Does a Digital Multimeter Have?
Resolution
The voltage value corresponding to one least‑significant digit on the lowest voltage range of a digital multimeter is defined as resolution, which reflects the instrument's sensitivity. The resolution of digital instruments improves as the number of display digits increases. Digital multimeters with different digit specifications deliver different maximum resolution values. For example, a 3‑1/2‑digit multimeter offers 100 μV resolution.
The resolution indicator of a digital multimeter can also be expressed as a percentage‑based resolution. It refers to the percentage ratio between the smallest non‑zero readable digit and the maximum readable digit of the instrument. For instance, a typical 3‑1/2‑digit digital multimeter has a minimum non‑zero digit of 1 and a maximum reading of 1999, so its percentage resolution equals 1 / 1999 ≈ 0.05 %.
It should be noted that resolution and accuracy are two distinct concepts. Resolution characterizes the instrument's sensitivity, namely its capability to detect tiny voltage changes. Accuracy reflects measurement trueness, that is, the consistency between measured values and true values. There is no inevitable correlation between them, so they must not be confused. Accuracy is determined by the combined errors of the internal A/D converter, functional converter and quantization error. From the measurement perspective, resolution is a nominal indicator independent of measurement error, while accuracy is a practical indicator that defines the magnitude of measurement error. Therefore, simply increasing display digits to improve resolution is not a feasible solution.
Measuring Range
For multi‑function digital multimeters, each function has its corresponding maximum and minimum measurable values. For example, for a 4‑1/2‑digit multimeter, the DC voltage measuring range is 0.01 mV ~ 1000 V.
Measuring Rate
Measuring rate refers to the number of measurements performed by the digital multimeter per second, in readings per second (readings/s). It mainly depends on the conversion speed of the A/D converter. Some handheld digital multimeters specify speed by measurement cycle, which is the time required to complete one single measurement.
There is a trade‑off between measuring rate and accuracy. Higher accuracy generally comes with lower measuring rate, and it is difficult to optimize both at the same time. This conflict can be resolved by equipping the multimeter with selectable display digits or a measuring‑speed switch: add a fast‑measurement mode supported by a high‑speed A/D converter, or drastically boost measuring rate by reducing active display digits. This widely‑adopted method satisfies different‑user requirements for measurement speed.
Input Impedance
For voltage measurement, the instrument shall feature high input impedance. It draws very little current from the circuit under test, avoids disturbing the operating state of the circuit or signal source, and reduces measurement errors. For example, the DC‑voltage input resistance of a typical 3‑1/2‑digit handheld digital multimeter is generally 10 MΩ. Affected by input capacitance, the input impedance of the AC‑voltage range is normally lower than that of the DC‑voltage range.
For current measurement, the instrument shall have low input impedance to minimize its influence on the circuit under test after connection. However, the low input impedance of the current range makes the multimeter vulnerable to burnout. Users shall pay close attention during operation.
