Optimizing Measurements; Auto-Zero - National Instruments NI 435 Series User Manual

Chapter 2 Operating the NI 435x Device

Optimizing Measurements

NI 435x User Manual
PXI and PCI has six additional ranges of 625 Ω, 1.25 kΩ, 2.5 kΩ, 3.75 kΩ,
7.5 kΩ, and 15 kΩ with the 1 mA current source.
To determine the most suitable input range for the application, estimate the
voltage developed across the resistor by following the procedure outlined
in Figures 2-10 and 2-11. Estimate the common-mode voltage at the inputs,
and verify that the range you select can handle that common mode voltage.
Choose the lowest range in the 4-wire ohms mode when you are measuring
RTDs and thermistors, for best results.
The NI 435x can measure resistances to its specified accuracy as long as the
voltage across the resistors is within the selected input range specified
above. To get the best resolution, noise rejection, and accuracy, choose the
smallest range in which the signals can be accommodated. Make sure that
each signal input to CH+ and CH– is within the input common-mode limits
of this input range. The input common-mode limits are ±2.5 V and ±15 V,
for the lower three and higher three input ranges, respectively.
For resistance higher than 25 kΩ, a settling time of over 1 s may be
required when changing channels, to achieve the specified accuracy.
In addition to the potential problems discussed in the sections on
connecting the RTDs and thermistors, also consider other problems
associated with AC noise effects, thermal EMF, and other errors as
discussed in the following sections.

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 is especially useful when
the NI 435x is operating at an ambient temperature other than that of
calibration (23 °C typical). Use the 4-wire mode in LabVIEW while
reading the offset for resistance measurements.
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