Auto-Zero; Programmable Ground-Referencing; Programmable Open-Thermocouple Detection; Source Impedance - National Instruments NI 435 Series User Manual

Chapter 2 Operating the NI 435x Device
Note When measuring the transducer channel with auto-zero, the NI 435x operates at its
multi-channel rate. Refer to Table 2-1 for this rate.
When using the MAX virtual channels, along with the NI 435x accessories—
Note
PSH32-TC6, CB-27T, CB-68T, TC-2190, or TBX-68T—the ground-referencing switch on
the auto-zero channel is automatically set appropriately.
NI 435x User Manual

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, TBX-68T,
and CB-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 435x may have.
This compensation is especially useful when the NI 435x is operating at an
ambient temperature other than that of calibration (23 °C typical).

Programmable Ground-Referencing

If you determine that the signal source is ground-referenced, switch off
ground-referencing on that channel.
If you determine that the 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 435x.
When you use the CB-27T, TBX-68T, and CB-68T accessories, always
switch on ground-referencing on CH1. Doing this ground-references the
auto-zero channel.

Programmable Open-Thermocouple Detection

When you measure voltage signals other than thermocouples, always
switch off the onboard open-thermocouple detection.

Source Impedance

For best results, maintain the source impedance and the lead wire resistance
of the signal at less than 100 Ω. If either of these is greater than 25 kΩ,
you should measure the same channel for up to 1 s, then switch to another
channel to achieve the specified accuracy.
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