Chapter 3
NI 4350 Operation
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National Instruments Corporation
higher in still air than in moving water. The self-heating in RTDs due
to the built-in 25
µ
A is negligible.
AC Noise Effects
The NI 4350 instruments reject AC noise as specified in NMR
in
However, if the amplitudes of the AC
noise are large compared to the DC signal, or if the peak value (AC +
DC) of the measured signal is outside the input range, the NI 4350
instrument may exhibit additional errors. To minimize these errors,
keep the signal source and the NI 4350 instrument and its accessory
away from strong AC magnetic sources and minimize the area of the
loop formed by the wires connecting the signal source with the
accessory. Choosing the notch filter frequency of 10 Hz will provide
you with the best AC noise rejection. If the peak value of the measured
voltage is likely to exceed the selected input range, select the next
higher input range.
Thermal EMF
Thermoelectric potentials or thermal EMFs are voltages generated at
the junctions of dissimilar metals and are functions of temperature.
Thermal EMFs in the source generating the signal can introduce errors
in measurements that change with variations in temperature.
To minimize thermal EMFs, use copper wires to connect the signal
to the NI 4350 instrument accessory. Avoid using dissimilar metal
wires in connections. Also, keep out temperature gradients in the space
enclosing the signal source, the NI 4350 instrument and its accessories.
Using the Current Source
The NI 4350 features a precision current source, which supplies 25
µ
A
and provides excitation to a total maximum resistance of 600 k
Ω
. This
resistance can be in the form of RTDs, thermistors, or any other resistor.
The calibrated value of the current source is stored on-board. Refer to
the sections Measuring Temperature with RTDs and Thermistors and
Measuring Resistance for details on how to use this current source.
