National Instruments Module SCXI-1503 - Manual (Page 35)

Chapter 4 Theory of Operation
© National Instruments Corporation 4-5 SCXI-1503 User Manual
The module includes first-in first-out (FIFO) memory for storing the
channel scan list defined in your application code. NI-DAQ drivers load the
FIFO based on the channel assignments you make in your application. You
need not explicitly program the module FIFO as this is done automatically
for you by the NI-DAQ driver.
When you configure a module for multiplexed mode operation, the routing
of multiplexed signals to the DAQ device depends on which module in the
SCXI system is cabled to the DAQ device. There are several possible
scenarios for routing signals from the multiplexed modules to the DAQ
device.
If the scanned SCXI-1503 module is not directly cabled to the DAQ device,
the module sends its signals through the SCXIbus to the cabled module.
The cabled module, whose routing is controlled by the SCXI chassis, routes
the SCXIbus signals to the DAQ device through the AI 0 pin on its rear
signal connector.
If the DAQ device scans the cabled module, the module routes its input
signals through the AI 0 pin on its rear signal connector to a single channel
on the DAQ device.
Measuring Temperature with Resistive Transducers
This section discusses RTDs and thermistors, and describes accuracy
considerations when connecting resistive transducers to the signal
conditioning system.
RTDs
A resistive-temperature detector (RTD) is a temperature-sensing device
whose resistance increases with temperature. An RTD consists of a wire
coil or deposited film of pure metal. RTDs are made of different metals and
have different resistances, but the most popular RTD is made of platinum
and has a nominal resistance of 100
Ω at 0 °C.
RTDs are known for their excellent accuracy over a wide temperature
range. Some RTDs have accuracies as high as 0.01
Ω (0.026 °C) at 0 °C.
RTDs are also extremely stable devices. Common industrial RTDs drift less
than 0.1
°C/year, and some models are stable to within 0.0025 °C/year.
RTDs are sometimes difficult to measure because they have relatively low
nominal resistance (commonly 100
Ω) that changes only slightly with
temperature (less than 0.4
Ω/°C). To accurately measure these small