National Instruments NI 435 Series, User Manual, Page: 43 / 77

Chapter 2 Operating the NI 435x Device
© National Instruments Corporation 2-25 NI 435x User Manual
Programmable Ground-Referencing
Always switch off ground-referencing on the channel connected to a
resistor excited by the current source. The current source return terminals,
I
EX–
, I
EX0–
, and I
EX1–
, are tied to –2.5 V through internal circuits.
This –2.5 V causes any resistor excited by the current source to be
ground-referenced. Otherwise, the leakage current flowing into the resistor
can cause erroneous measurements 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 measurements for all channels that use the current source.
Connecting to External Circuits
Refer to Figures 2-10 and 2-11 for examples 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.
2-Wire, 3-Wire, and 4-Wire Measurements
The discussion in the Connecting the RTD section on whether to use
2-wire, 3-wire, or 4-wire, applies to any resistance measurement. Choose
the appropriate measurement technique for the application as shown in
Table 2-5.
Table 2-5. Guidelines for Resistance Measurement
Resistance Being
Measured (
Ω
) Measurement Technique
R
≤
1 k
Ω
4-wire
1 k
Ω
< R
≤
10 k
Ω
4-wire or 3-wire
R > 10 k
Ω
4-wire, 3-wire, or 2-wire