Datasheet

3.0 Applications Information (Continued)
3.5 SAMPLED DATA SYSTEM CONSIDERATIONS
The LMF100 is a sampled data filter, and as such, differs in
many ways from conventional continuous-time filters. An im-
portant characteristic of sampled-data systems is their effect
on signals at frequencies greater than one-half the sampling
frequency. (The LMF100’s sampling frequency is the same
as its clock frequency.) If a signal with a frequency greater
than one-half the sampling frequency is applied to the input
of a sampled data system, it will be “reflected” to a frequency
less than one-half the sampling frequency. Thus, an input
signal whose frequency is f
s
/2 + 100 Hz will cause the sys-
tem to respond as though the input frequency was
f
s
/2 − 100 Hz. This phenomenon is known as “aliasing”, and
can be reduced or eliminated by limiting the input signal
spectrum to less than f
s
/2. This may in some cases require
the use of a bandwidth-limiting filter ahead of the LMF100 to
limit the input spectrum. However, since the clock frequency
is much higher than the center frequency, this will often not
be necessary.
Another characteristic of sampled-data circuits is that the
output signal changes amplitude once every sampling pe-
riod, resulting in “steps” in the output voltage which occur at
the clock rate (
Figure 25 ). If necessary, these can be
“smoothed” with a simple R-C low-pass filter at the LMF100
output.
The ratio of f
CLK
to f
c
(normally either 50:1 or 100:1) will also
affect performance. A ratio of 100:1 will reduce any aliasing
problems and is usually recommended for wide-band input
DS005645-39
R1 = 100 kΩ
R2=1kΩ
R3 = 100 kΩ
R4 = 100 kΩ
Rg=10kΩ
Rl=10kΩ
Rh=10kΩ
FIGURE 23. Second-Order Notch Filter
DS005645-13
FIGURE 24. Method for Trimming V
OS
LMF100
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