[sdiy] Non-inverting summing amplifier

[Karl Ekdahl]

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
> > 
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at dropmix.xs4all.nl
> > http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy


      ___________________________________________________
Sök efter kärleken!
Hitta din tvillingsjäl på Yahoo! Dejting: http://ad.doubleclick.net/clk;185753627;24584539;x?http://se.meetic.yahoo.net/index.php?mtcmk=148783