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| 型號(hào)： | MF6 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | 6th Order Switched Capacitor Butterworth Lowpass |
| 中文描述： | 6階巴特沃斯低通開關(guān)電容 |
| 文件頁數(shù)： | 15/20頁 |
| 文件大小： | 878K |
| 代理商： | MF6 |
1.0 MF6 Application Hints
(Continued)
2.0 Designing with the MF6
Given any lowpass filter specification two equations will
come in handy in trying to determine whether the MF6 will do
the job. The first equation determines the order of the low-
pass filter required:
where n is the order of the filter, A
is the minimum stop-
band attenuation (in dB) desired at frequency f
, and A
max
is
the passband ripple or attenuation (in dB) at frequency f
. If
the result of this equation is greater than 6, then more than a
single MF6 is required.
The attenuation at any frequency can be found by the follow-
ing equation:
Attn(f) = 10 log [1 + (10
0.1A max
1) (f/f
b
)
2n
] dB
where n = 6 (the order of the filter).
(2)
2.1 A LOWPASS DESIGN EXAMPLE
Suppose the amplitude response specification in Figure 11is
given. Can the MF6 be used The order of the Butterworth
approximation will have to be determined using eq. 1:
A
min
= 30 dB, A
max
= 1.0 dB, f
s
= 2 kHz, and f
b
= 1 kHz
Since n can only take on integer values, n = 6. Therefore the
MF6 can be used. In general, if n is 6 or less a single MF6
stage can be utilized.
Likewise, the attenuation at f
can be found using equation 2
with the above values and n = 6 giving:
Atten (2 kHz) = 10 log [ 1 + (10
0.1
1) (2 kHz/1 kHz)
12
]
= 30.26 dB
This result also meets the design specification given in
Figure 11 again verifying that a single MF6 section will be
adequate.
Since the MF6’s cutoff frequency f
, which corresponds to a
gain attenuation of 3.01 dB, was not specified in this ex-
ample it needs to be calculated. Solving equation 2 where
f = f
c
as follows:
To implement this example for the MF6-50 the clock fre-
quency will have to be set to f
= 50(1.116 kHz) = 55.8
kHz or for the MF6-100 f
CLK
= 100(1.116 kHz) = 111.6 kHz.
2.2 CASCADING MF6s
In the case where a steeper stopband attenuation rate is re-
quired two MF6’s can be cascaded (Figure 12) yielding a
12th order slope of 72 dB per octave. Because the MF6 is a
Butterworth filter and therefore has no ripple in its passband,
when MF6s are cascaded the resulting filter also has no
ripple in its passband. Likewise the DC and passband gains
will remain at 1V/V. The resulting response is shown in
Figures 13, 14
In determining whether the cascaded MF6s will yield a filter
that will meet a particular amplitude response specification,
as above, equations 3 and 4 can be used, shown below.
where n = 6 (the order of each filter).
Equation 3 will determine whether the order of the filter is ad-
equate (n
≤
6) while equation 4 can determine if the required
stopband attenuation is met and what actual cutoff frequency
(f
) is required to obtain the particular frequency response
desired. The design procedure would be identical to the one
shown in section 2.1.
DS005065-23
FIGURE 10. MF6-50
±
2.5V Supplies
Amplitude Response
DS005065-24
FIGURE 11. Design Example Magnitude Response
Specification Where the Response of the Filter Design
Must Fall Within the Shaded Area of the Specification
www.national.com
15
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