From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Historical question: negative feedback and the op amp
References: <3DD25F72.5793B8D5@webaccess.net> <6PuA9.5701$XF5.firstname.lastname@example.org>
Date: Wed, 13 Nov 2002 16:34:28 GMT
NNTP-Posting-Date: Wed, 13 Nov 2002 08:34:28 PST
> "Jeroen" wrote in message
> > "Mike" wrote in message
> > news:NluA9.5692$XF5.email@example.com...
> > >
> > > "Chuck Simmons" wrote in message
> > > news:3DD25F72.5793B8D5@webaccess.net...
> > > > Ban wrote:
> > >
> > > > > digital circuits work with saturated transistors, no feedback there.
> > > FIR
> > > > > filter does not use feedback.
> > > >
> > > > I beg your pardon, in an FIR filter, what is the output term(s) doing
> > > > the equation if not feedback?
> > >
> > > Depends on the filter. In the filter Ban's thinking of, the output is a
> > > function of the inputs, but not the ouput - it's strictly feedforward.
> > > the more general digital filter structure, of course, there are
> > feedforward
> > > and feedback terms.
> > >
> > In case of IIR filters, feedback reduces the number of calculations
> > In this case feedback isn't a thing that slows things down.
> Actually, it does slow down the maximum operating frequency. I can pipeline
> an FIR filter almost without limit, and obtain very high clock frequencies.
> In an IIR filter, the maximum operating frequency is often determined by the
> feedback loop. In the general case, the feedback term has to be fully
> calculated in one clock cycle.
An IIR can produce output with one multiply and one add at the arrival
of the sample for that iteration. Does anyone not do this? It is basic.
This means that there is no difference in output speed of an FIR versus
an IIR with fixed sampling and infinite speed arithmetic. The big
trouble is that when a low order IIR is optimal, the required FIR must
be infinite order. This turns into a major can of worms because a simple
integrator has no FIR implementation. At all. Forget it.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org