Chapter 2
Operating the NI 435x Device
©
National Instruments Corporation
2-11
minimize temperature gradients in the space enclosing the thermocouples,
the NI 435
x
, and its accessories.
Measuring DC Voltage
Connecting the DC Voltage Signal
The NI 435
x
accessories—the TBX-68T, CB-68T, and TBX-68 for the
NI 435
x
for USB, PXI, and PCI—are designed to be used with any
DC voltage signal. Consult the accessory installation guide for instructions
on how to connect the voltage signals.
The NI 435
x
analog inputs are protected against damage from voltages
within
±
42 VDC in all ranges when powered on and
±
17 VDC when the
NI 435
x
is powered off. Never apply voltages above these levels to the
inputs.
Caution
To prevent possible safety hazards, the maximum voltage between any of the
analog inputs and the computer ground should
never
exceed
±
42 VDC when the NI 435
x
is powered on and
±
17 VDC when the NI 435
x
is powered off.
Input Ranges
The NI 435
x
has six bipolar input ranges available for measuring DC
voltage. These ranges are
±
625 mV,
±
1.25 V,
±
2.5 V,
±
3.75 V,
±
7.5 V,
and
±
15 V. The NI 435
x
can measure DC voltage to the specified accuracy
as long as the voltage is within the selected input range. To get the best
resolution, noise rejection, and accuracy, choose the smallest possible
range. 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.
Note
If scanning voltages in different ranges, the NI 435
x
uses the widest range for all
channels.
Optimizing Measurements
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.