Programmable Open-Thermocouple Detection - National Instruments NI 4350 User Manual

Chapter 3 NI 4350 Operation

Programmable Open-Thermocouple Detection

When you measure voltage signals other than thermocouples, always
switch off the onboard, open-thermocouple detection.
Source Impedance
For best results, maintain the source impedance and the lead wire
resistance of your signal at less than 100 Ω. If either of these is greater
than 25 kΩ, you should measure the same channel for up to 1 s, then
switch to another channel to achieve the specified accuracy.
AC Noise Effects
Your NI 4350 instrument rejects AC voltages as specified in NMR in
Appendix A, Specifications. 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 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
accessories away from strong AC magnetic sources and minimize the
area of the loop formed by the wires that connect the signal source
with the accessories. 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, minimize temperature gradients in the space
enclosing the signal source, the NI 4350 instrument, and its accessories.
© National Instruments Corporation 3-13 NI 4350 User Manual