filters

[brad sanders]

>> Hi all, I was paging through the national semi data accuisition book and 
>> found the lmf40, lmf60 4th & 6th order butterworth low pass filter....

>   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.

Ummm... the MF10, MF40, MF60, etc are NOT switched cap filters. They
are rather like the MAX7-something in that they are programmed via
resistors and may (depending on model) have "built in" caps. 

Analog Devices also has appnotes on using these with D/A convertors to
create "digitally programmed" filters. Seems one could just as easily
substitute OTAs to create "voltage programmable" filters.

Problem is, the damn things are expensive. If you're ALREADY using
OTAs (for the VC part) you might as well just go ahead and use a
single opamp and a coupla discrete caps to make a VCF; it'd sure be
cheaper.

> (a switched cap filter) is 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.

This is completely false. Switching ("chopping") in the time domain
does NOT guarantee "stairstepped" outputs (some of the most expensive,
highest performance opamps are "choppers" - look in your AD databook).
Hell, you shouldn't even be getting "stairsteps" from a properly
designed DAC interface - but that's another discussion.

Switched filter caps are as "analog" as a BBD delay line. They
discretize TIME, but they do NOT quantize level. A switched cap filter
merely switches one or more caps in and out of the circuit many
times/sec as a means of setting "effective impedance" of the filter. 

Many companies DO make these things, and many are very easy to use 8
pin DIPs. They use a switching freq typically many times the bandpass
(>100KHz for 10KHz response) and you DO need to "post filter" (as well
as "pre filter") these to properly control aliasing.

But again: aliasing and quantizing are NOT the same thing.

Many HIFI components (and quite possibly even *your* CD player) use
SWITCHED CAPACITOR filters after the (one bit) DAC. At least one of
the highest performance "one bit" DACs (the CS4328) features a built
in switched cap "post" filter; tack on a single pole RC (output)
shaping network and you have a DAC with >100db S/N ratio, and very low
distortion.

Switched cap filters can be VERY high performance. The "off the shelf"
devices are limited in a synth application because of the difficulty
varying Q, and (quite possibly) because they're just "too much filter"
(most run fourth to TENTH order response - WAY too much to be of use
in a synth). They might also just be too durn *clean* to be any fun.

The biggest problem is you have to band limit the INPUT waveform (to
prevent aliasing), which means those nice 5KHz triangles your VCO is
making will give these things hell unless you put a 4th or 5th order
band limiting filter on the INPUT to your (switched cap) VCF. Can
anyone here say "irony?"