Programmable Ground-Referencing - National Instruments NI 4350 User Manual

Chapter 3 NI 4350 Operation
Note:
NI 4350 User Manual
especially useful when your NI 4350 instrument is operating at an
ambient temperature other than that of calibration (23° C typical). Use
the 4-wire mode in LabVIEW while reading the offset for resistance
measurements.
When using VirtualBench-Logger along with NI 4350 accessories—
PSH32-TC6, CB-27T, TC-2190, or TBX-68T—auto-zeroing is
implemented automatically.

Programmable Ground-Referencing

Always switch off ground-referencing on the channel connected to a
resistor excited by the current source. The current source return
terminal IEX– is tied to –2.5 V through a resistor. This causes any
resistor excited by the current source to be ground-referenced.
Otherwise, the leakage current flowing into the resistor can cause
erroneous measurement for all channels that use the current source.
Programmable Open-Thermocouple Detection
Always switch off open-thermocouple detection on the channel
connected to a resistor. Otherwise, the leakage current flowing into the
resistor can cause erroneous measurement for all channels that use the
current source.
Connecting to External Circuits
See Figure 3-10 for an example of how different transducers connect to
analog channels in the same measurement setup. To measure the value
of a resistor accurately, make sure the resistor is not electrically
connected to any other circuits. Erroneous or misleading readings can
result if the resistor you are measuring is electrically connected to
external circuits that supply voltages or currents or is connected to
external circuits that change the effective resistance of that resistor.
Two-Wire, Three-Wire, and Four-Wire
Measurements
The discussion in
three-wire, or four-wire, earlier in this chapter, applies to any resistance
measurement. Choose the appropriate measurement technique for your
application as shown in Table 3-5.
Connecting Your RTD on whether to use two-wire,
3-24 © National Instruments Corporation