Chapter 4
Theory of Operation
©
National Instruments Corporation
4-15
SCXI-1141/1142/1143 User Manual
The stopband for the SCXI-1143 module begins at 3.2 times the cutoff
frequency, so you should sample it at a rate of 6.4 times the cutoff frequency
to acquire meaningful data.
For example, if a DAQ device is scanning all eight channels of the
SCXI-1141 at a rate of 120,000 channels/s, the sample rate for each of the
eight channels is:
and the cutoff frequency for the filters should be set no higher than:
Using this stopband, the filter attenuates the input signal by 80 dB or more.
This is enough attenuation to prevent aliasing on DAQ systems with 12 bits
of precision or less. On systems with more than 12 bits of precision or
systems with extremely high amounts of out-of-passband noise, higher
sampling rates or lower cutoff frequencies are necessary to prevent aliasing.
You can set the filter cutoff frequency closer to the sampling rate with the
consequence of having some aliasing. If you can tolerate aliasing in the
transition band, you can reduce the sampling rate to 2.6 times the cutoff
frequency for the SCXI-1141 module, five times the cutoff frequency for
the SCXI-1142 module, and 3.5 times the cutoff frequency for the
SCXI-1143 module.
Using the External Clock Input
You can set the cutoff frequencies of filters in the SCXI-1141/1142/1143
module by using the external clock input in applications that require
external control of the cutoff frequency or that require finer resolution than
the module provides internally. The cutoff frequency for each filter using
the external clock as a base is:
f
ext
/(100
×
n
)
where
f
ext
is the frequency of the external clock and
n
is an integer you
select such that 2
≤
n
≤
2
16
.
When the frequency of the external clock changes, the cutoff frequency
changes proportionally.
120,000
8
-------------------
15,000 S/s
=
15,000
3
----------------
5,000 Hz
=
