Auto-Zero - National Instruments NI 4350 User Manual

Note:
Note:
© 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, and TBX-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 4350 instrument may have. This is
especially useful when your NI 4350 instrument is operating at an
ambient temperature other than that of calibration (23° C typical).
When using the VirtualBench-Logger along with NI 4350 accessories—
PSH32-TC6, CB-27T, TC-2190, or TBX-68T—auto-zeroing is
implemented automatically.
Programmable Ground-Referencing
If you determine that your thermocouple is ground-referenced, switch
off ground-referencing on that channel.
If you determine that your 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 4350
instrument.
When you use the PSH32-TC6, CB-27T, TC-2190, and TBX-68T
accessories, always switch on ground-referencing on CH1. Doing this
ground-references the auto-zero channel.
On all the NI 4350 instrument accessories used with thermocouples,
analog channel CH0 is dedicated to the thermistor cold-junction sensor.
The built-in current source return terminal IEX- is tied to –2.5 V
through a resistor. This 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.
When using VirtualBench-Logger, the DAQ Channel Wizard, or the
NI435X Instrument Driver, the ground-referencing switch on the
cold-junction sensor channel and auto-zero channel is set appropriately,
automatically.
3-9
Chapter 3 NI 4350 Operation
NI 4350 User Manual