MF6 datasheet(15/20 Pages) NSC

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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

min is the minimum stop-

band attenuation (in dB) desired at frequency f

s, and Amax is

the passband ripple or attenuation (in dB) at frequency f

b.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

2n] dB

(2)

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:

min = 30 dB, Amax = 1.0 dB, fs = 2 kHz, and fb = 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

s 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

c, 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

CLK = 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

c) 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

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