MF6CWM-50 National Semiconductor, MF6CWM-50 Datasheet - Page 15

• Manufacturer Part Number: MF6CWM-50
• Description: IC FILTER 6TH ORER SW CAP SO14
• Manufacturer: National Semiconductor

Specifications of MF6CWM-50

• Filter Type: Butterworth, Lowpass Switched Capacitor
• Frequency - Cutoff Or Center: 20kHz
• Number Of Filters: 1
• Max-order: 6th
• Voltage - Supply: 5 V ~ 14 V
• Mounting Type: *
• Package / Case: *
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *MF6CWM-50

1.0 MF6 Application Hints
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
band attenuation (in dB) desired at frequency f
the passband ripple or attenuation (in dB) at frequency f
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:
where n = 6 (the order of the filter).
2.1 A LOWPASS DESIGN EXAMPLE
Suppose the amplitude response specification in Figure 11 is
given. Can the MF6 be used? The order of the Butterworth
approximation will have to be determined using eq. 1:
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
with the above values and n = 6 giving:
This result also meets the design specification given in
Figure 11 again verifying that a single MF6 section will be
adequate.
A
Attn(f) = 10 log [1 + (10
Atten (2 kHz) = 10 log [ 1 + (10
min
= 30 dB, A
FIGURE 10. MF6-50
= 30.26 dB
max
Amplitude Response
= 1.0 dB, f
0.1A max
s
can be found using equation 2
s
±
−1) (f/f
2.5V Supplies
= 2 kHz, and f
0.1
min
− 1) (2 kHz/1 kHz)
DS005065-23
is the minimum stop-
b
)
2n
] dB
(Continued)
s
, and A
b
= 1 kHz
(2)
max
b
12
. If
is
]
15
Since the MF6’s cutoff frequency f
gain attenuation of −3.01 dB, was not specified in this ex-
ample it needs to be calculated. Solving equation 2 where
f = f
To implement this example for the MF6-50 the clock fre-
quency will have to be set to f
kHz or for the MF6-100 f
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
stopband attenuation is met and what actual cutoff frequency
(f
desired. The design procedure would be identical to the one
shown in section 2.1.
Specification Where the Response of the Filter Design
Must Fall Within the Shaded Area of the Specification
c
) is required to obtain the particular frequency response
FIGURE 11. Design Example Magnitude Response
c
as follows:
6) while equation 4 can determine if the required
CLK
= 100(1.116 kHz) = 111.6 kHz.
CLK
c
, which corresponds to a
= 50(1.116 kHz) = 55.8
www.national.com
DS005065-24