Chapter 4
Theory of Operation
©
National Instruments Corporation
4-7
The analog input consists of eight isolated single-ended noninverting
amplifiers. In addition, lowpass filtering is available at the inputs. You can
jumper select one of two bandwidths; 10 kHz or 4 Hz for the SCXI-1120,
and 22.5 kHz or 4.5 kHz for the SCXI-1120D. The amplifier gain is divided
into two stages; a first stage providing gains of 1, 10, 50, and 100, and a
second stage providing gains of 1, 2, 5, 10, and 20. Each channel is
configurable to a different bandwidth and gain. In addition to these two
stages, the SCXI-1120D has a fixed input gain stage of 0.5.
Use the following formula to determine the overall gain of a given amplifier
input channel:
for the SCXI-1120
for the SCXI-1120D
where G
total
is the overall gain and G
1st
and G
2nd
are the first- and
second-stage gains. Here it is important to note that the choice of gain in
each stage will affect the amplifier bandwidth. To determine the bandwidth
of a given gain stage use the following formula:
where BW is a given amplifier stage bandwidth, GBWP is the gain
bandwidth product (typically 800 kHz for the SCXI-1120 and 8 MHz for
the SCXI-1120D), and G is the gain at this stage. This BW might be of
concern at high first-stage gains such as 50 and 100. In this case, the
first-stage amplifier has a BW equal to 16 kHz and 8 kHz, respectively, for
the SCXI-1120, and 60 kHz and 80 kHz for the SCXI-1120D. Due to this
decrease in the amplifier bandwidth, you will notice a decrease in the
channel overall bandwidth, but a better noise immunity. If this bandwidth
limitation is unacceptable, you should spread the gains over both stages,
thus increasing the BW of each amplifier stage. This will introduce, in most
cases, a negligible effect on the channel bandwidth. For example, to achieve
a gain of 100 for the SCXI-1120, use G
1st
= 10 and G
2nd
= 10; for a gain of
1,000, use G
1st
= 50 and G
2nd
= 20. However, for the SCXI-1120D, minimal
effects are introduced at G
≤
100. Refer to Appendix A,
, for
Gain versus BW for the SCXI-1120D.
All the amplifier input channels are overvoltage protected to 250 V
rms
with
power on or off.
G
Total
G
1ST
G
2ND
×
=
G
TOTAL
G
1ST
2
-----------
G
2ND
×
=
BW
GPWP
G
----------------
=
