Chapter 4
Theory of Operation
©
National Instruments Corporation
4-31
VXI-SC-1102 User Manual
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. The
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 with a cutoff frequency of 2 Hz.
Note:
Because of the 2 Hz bandwidth of the 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.
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
multiplexed to the output buffer, where 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
µ
s. This is the smallest interval in which you can switch
between analog channels on the submodule and still measure accurate
voltages. The 3
µ
s scan interval gives you a maximum sampling rate of
333 kHz. Because each VXI-SC-1102 channel contains a 2 Hz lowpass
filter, the 333 kHz sample rate allows you to sample multiple channels
without undersampling any channel.
Analog Output
The 32 input channels and the CJSENSOR are multiplexed into one
output. This output is passed to the VXI-SC-1000 carrier module, where
it is connected to the Analog Bus for measurement by a VXI-MIO
module.
01Title.book : 08Ch4.frm Page 31 Friday, March 21, 1997 3:40 PM
