Eight Tips for Better Measurements with Digital Multimeters
Tip 1 Avoid Measurement Errors Caused by Connections, Test Leads and Digital Multimeter Wiring
The simplest way to eliminate errors introduced by wiring is to perform a null measurement. For DC voltage or resistance measurement, select an appropriate measuring range, short the probes together and take a reading - this simulates a near‑zero‑input condition - then press the null button. Subsequent readings will subtract the result of the null measurement. Null measurement works well for DC and resistance functions, but this technique is not applicable to AC measurements. AC converters perform poorly at the lower end of the range; the analog converter of the Agilent 34401A digital multimeter has no specified performance below 10 % of full‑scale value. The Agilent 34410A and 34411A digital multimeters employ digital technology and can measure down to 1 % full‑scale, yet they are still not intended for short‑circuit measurements.
Connections
Junctions formed by dissimilar metals create thermocouple junctions, which generate temperature‑dependent voltages. Although such voltages are low, they must be taken seriously when measuring small voltages or working with systems featuring numerous connections. Thermocouple junctions can occur at the DUT, relays (multiplexers), and the digital multimeter itself. Copper‑to‑copper junctions minimize this offset.
For resistance measurements, offset compensation can be used to measure and subtract any offset voltage. Figure 1 illustrates the two measurements carried out during offset‑compensated measurement: the first reading is taken with the current source enabled, and the second with the current source disabled. The actual resistance value is obtained by subtracting the second reading from the first and dividing by the known current‑source output current. Taking two readings reduces measurement speed but improves accuracy. Offset compensation applies to both 2‑wire and 4‑wire resistance measurements.
Figure 1
Offset compensation using two‑step measurement. The first measurement is a standard ohm measurement; the second measures the offset caused by thermoelectric EMF. The voltmeter output equals the difference between the two readings divided by the known source current.
