Chapter 2
Operating the NI 435x Device
2-12
ni.com
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 435
x
may have.
This compensation is especially useful when the NI 435
x
is operating at an
ambient temperature other than that of calibration (23 °C typical).
Note
When measuring the transducer channel with auto-zero, the NI 435
x
operates at its
multi-channel rate. Refer to Table 2-1 for this rate.
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 435
x
.
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.
Note
When using the MAX virtual channels, along with the NI 435
x
accessories—
PSH32-TC6, CB-27T, CB-68T, TC-2190, or TBX-68T—the ground-referencing switch on
the auto-zero channel is automatically set appropriately.
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 k
Ω,
you should measure the same channel for up to 1 s, then switch to another
channel to achieve the specified accuracy.