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Subject: Re: Historical question: negative feedback and the op amp
X-Newsreader: Microsoft Outlook Express 5.50.4920.2300
Date: Wed, 13 Nov 2002 17:04:52 GMT
NNTP-Posting-Date: Wed, 13 Nov 2002 12:04:52 EST
Organization: Cox Communications
"Chuck Simmons" wrote in message
> Mike wrote:
> > "Jeroen" wrote in message
> > news:GyuA9.80964$I6.email@example.com...
> > >
> > >
> > > In case of IIR filters, feedback reduces the number of calculations
> > needed.
> > > In this case feedback isn't a thing that slows things down.
> > Actually, it does slow down the maximum operating frequency. I can
> > an FIR filter almost without limit, and obtain very high clock
> > In an IIR filter, the maximum operating frequency is often determined by
> > 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.
Yes, but if your arithmetic isn't infinite speed, then there's a practical
limitation due to the feedback loop in the IIR filter. As a result, the FIR
clock rate can be significantly faster than for an IIR filter with
equivalent word widths.
> 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.
I had never thought of a capacitor as a self contained feedback device until
I started working on digital. A simple capacitor has (virtually) infinite
precision and infinite bandwidth compared to the digital implementation. At
this point, I suppose someone will argue that since electrons are quantized,
that you could actually build a digital integrator with better precision...
-- Mike --
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