[sdiy] Non-inverting summing amplifier

[David G. Dixon]

Karl,

If the voltages are coming from opamp outputs (with their typically
infinitesimal output impedances), then no worries!  Just bear in mind that
the feedback resistor value is different for a non-inverting summer.  If you
feed n different voltages into an inverting summer, all with input resistors
of value R, then you get a proper sum with an R-valued feedback resistor.
However, if you do the same to a non-inverting summer, then the feedback
resistor must be R * (n - 1).  In other words, for 3 voltages, you would
need a feedback resistor of 2*R.  Clear as mud?

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
Fax 1-604-822-3619
 
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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 Karl Ekdahl
> Sent: Wednesday, January 28, 2009 8:01 PM
> To: 'Synth DIY'
> Subject: RE: [sdiy] Non-inverting summing amplifier
> 
> So,
> 
> Am i right in saying that as long as the driving sources are stiff (opamp)
> and the input resistors to the summing network are large (>100K)
> everything should be OK?
> 
> In what kind of applications would a non-inverting summing amplifier be
> OK? What about summing a couple of 20K voltage-divider pots through for
> instance 330K resistors, would that be ok for say LFO speed. Just
> considering this is a parameter that doesn't really need precision.
> 
> Just trying to avoid having to double my opamp usage for a poly synth i'm
> probably never gonna build :)
> 
> Karl
> 
> 
> --- Den tors 2009-01-29 skrev David G. Dixon <dixon at interchange.ubc.ca>:
> 
> > Från: David G. Dixon <dixon at interchange.ubc.ca>
> > Ämne: RE: [sdiy] Non-inverting summing amplifier
> > Till: "'Ingo Debus'" <igg.debus at t-online.de>, "'Tom Wiltshire'"
> <tom at electricdruid.net>
> > Kopia: "'Synth DIY'" <synth-diy at dropmix.xs4all.nl>
> > Datum: torsdag 29 januari 2009 02.20
> > Yeah, that's what I meant!  ;->
> >
> > Non-inverting summer: inputs affect each other: bad!
> >
> > Inverting summer: inputs independent: good!
> >
> > End of story!
> >
> > 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
> > Fax 1-604-822-3619
> >
> > "PERFECTA FINGAMUS SERVIAT NATURA"
> >
> > The information in this email and in any attachments is
> > confidential and
> > intended solely for the attention and use of the named
> > addressee(s).  It
> > must not be disclosed to any person without the
> > writer's authority.  If you
> > are not the intended recipient, or a person responsible for
> > delivering it to
> > the intended recipient, you are not authorized to and must
> > not disclose,
> > copy, distribute, or retain this message or any part of it.
> >
> > > -----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 12:26 PM
> > > To: Tom Wiltshire
> > > Cc: Synth DIY
> > > Subject: Re: [sdiy] Non-inverting summing amplifier
> > >
> > >
> > > Am 28.01.2009 um 20:38 schrieb Tom Wiltshire:
> > >
> > > >
> > > > On 28 Jan 2009, at 18:40, Ingo Debus wrote:
> > > >
> > > >>
> > > >> 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.
> > > >
> > > > I thought he meant that the inv amp arrangement
> > makes it look like
> > > > the *output* impedance of the thing you feed into
> > it is infinite.
> > > > So you get all the voltage that you sent out,
> > without any losses.
> > >
> > > Now I'm really confused.
> > > Why would the output impedance of something look
> > different from what
> > > it really was?
> > > And something you get losslessly "all the
> > voltage" from would be a
> > > voltage source, not a current source, no?. A voltage
> > source has zero
> > > output impedance.
> > >
> > > As I view it, the difference between a inverting
> > summer and a non-
> > > inverting summer (which is nothing else but a passive
> > mixer followed
> > > by a non-inverting amplifier) is that the impedance of
> > one input of
> > > the inverting summer is independent of what is
> > connected to the other
> > > input(s). But both have finite input impedances.
> > >
> > > Ingo
> > >
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