Chapter 3
DMM Operation
DAQMeter DAQCard-4050 User Manual
3-4
©
National Instruments Corporation
Figure 3-2 illustrates the input impedance of the DAQCard-4050 and its
effect on the measurement of a circuit under test. If the source
impedance of the circuit being tested is known, you can correct for the
attenuation caused by the DAQCard-4050 in software.
Figure 3-2.
The Effect of Input Impedance on Signal Measurement
AC Noise Effects
The DAQCard-4050 filters out AC voltages while in the DC voltage
measurement ranges. However, if the amplitudes of the AC voltages are
large compared to the DC voltages, or if the peak value (AC+DC) of the
measured voltage is outside the overrange limits, the DAQCard-4050
may exhibit additional errors. To minimize these errors, keep the circuit
under test and the DAQCard-4050 away from strong AC magnetic
sources and minimize the area of the loop formed by the test leads.
Choosing the 10 readings/s reading rate will also help minimize noise
from AC sources. If the peak value of the measured voltage is likely to
exceed the selected input range, select the next high input range.
Thermal EMF
Thermal EMFs, or thermoelectric potentials, are voltages generated at
the junctions of dissimilar metals and are functions of temperature.
Thermal EMFs in a circuit under test can cause higher than expected
offsets that change with variances in temperature. Occasionally
these voltages are desirable, such as when the thermal EMF of a
thermocouple is used to measure temperature changes. Other times
these voltages are sources of error.
Source
Voltage Vs
Vs
V
m
=
Rin
Rs +Rin
Measured
Voltage
Vm
External
Source
Impedance Rs
Input
Impedance
Rin
DAQCard-4050
+
+
-
-