[sdiy] Non-inverting summing amplifier

[David G. Dixon]

Ingo,

I was referring to the output impedance looking back from the opamp input
into the voltage source (this may be inappropriate -- I was just trying to
say that the voltage sources and their resistors look more or less like
perfect current sources to the opamp's inverting input).

Of course, the impedance looking from the voltage source to an opamp's
inverting input is simply the input resistance.  This would imply that the
inverting input's impedance is zero (since it adds no additional impedance
in series with the input resistor, but passes all the current into the
feedback loop).  This means that impedances in parallel at the inverting
input are zero, which means that the various inputs to an inverting summer
cannot interfere with each other, and the summation is exact.

On the other hand, the input impedance to an opamp's noninverting input is
infinity, which means that multiple inputs at the noninverting input result
in an effective input impedance which is the parallel impedance of all the
other inputs, meaning that multiple inputs at the noninverting input
interact with one another, which means that a noninverting summer attenuates
as it adds, and this is why noninverting summers are avoided.

Have I got this approximately right?

David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C. V6T 1Z4
Canada
 
Tel 1-604-822-3679
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> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of Ingo Debus
> Sent: Wednesday, January 28, 2009 10:40 AM
> To: Synth DIY
> Subject: Re: [sdiy] Non-inverting summing amplifier
> 
> 
> Am 28.01.2009 um 08:23 schrieb David G. Dixon:
> 
> > In an inverting summer, on the other hand, the inputs act as
> > current sources
> > into the negative input terminal, which in turn acts as a summed
> > current
> > source through the feedback resistor.  Hence, the opamp makes the
> > voltage
> > inputs look like current sources with infinite impedance.
> 
> You mean, the input impedance of an inverting amplifier is infinite?
> Sure aout this?
> As I understand it, the input impedance is just the value of the
> input resistor, since its other end is tied to virtual ground.
> 
> Ingo
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