Optimizing Measurements - National Instruments NI 4350 User Manual

Chapter 3 NI 4350 Operation

Optimizing Measurements

Note:
Note:
NI 4350 User Manual
To make accurate voltage measurements, program the onboard
ground-referencing and open-thermocouple detection appropriately.
Also consider problems associated with AC noise effects, thermal
EMFs, and other errors as discussed in the following sections.
Auto-Zero
Auto-zero is a method that instruments use to remove offset errors
in the measurement. Analog channel 1 (CH1) on the CB-27T and
TBX-68T is dedicated for auto-zero. CH1+ is connected to CH1– on
these accessories. When using a CB-27 or TBX-68 accessory for RTDs,
connect CH– to CH+ (any channel) to make that channel useful for
auto-zero. 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 the 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 signal source is ground-referenced, switch
off ground-referencing on that channel.
If you determine that your signal source is floating, switch on
ground-referencing on that channel. Otherwise, the inputs may float
out of the input common mode limits of the NI 4350 instrument.
When you use the CB-27T and TBX-68T accessories, always switch on
ground-referencing on CH1. Doing this ground-references the
auto-zero channel.
When using the VirtualBench-Logger, or NI435X Instrument Driver, or
the DAQ Channel Wizard, along with the NI 4350 accessories—
PSH32-TC6, CB-27T, TC-2190, or TBX-68T— the ground-referencing
switch on the auto-zero channel is set appropriately, automatically.
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