Chapter 4 Connecting and Using Accessories and Transducers
Getting Started with SCXI 4-24 www.ni.com
Analog Input Channels
A relay multiplexer routes input signals to the isolation amplifier. This
relay multiplexer can be configured in two-wire or four-wire mode
scanning. In the two-wire scan mode, all 16 channels operate as voltage
sense channels. At any time only one of 16 channels is connected to the
isolation amplifier. In the four-wire scan mode, the 16 channels are divided
into two banks that switch synchronously. The eight upper channels
(0 through 7) operate as voltage sense channels and one out of eight is
connected to the amplifier at any time. In addition, the eight lower channels
(8 through 15) operate as current output channels that switch in tandem
with the sense channels. At any time only one channel is connected to the
current excitation channel. Table 4-5 lists the sense and current output
channel associations.
The temperature sensor consists of a thermistor located on the SCXI-1322
terminal block. This thermistor connects through the temperature channel
to the isolation amplifier. The temperature sensor is for cold-junction
compensation of thermocouples. When measuring the temperature sensor
output, set your SCXI-1122 for a gain of 5 and a 4 Hz filter. This increases
the measurement resolution and accuracy and reduces noise.
Note
With a 4 Hz bandwidth, you must wait 1 s before you take the temperature
measurement to permit the system to settle. If you want to use the 4 kHz filter, take a large
number of samples and average them. To reject 50 or 60 Hz noise, you should acquire
multiple samples over an integral number of power-line cycles, then average these samples.
Table 4-5. Sense and Current Output Channel Associations
Sense Current Output
Channel 0 Channel 8
Channel 1 Channel 9
Channel 2 Channel 10
Channel 3 Channel 11
Channel 4 Channel 12
Channel 5 Channel 13
Channel 6 Channel 14
Channel 7 Channel 15
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