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参数资料
| 型号： | MF5CWM |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 运动控制电子 |
| 英文描述： | MF5 Universal Monolithic Switched Capacitor Filter |
| 中文描述： | SWITCHED CAPACITOR FILTER, RESISTOR PROGRAMMABLE, UNIVERSAL, PDSO14 |
| 封装： | SO-14 |
| 文件页数： | 11/16页 |
| 文件大小： | 293K |
| 代理商： | MF5CWM |
2.0 Modes of Operation
(Continued)
TABLE I. Summary of Modes. Realizable filter types (e.g. low-pass) denoted by asterisks. Unless otherwise noted,
gains of various filter outputs are inverting and adjustable by resistor ratios.
Mode
BP
LP
HP
N
AP
Number of
resistors
Adjustable
f
CLK
/f
o
Notes
1
(2)
*
*
3
No
May need input buf-
fer. Poor dynamics
for high Q.
1a
H
OBP1
eb
Q
H
OBP2
ea
1
H
OLP
ea
1
2
No
Yes (above
f
CLK
/50 or
f
CLK
/100)
2
*
*
*
3
Universal State-
Variable Filter. Best
general-purpose mode.
3
*
*
*
4
Yes
As above, but also
includes resistor-
tuneable notch.
3a
*
*
*
*
7
Yes
Gives Allpass res-
ponse with H
OAP
eb
1
and H
OLP
eb
2.
4
*
*
*
3
No
Gives flatter allpass
response than above
if R
1
e
R
2
e
0.02R
4
.
5
*
*
(2)
*
4
6a
*
3
Single pole.
6b
H
OLP
ea
1
H
OLP2
eb
R3
2
Single pole
R2
3.0 Applications Information
The MF5 is a general-purpose second-order state variable
filter whose center frequency is proportional to the frequen-
cy of the square wave applied to the clock input (f
CLK
). By
connecting pin 9 to the appropriate DC voltage, the filter
center frequency f
o
can be made equal to either f
CLK
/100
or f
CLK
/50. f
o
can be very accurately set (within
g
0.6%) by
using a crystal clock oscillator, or can be easily varied over
a wide frequency range by adjusting the clock frequency. If
desired, the f
CLK
/f
o
ratio can be altered by external resis-
tors as inFigures 9, 10, 11, 13, 14, and15. The filter Q and
gain are determined by external resistors.
All of the five second-order filter types can be built using the
MF5. These are illustrated inFigures 1 through5 along with
their transfer functions and some related equations. Figure
6 shows the effect of Q on the shapes of these curves.
When filter orders greater than two are desired, two or more
MF5s can be cascaded. The MF5 also includes an uncom-
mitted CMOS operational amplifier for additional signal pro-
cessing applications.
3.1 DESIGN EXAMPLE
An example will help illustrate the MF5 design procedure.
For the example, we will design a 2nd order Butterworth
low-pass filter with a cutoff frequency of 200 Hz, and a pass-
band gain of
b
2. The circuit will operate from a
g
5V power
supply, and the clock amplitude will be
g
5v (CMOS) levels).
From
f
o
e
200 Hz, H
OLP
eb
2, and, for Butterworth response,
Q
e
0.707.
In section 2.0 are several modes of operation for the MF5,
each having different characteristics. Some allow adjust-
ment of f
CLK
/f
o
, others produce different combinations of
filter types, some are inverting while others are non-invert-
ing, etc. These characteristics are summarized in Table I. To
keep the example simple, we will use mode 1, which has
notch, bandpass, and lowpass outputs, and inverts the sig-
nal polarity. Three external resistors determine the filter’s Q
and gain. From the equations accompanying Figure 7,
Q
e
R
3
/R
2
and the passband gain H
OLP
e b
R
2
/R
1
. Since
the input signal is driving a summing junction through R
1
,
the input impedance will be equal to R
1
. Start by choosing a
value for R
1
. 10k is convenient and gives a reasonable input
impedance. For H
OLP
e b
2, we have:
R
2
e b
R
1
H
OLP
e
10k
c
2
e
20k.
For Q
e
0.707 we have:
R
3
e
R
2
Q
e
20k
c
0.707
e
14.14k. Use 15k.
For operation on
g
5V supplies, V
a
is connected to
a
5V,
V
b
to
b
5V, and AGND to ground. The power supplies
should be ‘‘clean’’ (regulated supplies are preferred) and
0.1
m
F bypass capacitors are recommended.
the
specifications,
the
filter
parameters
are:
11
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