Signametrics
16
4.0 DMM Operation and Measurement Tutorial
Most of the 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 DMM, 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
V to 240 VDC or VAC. Use the
V,
+
and
V,
-
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,
+
and
V,
-
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
Volts 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 front panel powers up in 8rps, DCV,
240
V range. If the DMM is operated in Autorange, with an open
input, The DMM will keep changing ranges. This is perfectly normal with ultra high impedance DMM’. The
virtually infinite input impedance of the 240 mV and 2.4 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 accumulates, the DMM will change ranges.
4.1.2 True RMS AC Voltage Measurements
ACV is specified for signals greater than 1mV, from 10 Hz to 50 kHz. The ACV function is AC coupled, and
measures the true RMS value of the waveform.
ACV measurements, if possible, should have the NEUTRAL or GROUND attached to the
V,
-
terminal. See
Figure 4-1, below. This prevents any “Common Mode” problems from occurring (Common Mode refers to floating
the SM2060
V,
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.
The settling time and low end bandwidth of the RMS function are effected by the status of the Fast RMS control
circuit. When fast RMS is selected, the RMS settling time is about 10 times faster, but the low end frequency is
significantly increased.
