![background image](http://html1.mh-extra.com/html/national-instruments/scxi-1100/scxi-1100_user-manual_3426713057.webp)
Chapter 3
Theory of Operation
© National Instruments Corporation
3-11
SCXI-1100 User Manual
Table 3-2. Multiplexer/Input Channel Correspondence
Multiplexer
Input Channels
0
0 through 3
1
4 through 7
2
8 through 11
3
12 through 15
4
16 through 19
5
20 through 23
6
24 through 27
7
28 through 31
The analog input overvoltage protection is
±
25 V when powered on and
±
15 V when powered
off.
The channel-select hardware consists of a 5-bit counter, MUXCOUNTER, and a three-to-eight
decoder. The module sends to the decoder the three MSBs of the counter to determine which
multiplexer is addressed, and the two LSBs to determine which of the four channels of the
selected multiplexer is to be read. In the Single-Channel Read mode, the module loads the
MUXCOUNTER with the desired channel number. In the Scanning mode, the module loads the
counter with the first channel to be read. During the scan, the counter is clocked by SCANCLK
from the data acquisition board, or TRIG0 from the SCXIbus, depending on the state of the
CLKSELECT bit in the Configuration Register. During scanning operations, MUXCOUNTER
is reloaded with the channel value stored in the Configuration Register when SCANCON is high
(inactive) and counts upwards on each rising clock edge when SCANCON is low (active).
The instrumentation amplifier on the SCXI-1100 board fulfills two purposes. The PGIA
converts a differential input signal into a single-ended signal with respect to the SCXI-1100
ground for input common-mode signal rejection. With this conversion, the module can extract
the input analog signal from a common-mode voltage or noise before the data acquisition board
samples and converts the signal. The instrumentation amplifier also amplifies and conditions an
input signal, which results in an increase in measurement resolution and accuracy. Furthermore,
software-selectable gains of 1, 2, 5, 10, 20, 50, 100, 200, 1,000, and 2,000 are available through
the SCXI-1100 instrumentation amplifier. The instrumentation amplifier is made up of two
cascaded stages with independent gain control as described in Chapter 5, Programming. The
first stage has gains of 1, 10, and 100 and the second stage has gains of 1, 2, 5, 10, and 20. The
choice of first-stage and second-stage gains affects the settling time of the PGIA. For example,
to achieve a settling time of 5.6
µ
s at an overall gain of 100, you must set the first and second
stages to 10 and 10 and not to 100 and 1.
The input circuitry also includes two one-pole filters at 4 Hz and 10 kHz. These filters are
jumper selectable as described in Chapter 2, Configuration and Installation. For further filtering,
each input channel has C320-type capacitor pads for differential filtering at each channel. Notice
that the filter cut-off frequencies can differ for each channel depending on the capacitor used.
You can determine the -3 dB point of these filters from the following equation:
f
c
= 1/(4
π
R
p
C)
where R
p
is equal to 820
Ω
±
2% and C is the capacitor you installed.