Chapter 4 Theory of Operation
©
National Instruments Corporation 4-3 VXI-SC-1102/B/C User Manual
Introduction, of this manual to learn about opt ions for programming the
control circuitry.
Analog Circuitry
The analog circuitry per channel consists of a lowpass filter and an
amplifier with a software-selectable gain of 1 or 100. The CJSENSOR
channel also has a buffered lowpass filter but has no amplifier. T he
channels and CJSENSOR are multiplexed to a single output buffer.
Analog Input Channels
Each of the 32 analog input channels drives a separate amplifier with a
programmable gain of 1 or 100. Then the signal passes through a
three-pole lowpass filter.
Note: Because of the 2 Hz bandwidth of the VXI-SC-1102 input channels, you
must wait approximately 3 s after changing the gains before the channels
settle and you can take an accurate measurement. NI-DAQ automatically
reads the Status Register to determine when the module output has settled.
This time is approximately 100 ms and 1 ms for the VXI-SC-1102B and
VXI-SC-1102C, respectively.
The temperature sensor consists of a thermistor located on a National
Instruments VXI terminal block. 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 32 input channels, the CJSENSOR is
multiplex ed to the output buffer, wh ere it can be read by the VXI-MIO
module.
For a measurement accuracy of 0.012% of full scale, the minimum scan
interval is 3 µs. This is the smallest interval in which you can switch
between analog channels on the submodule and still measure accurate
voltages. The 3 µs scan interval gives yo u a maximum sampling rate of
333 kHz. Because each VXI-SC-1102/B/C channel contains a lowpass
filter, the 333 kHz sample rate allows you to sample multiple channels
without undersampling any channel.