Chapter 4
Theory of Operation
©
National Instruments Corporation
4-3
Analog Input Channels
Each of the eight analog input channels feeds to a separate amplifier with a
fixed gain of 100. Then the signal passes through a three-pole lowpass
filter.
The temperature sensor consists of a thermistor located inside the module
next to its respective channel. The temperature sensor is for cold-junction
compensation of thermocouples. The CJSENSOR channel also passes
through a 2 Hz lowpass filter to reject unwanted noise. Along with the other
eight input channels, the CJSENSOR is multiplexed to the output buffer,
where it can be read by the DAQ device.
For measurement accuracy of 0.012% of full scale, the minimum
interchannel delay is 3
µ
s. This is the smallest interval in which you can
switch between analog channels on the module and still measure voltages
accurately. The 3
µ
s interchannel delay gives you a maximum sampling
rate of 333 kHz. Because this rate is higher than the bandwidth of a single
SCXI-1112 channel (2 Hz), you can sample multiple channels on multiple
SCXI modules without undersampling one of the SCXI-1112 channels.
Analog Bus Switch
The SCXI-1112 modules contain switches to place their analog output on
the SCXIbus to pass the output to the SCXI module cabled to the DAQ
device. The SCXI-1112 modules also contain switches to receive a signal
placed on the SCXIbus by another SCXI module. The output buffer of the
cabled module drives a received signal onto the MCH0+ line of the rear
signal connector so that the DAQ device can read it. When a signal passes
on the SCXIbus from the scanned SCXI module to the cabled SCXI
module, the measurement is known as indirect scanning.