has anyone used the lmf40,lmf60,etc

[Tony Clark]

> Hi all, I was paging through the national semi data accuisition book and 
> found the lmf40, lmf60 4th & 6th order butterworth low pass filter.  
> packaged nicely with two spare op amps, one nicely biased to ground(lmf60).  
> controllable with either cmos clock or voltage adjust(res cap combo) for 
> the cutoff frequency.  
> 
> Are these too good to be true? 

   These must be similar to the MF10 from....Harris?  Or is it Maxim?  In 
any case, these are switched capacitor filters which is bad right from 
the beginning.   It's about the equivilant of taking an analog signal 
passing it through a A/D, performing a software butterworth, and then 
spitting it back out through a D/A.  In other words, you are going to get 
a stairstepped output mostlikely and (as with all sampling technology) 
you run into Nyquist's theorum rearing its ugly head.
   The second setback is that there is no resonance control.  Of course 
that's because they wanted the circuit to emulate a butterworth 
response.
   Now depending on what you want to use this for, it might be okay to 
use.  I'll leave that up to you.  :)

   Tony

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I can't drive (my Moog) 55!
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Tony Clark -- clark at andrews.edu 
http://aupe.phys.andrews.edu/~clark
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