Chapter 3
DMM Operation
©
National Instruments Corporation
3-5
DAQMeter DAQCard-4050 User Manual
To minimize thermal EMFs, use copper leads to connect the signal to
the DAQCard-4050. Also, try to maintain a constant temperature in the
circuits under test, perhaps by using an enclosure to minimize air
currents.
AC Voltages
True RMS Method
In the AC voltage ranges, the DAQCard-4050 measures the AC-coupled
root mean square (RMS) value of a signal. The RMS value of a signal
is a fundamental measurement of the magnitude of an AC signal. The
RMS value of an AC signal can be defined mathematically as the square
root of the average of the square of a signal. In practical terms, the RMS
value of an AC signal is the DC value required to produce an equivalent
amount of heat in the same load.
The DAQCard-4050 first AC-couples the measured signal to remove
any DC components and then measures the RMS value of the AC
component. This method lets you measure a small AC signal in the
presence of a large DC offset.
Input Ranges
The DAQCard-4050 has five input ranges available for measuring AC
voltages. These ranges are 20 mV
rms
, 200 mV
rms
, 2.0 V
rms
, 25 V
rms
,
and 250 V
rms
. The DAQCard-4050 can measure AC voltages to its
specified accuracy as long as the voltage is at least 10% and no more
than 100% of the selected input range. The DAQCard-4050 can
measure voltages outside of the input range (the overrange) with
reduced accuracy. Table 3-2 shows the accuracy and typical overrange
of each of the input ranges. The accuracy is based on a 1 kHz sine wave
of full-scale input range.
