©
National Instruments Corp.
15
NI 9235/9236
Understanding NI 9235/9236 Filtering
The NI 9235/9236
u
ses a combination of analog and digital
filtering to provide an acc
u
rate representation of in-band signals
while rejecting o
u
t-of-band signals. The filters discriminate
between signals based on the freq
u
ency range, or bandwidth,
of the signal. The three important bandwidths to consider are the
passband, the stopband, and the alias-free bandwidth.
The NI 9235/9236 represents signals within the passband, as
q
u
antified primarily by passband flatness and phase nonlinearity.
All signals that appear in the alias-free bandwidth are either
u
naliased signals or signals that have been filtered by at least the
amo
u
nt of the stopband rejection.
Passband
The signals within the passband have freq
u
ency-dependent gain or
atten
u
ation. The small amo
u
nt of variation in gain with respect to
freq
u
ency is called the passband flatness. The digital filters of the
NI 9235/9236 adj
u
st the freq
u
ency range of the passband to match
the data rate. Therefore, the amo
u
nt of gain or atten
u
ation at a
given freq
u
ency depends on the data rate. Fig
u
re 7 shows typical
passband flatness for the 10 kS/s data rate.
