MF4CN-50 National Semiconductor, MF4CN-50 Datasheet - Page 8

• Manufacturer Part Number: MF4CN-50
• Description: IC LOWPASS FILTER 4TH ORD SW CAP
• Manufacturer: National Semiconductor

Specifications of MF4CN-50

• Filter Type: Butterworth, Lowpass Switched Capacitor
• Frequency - Cutoff Or Center: 20kHz
• Number Of Filters: 1
• Max-order: 4th
• Voltage - Supply: 5 V ~ 14 V
• Mounting Type: *
• Package / Case: *
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *MF4CN-50; MF4NC-50

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1.0 MF4 Application Hints
If the MF were set up for a cutoff frequency of 10 kHz the in-
put impedance would be:
In this example with a source impedance of 10K the overall
gain, if the MF4 had an ideal gain of 1 or 0 dB, would be:
Since the maximum overall gain error for the MF4 is
±
would be +0.06 dB to −0.24 dB.
1.4 CUTOFF FREQUENCY RANGE
The filter’s cutoff frequency (f
age currents through the internal switches draining the
charge stored on the capacitors. At lower clock frequencies
these leakage currents can cause millivolts of error, for ex-
ample:
The propagation delay in the logic and the settling time re-
quired to acquire a new voltage level on the capacitors limit
the filter’s accuracy at high clock frequencies. The amplitude
characteristic on
exceeds 750 kHz and then peak at about 0.5 dB at the cor-
ner frequency with a 1 MHz clock. As supply voltage drops to
±
noticeable when the clock frequency exceeds 250 kHz. The
response of the MF4 is still a good approximation of the ideal
Butterworth low-pass characteristic shown in Figures 6, 7 .
2.0 Designing With The MF4
Given any low-pass filter specification, two equations will
come in handy in trying to determine whether the MF4 will do
the job. The first equation determines the order of the
low-pass filter required to meet a given response specifica-
tion:
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 cutoff frequency
f
single MF4 is required.
The attenuation at any frequency can be found by the follow-
ing equation:
where n = 4 for the MF4.
b
0.15 dB with R
. If the result of this equation is greater than 4, more than a
2.5V, a shift in the f
Attn (f) = 10 log [1 + (10
s
±
5V supplies will typically stay flat until f
2 k
CLK
/f
the actual gain error for this case
c
ratio occurs which will become
c
) has a lower limit due to leak-
0.1A max
min
− 1) (f/f
is the minimum stop-
(Continued)
b
s
)
, and A
2n
] dB
max
CLK
(3)
(4)
is
8
2.1 A LOW-PASS DESIGN EXAMPLE
Suppose the amplitude response specification in Figure 8 is
given. Can the MF4 be used? The order of the Butterworth
approximation will have to be determined using Equation (1) :
Since n can only take on integer values, n = 4. Therefore the
MF4 can be used. In general, if n is 4 or less a single MF4
stage can be utilized.
Likewise, the attenuation at f
(4) with the above values and n = 4:
Attn (2 kHz) = 10 log [1 + 10
18.28 dB
This result also meets the design specification given in Fig-
ure 8 again verifying that a single MF4 section will be ad-
equate.
Since the MF4’s cutoff frequency (f
a gain attenuation of −3.01 dB, was not specified in this ex-
ample, it needs to be calculated. Solving Equation (4) where
f = f
where f
MF4-50 the clock frequency will have to be set to f
50(1.184 kHz) = 59.2 kHz, or for the MF4-100, f
(1.184 kHz) = 118.4 kHz.
2.2 CASCADING MF4s
When a steeper stopband attenuation rate is required, two
MF4s can be cascaded ( Figure 9 ) yielding an 8th order slope
of 48 dB per octave. Because the MF4 is a Butterworth filter
and therefore has no ripple in its passband when MF4s are
cascaded, the resulting filter also has no ripple in its pass-
band. Likewise the DC and passband gains will remain at
1V/V. The resulting response is shown in Figure 10 , Figure
11 .
In determining whether the cascaded MF4s will yield a filter
that will meet a particular amplitude response specification,
as above, Equations (5), (6) can be used, shown below.
where n = 4 (the order of each filter).
Equation (5) will determine whether the order of the filter is
adequate (n
stopband attenuation and cutoff frequency (f
obtain the desired frequency response. The design proce-
dure would be identical to the one shown in section 2.0.
c
as follows:
c
= f
CLK
4) while Equation (6) can determine the actual
/50. To implement this example for the
s
0.1
can be found using Equation
− 1) (2 kHz/1 kHz)
c
), which corresponds to
c
) necessary to
CLK
8
] =
CLK
= 100
(5)
(6)
=