lmf60 National Semiconductor Corporation, lmf60 Datasheet - Page 17

• Manufacturer Part Number: lmf60
• Description: High Performance 6th-order Switched Capacitor Butterworth Lowpass Filter
• Manufacturer: National Semiconductor Corporation

2 0 Designing with the LMF60
2 4 CHANGING CLOCK FREQUENCY
INSTANTANEOUSLY
The LMF60 will respond well to a sudden change in clock
frequency Distortion in the output signal occurs at the tran-
sition of the clock frequency and lasts approximately three
cutoff frequency (f
control signal is low the LMF60-50 has a 100 kHz clock
making f
frequency changes to 50 kHz yielding 1 kHz f
The transient response of the LMF60 seen in Figure 13 is
also dependent on the f
filter The LMF60 responds as a classical sixth order Butter-
worth lowpass filter
f
2 5 ALIASING CONSIDERATIONS
Aliasing effects have to be taken into consideration when
input signal frequencies exceed half the sampling rate For
the LMF60 this equals half the clock frequency (f
When the input signal contains a component at a frequency
higher than half the clock frequency as in Figure 14a that
IN
FIGURE 12 LMF60-50 Abrupt Clock Frequency Change
e
1 5 kHz (Scope Time Base
C
FIGURE 14 The phenomenon of aliasing in sampled-data systems An input signal whose frequency
e
2 kHz when this signal goes high the clock
(a) Input Signal Spectrum
C
at a frequency lower than one-half the sampling frequency In the LMF60 f
) cycles As shown in Figure 12 if the
is greater than one-half the sampling frequency will cause an output to appear
c
and thus the f
e
2 ms Div)
CLK
applied to the
C
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(Continued)
CLK
)
17
component will be ‘‘reflected’’ about f
quency range below f
nent is within the passband of the filter and of large enough
amplitude it can cause problems Therefore if frequency
components in the input signal exceed f
attenuated before being applied to the LMF60 input The
necessary amount of attenuation will vary depending on
system requirements In critical applications the signal com-
ponents above f
the filter’s residual noise level An example circuit is shown
in Figure 15 using one of the uncommitted Op-Amps avail-
able in the LMF60
(b) Output Signal Spectrum Note that the input signal at
f
s
2
a
FIGURE 13 LMF60-50 Step Input Response
f causes an output signal to appear at f
Vertical
1 ms Div f
CLK
2 will have to be attenuated at least to
e
CLK
2V Div Horizontal
2 as in Figure 14b If this compo-
CLK
s
e
e
f
100 kHz
CLK
CLK
CLK
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2 into the fre-
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TL H 9294 – 28
TL H 9294 – 30
s
2
b
f