National Instruments NI 435 Series, User Manual, Page: 27 / 77

Chapter 2 Operating the NI 435x Device
© National Instruments Corporation 2-9 NI 435x User Manual
Auto-Zero
Auto-zero is a method that instruments use to remove any offset errors in
the measurement. Analog channel 1 (CH1) on the PSH32-TC6, CB-27T,
TC-2190, TBX-68T, and CB-68T is dedicated for auto-zero. CH1+ is
connected to CH1– on these accessories. You can measure the voltage
offset on this auto-zero channel and subtract it from the voltage
measurements on other channels. This way you can compensate for any
residual offset error the NI 435
x may have. This compensation is especially
useful when the NI 435 x device is operating at an ambient temperature
other than that of calibration (23 °C typical).
Note When measuring the transducer channel with auto-zero and/or cold-junction
compensation, the NI 435 x device operates at its multi-channel rate. Refer to Table 2-1
for this rate.
Programmable Ground-Referencing
If you determine that the thermocouple is ground-referenced, switch off
ground-referencing on that channel.
If you determine that the thermocouple is floating, switch on
ground-referencing on that channel. Otherwise, the thermocouple inputs
may float out of the input common-mode limits of the NI 435
x device.
On all the NI 435 x accessories used with thermocouples, analog channel
CH0 is dedicated to the thermistor cold-junction sensor. The built-in
current source return terminal I
EX–
or I
EX0–
is tied to –2.5 V through a
resistor. This –2.5 V references any resistor excited by the current source to
ground. Since this current source excites the cold-junction thermistor,
CH0 is automatically ground-referenced. Therefore, when measuring the
voltage across this thermistor, always switch off programmable
ground-referencing on CH0. Otherwise, the leakage current flowing into
the thermistor may cause erroneous measurements in all the channels that
use the current source. Current source terminal I
EX1–
also is tied to –2.5 V
through a resistor.
Note When using VirtualBench-Logger or the MAX virtual channels, the
ground-referencing switch on the cold-junction sensor channel and auto-zero channel is
automatically set appropriately.
Programmable Open-Thermocouple Detection
To detect open or broken thermocouples, switch on open-thermocouple
detection on that channel. Then, if the thermocouple breaks, the voltage on