National Instruments SCXI-1121 User Manual
Page 72
Chapter 3
Theory of Operation
3-10
www.natinst.com
The analog input consists of four 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. 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. Also, the module
has an internal completion network that can be used with half-bridge or
quarter-bridge networks. Each channel is configurable to a different
bandwidth, gain, or completion network operation.
Use the following formula to determine the overall gain of a given amplifier
input channel:
where G
total
is the overall gain and G
1st
and G
2nd
are the first and
second-stage gains. 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), 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. Because of this decrease in the amplifier bandwidth, the
channel overall bandwidth decreases, but noise immunity improves. If this
bandwidth limitation is unacceptable, you should spread the gains over
both stages, thus increasing the BW of each amplifier stage. In most cases
this will introduce a negligible effect on the channel bandwidth. For
example, to achieve a gain of 100, use G
1st
= 10 and G
2nd
= 10 for a gain of
1,000 use G
1st
= 50 and G
2nd
= 20.
All the amplifier input channels are overvoltage-protected to 240 V
rms
with
power on or off.
The isolated amplifiers fulfill two purposes on the SCXI-1121 module.
They convert a small signal riding on a high common-mode voltage into a
single-ended signal with respect to the SCXI-1121 chassis ground. With
this conversion, the input analog signal can be extracted from a high
common-mode voltage or noise before being sampled and converted by the
data acquisition board. The isolated amplifier also amplifies and conditions
an input signal, which results in an increase in measurement resolution and
accuracy.
G
total
G
1st
G
2nd
×
=
BW
GBWP
G
-----------------
=