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參數(shù)資料
| 型號(hào)： | LMF60CIWM-50 |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 模擬濾波器 |
| 英文描述： | LMF60 High Performance 6th-Order Switched Capacitor Butterworth Lowpass Filter |
| 中文描述： | SWITCHED CAPACITOR FILTER, BUTTERWORTH, LOWPASS, PDSO14 |
| 封裝： | 0.300 INCH, PLASTIC, SOP-14 |
| 文件頁(yè)數(shù)： | 17/20頁(yè) |
| 文件大?。?/td> | 391K |
| 代理商： | LMF60CIWM-50 |
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2.0 Designing with the LMF60
(Continued)
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
C
) cycles. As shown in Figure 12, if the
control signal is low the LMF60-50 has a 100 kHz clock
making f
C
e
2 kHz; when this signal goes high the clock
frequency changes to 50 kHz yielding 1 kHz f
C
.
The transient response of the LMF60 seen in Figure 13 is
also dependent on the f
c
and thus the f
CLK
applied to the
filter. The LMF60 responds as a classical sixth order Butter-
worth lowpass filter.
TL/H/9294–27
f
IN
e
1.5 kHz (Scope Time Base
e
2 ms/Div)
FIGURE 12. LMF60-50 Abrupt Clock Frequency Change
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
CLK
).
When the input signal contains a component at a frequency
higher than half the clock frequency, as in Figure 14a, that
component will be ‘‘reflected’’ about f
CLK
/2 into the fre-
quency rangebelow f
CLK
/2 as inFigure 14b. If this compo-
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
CLK
/2 they must be
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
CLK
/2 will have to be attenuated at least to
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.
TL/H/9294–28
FIGURE 13. LMF60-50 Step Input Response,
Vertical
e
2V/Div., Horizontal
e
1 ms/Div., f
CLK
e
100 kHz
TL/H/9294–29
(a) Input Signal Spectrum
TL/H/9294–30
(b) Output Signal Spectrum. Note that the input signal at
f
s
/2
a
f causes an output signal to appear at f
s
/2
b
f.
FIGURE 14. The phenomenon of aliasing in sampled-data systems. An input signal whose frequency
is greater than one-half the sampling frequency will cause an output to appear
at a frequency lower than one-half the sampling frequency. In the LMF60, f
s
e
f
CLK
.
http://www.national.com
17
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