Chapter 4
Theory of Operation
©
National Instruments Corporation
4-3
SCXI-1102/B/C User Manual
Software Programming Choices in Chapter 1, Introduction, to learn
about options 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. The
channels and CJSENSOR are multiplexed to a single output buffer.
Analog Input Channels
Each of the 32 analog input channels feeds to 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 SCXI-1102 module 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. For the SCXI-1102B and SCXI-1102C modules, this
time is approximately 100 ms and 1 ms, respectively.
The temperature sensor consists of a thermistor located on the
SCXI-1300 or SCXI-1303 terminal block. The temperature sensor is for
cold-junction compensation of the SCXI-1102/B/C thermocouples. The
CJSENSOR channel also passes through a 2 Hz lowpass filter to reject
unwanted noise on the SCXI-1102/B/C. Along with the other 32 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 scan
interval is 3
µ
s. This is the smallest interval in which you can switch
between analog channels on the module and still measure accurate
voltages. The 3
µ
s scan interval gives you a maximum sampling rate of
333 kHz. Because this rate is higher than the bandwidth of a single
SCXI-1102/B/C channel, you can sample multiple channels on multiple
SCXI modules without undersampling one of the SCXI-1102/B/C
channels.
