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Note
When measuring the transducer channel with auto-zero and/or cold-junction
compensation, the NI 435x device operates at its multi-channel rate. Refer to Table 2-1
for this rate.
When using VirtualBench-Logger or the MAX virtual channels, the
Note
ground-referencing switch on the cold-junction sensor channel and auto-zero channel is
automatically set appropriately.
© National Instruments Corporation
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 435x may have. This compensation is especially
useful when the NI 435x device is operating at an ambient temperature
other than that of calibration (23 °C typical).
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 435x device.
On all the NI 435x accessories used with thermocouples, analog channel
CH0 is dedicated to the thermistor cold-junction sensor. The built-in
current source return terminal I
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
through a resistor.
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
Chapter 2
or I
is tied to –2.5 V through a
EX–
EX0–
2-9
Operating the NI 435x Device
also is tied to –2.5 V
EX1–
NI 435x User Manual
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