Operator`s manual

Signametrics 24
4.0 DMM Operation and Measurement Tutorial
Most of the SM2040 measurement functions are accessible from the Windows Control Panel (Figure above). All of
the functions are included in the Windows DLL driver library. To gain familiarity with the SM2040 series DMMs,
run the Windows ‘SETUP.EXE’ to install the software, then run the DMM, as described in the previous section.
This section describes in detail the DMM’s operation and measurement practices for best performance.
4.1 Voltage Measurement
Measures from 0.1 µV to 300 VDC or 250 VAC. Use the V, Ω + and V, Ω - terminals, being certain to always leave
the I+, I- and DIN-7 terminals disconnected. Use the AC/DC button on the Control Panel to switch between AC
and DC.
Making Voltage Measurements is straightforward. The following tips will allow you to make the most accurate
voltage measurements.
4.1.1 DC Voltage Measurements
When making very low-level DCV measurements (<1 mV), you should first place a copper wire shorting plug
across the V, Ω + and V, Ω - terminals and perform Relative function to eliminate zero errors before making your
measurements. A common source of error can come from your test leads, which can introduce several µVolts of
error due to thermal voltages. To minimize thermal voltaic effects after handling the test leads; you should wait a
few seconds before making measurements. Signametrics offers several high quality probes that are optimal for low-
level measurements.
Note: The SM2040 front panel powers up in DCV, 2 readings per second, 330 V range. If the DMM is operated in
Autorange, with an open input, you may hear the SM2040 relays clicking every few seconds, as a range change
occurs. This is perfectly normal with ultra high impedance DMMs such as the SM2040. The virtually infinite input
impedance of the 330 mV and 3.3 V DCV ranges causes this phenomenon. On these ranges, an open input will read
whatever charge is associated with the signal conditioning of the DMM. As this electrical charge changes, the
SM2040 will change ranges, causing the relays to click. This is normal.
4.1.2 True RMS AC Voltage Measurements
ACV is specified for signals greater than 1mV, from 10 Hz to 100 kHz. The ACV function is AC coupled, and
measures the true RMS value of the waveform. As with virtually all true-RMS measuring meters, the SM2040 may
not read a perfect zero with a shorted input. This is normal.
ACV measurements, if possible, should have the NEUTRAL or GROUND attached to the SM2040 V,Ω - terminal.
See Figure 4-1, below. This prevents any “Common Mode” problems from occurring (Common Mode refers to
floating the SM2040 V,Ω LO above Earth Ground.) Common Mode problems can result in noisy readings, or even
cause the PC to hang-up under high V x Hz input conditions. In many systems, grounding the source to be
measured at Earth Ground (being certain to avoid any ground loops) can give better results.