lmf90 National Semiconductor Corporation, lmf90 Datasheet - Page 13

• Manufacturer Part Number: lmf90
• Description: 4th-order Elliptic Notch Filter
• Manufacturer: National Semiconductor Corporation

2 0 Applications Information
ALIASING
Another important characteristic of sampled-data systems is
their effect on signals at frequencies greater than one-half
the sampling frequency (The LMF90’s sampling frequency
is the same as the filter’s clock frequency This is the fre-
quency at the CLK pin) 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 frequen-
cy less than one-half the sampling frequency Thus an input
signal whose frequency is f
tem to respond as though the input frequency was f
10 Hz This phenomenon is known as ‘‘aliasing’’ Aliasing
can be reduced or eliminated by limiting the input signal
spectrum to less than f
In some cases it may be necessary to use a bandwidth
limiting filter (often a simple passive RC low-pass) ahead of
the bandpass input Although the summing amplifier uses
switched-capacitor techniques it does not exhibit aliasing
behavior and the anti-aliasing filter need not be in its input
signal path The filter can be placed ahead of pin 12 as
shown in Figure 4 with the non-band limited input signal
applied to pin 11 The output spectrum will therefore be
wideband although limited by the bandwidth of the sum-
ming amplifier’s output buffer amplifier (typically 1 MHz)
even if f
ing filter will affect the accuracy of the notch transfer func-
FIGURE 4 Using a simple passive low-pass filter to prevent aliasing in the presence of high-frequency input signals
CLK
is less than 1 MHz Phase shift in the anti-alias-
s
2
S
2
a
10 Hz will cause the sys-
(Continued)
s
2
b
13
tion however so it is best to use the highest available
clock-to-center-frequency ratio (100 1) and set the RC filter
cutoff frequency to about 15 to 20 times the notch frequen-
cy This will provide reasonable attenuation of high-frequen-
cy input signals while avoiding degradation of the overall
notch response If the anti-aliasing filter’s cutoff frequency is
too low it will introduce phase shift and gain errors large
enough to shift the frequency of the notch and reduce its
depth A cutoff frequency that is too high may not provide
sufficient attenuation of unwanted high-frequency signals
FIGURE 3 Output waveform of a switched-capacitor
filter Note the voltage steps caused by sampling
at the clock frequency
TL H 10354 – 8
TL H 10354 – 7