Signametrics
18
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, 2
+
and
V, 2
-
terminals, being certain to
always leave the
I,4
+ and I,4
-
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, 2
+
and
V, 2
-
terminals and perform
Relative
operation to eliminate zero errors.
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 2rps, 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. As with virtually all true-RMS measuring
meters, the DMM 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
V,2
-
terminal. This prevents any “Common Mode” problems from occurring (Common Mode refers to
floating the DMM
V,2
-
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 frequency limits of the RMS functions (AC Voltage and current) are effected
by both, the value of the input as a percentage of range, and the frequency of the signal.
Take into account the amount of time it takes for the RMS circuit to settle. With a setteling time of 0.5s,
you may use a measurement rate of 2 readings/s and take two readings. The first will provide the 0.5s to
settele, and the second will make a stable reading. Do not average since the first reading will introduce an
error.
For improved accuracy and stability while measuring low frequency signals, the measurement time
(1/rate) should be at least ten (10) times the period of the measured signal.
4.2 Current Measurements
The DMM measures AC and DC currents between 100
A and 2.5 A. Use the +
I, 4W
terminals, being
certain to always leave all other terminals disconnected. Use the AC/DC button to switch between AC
and DC. The AC current is an AC coupled True RMS measurement function. See figure 4-2 for
connection.
