MF8 datasheet(12/24 Pages) NSC

12 / 24 page

10 Application Information (Continued)

Higher-order filters are designed in a similar manner An

eighth-order Chebyshev with 01 dB ripple center frequency

equal to 1 kHz and 100 Hz bandwidth for example could

be built as in

Figure 11 with the following component values

R0 e 7986k

RF e 100k

R2 e 5782k

R3 e 18808k

R4 e 20342k

Pins 1 3 17 and 18 high pin 2 low For 1001 clock-to-cen-

ter-frequency ratio pin 10 is tied to Va and the clock fre-

quency is 100 kHz For 501 clock-to-center-frequency ratio

pin 10 is tied to Vb and the clock frequency is 50 kHz

When building filters of order 4 or higher best performance

will always be realized when the filter blocks are cascaded

in numerical order Filter 1 (pins 16 and 15) should always

precede Filter 2 (pins 5 and 6) If a second MF8 is used

Filter 2 of the first MF8 should precede Filter 1 of the sec-

ond MF8 and so on

Dynamic Considerations

Some filter response characteristics will result in high gain

at certain internal nodes particularly at the op amp output

This can cause clipping in intermediate stages even when

no clipping is evident at the filter output The consequences

are significant distortion and degradation of the overall

transfer function The likelihood of clipping at the op amp

output becomes greater as RF R0 increases As the design

tables show RF R0 increases with increasing filter order

and increasing ripple It is good practice to keep out-of-band

input signal levels small enough that the first stage can’t

overload

MF8 Datasheet(PDF) 12 Page - National Semiconductor (TI)