Operation of the current sources, Theory of multiplexed operation – National Instruments Module SCXI-1503 User Manual
Page 34
Chapter 4
Theory of Operation
4-4
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The CJ SENSOR input channel is used to read the sensor temperature from
the terminal block. The temperature sensor is for cold-junction
compensation of thermocouple measurements. The CJ SENSOR channel
also passes through a 5 Hz lowpass filter to reject unwanted noise on the
SCXI-1503. Along with the other 16 input channels, the CJ SENSOR is
multiplexed to the output buffer, where it can be read by the DAQ device.
Operation of the Current Sources
The current sources on the SCXI-1503 continuously provide 16 channels
of 100
μA current excitation. These current sources are on whenever the
SCXI chassis is powered-on. The current sources on the SCXI-1503 are
designed to be accurate to within ±0.05% of the specified value with a
temperature drift of no more than ±5 ppm/°C. The high accuracy and
stability of these current sources makes them especially well suited to
measuring resistance to a high degree of accuracy.
Theory of Multiplexed Operation
In multiplexed mode, all input channels of an SCXI module are
multiplexed into a single analog input channel of the DAQ device.
Multiplexed mode operation is ideal for high channel count systems.
Multiplexed mode is typically used for performing scanning operations
with the SCXI-1503. The power of SCXI multiplexed mode scanning is its
ability to route many input channels to a single channel of the DAQ device.
The multiplexing operation of the analog input signals is performed
entirely by multiplexers in the SCXI modules, not inside the DAQ device
or SCXI chassis. In multiplexed mode the SCXI-1503 scanned channels are
kept by the NI-DAQ driver in a scan list. Immediately prior to a multiplexed
scanning operation, the SCXI chassis is programmed with a module scan
list that controls which module sends its output to the SCXIbus during a
scan through the cabled SCXI module.
The list can contain channels in any physical order and the multiplexer can
sequence the channel selection from the scan list in any order. The ordering
of scanned channels need not be sequential. Channels can occur multiple
times in a single scan list. The scan list can contain an arbitrary number of
channels for each module entry in the scan list, limited to a total of
512 channels per DAQ device. This is referred to as flexible scanning
(random scanning). Not all SCXI modules provide flexible scanning.