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Chapter 3
DMM Operation
DAQMeter DAQCard-4050 User Manual
3-8
©
National Instruments Corporation
AC Voltage Offset
The AC measurements of the DAQCard-4050 are specified over the
range of 10% to 100% of the full scale input range. Below 10% of the
input range, errors due to the AC voltage offset become significant.
This offset, unlike DC voltage offsets, cannot simply be subtracted from
the readings or zeroed out because the offset gets converted in the RMS
conversion. The relationship between the input voltage, AC offset
voltage, and the measured voltage is as follows:
For example:
Range = 200 mV
rms
Input voltage = 100 mV
rms
AC offset voltage = 0.5 mV
rms
The measured voltage in this example would be
V
measured
= 100.001 mV
rms
; the 0.5 mV
rms
offset contributes only
0.001 mV
rms
of error and is, therefore, negligible. However, if the input
voltage were only 10 mV
rms
, the measured voltage would be
10.012 mV
rms
. To minimize the errors due to the AC offset voltage,
choose an input range that keeps the measured voltage between 10%
and 100% of full scale.
Measuring Resistance
Signal Connections
Connect the test probes to resistors as shown in Figure 3-3. To
accurately measure the value of a resistor, make sure the resistor is not
connected to any other circuits. Erroneous or misleading readings may
result if the resistor you are measuring is connected to external circuits
that supply voltages or currents or to external circuits that change the
effective resistance of that resistor.
Figure 3-3.
Connections for Resistance Measurement
Vmeasured
Vin
(
)
2
Voffset
(
)
2
+
=
Resistor
250 V
MAX.
LO
HI