Input Ranges - National Instruments NI 4350 User Manual

Input Ranges

Optimizing Measurements
© National Instruments Corporation
The NI 4350 has six ranges for resistance measurements. These ranges
are 25 kΩ, 50 kΩ, 100 kΩ, 150 kΩ, 300 kΩ, and 600 kΩ.These ranges
correspond to the six input ranges available for measuring DC voltages
developed across resistors. These ranges are ±625 mV, ±1.25 V,
±2.5 V, ±3.75 V, ±7.5 V, and ±15 V. To determine the most suitable
input range for your application, estimate the voltage developed across
the resistor by following the procedure outlined in Figure 3-10. Also
estimate the common-mode voltage at the inputs and verify that the
range you select can handle that common mode voltage. Also estimate
the common-mode voltage at the inputs and verify that the range you
select can handle that common-mode voltage. Choose the 25 kΩ range
in the 4-wire ohms mode when you are measuring RTDs and
thermistors, for best results.
The NI 4350 instrument 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 your signals will 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 your 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, 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
3-23
Chapter 3 NI 4350 Operation
NI 4350 User Manual