[sdiy] Non-inverting summing amplifier

[George Hearn]

Bit of a hack agreed, but to make a non-inverting summer you can have
multiple large resistances sum into a small resistance (perhaps 30k-100k
into 200R) then amplify the voltage across the 200R resistor by the correct
amount with a non-inverting opamp to provide the required gain.  For many
applications where a low voltage input must be driven and where absolute
summing accuracy is not required, summing can be purely resistive if it is
desired non-inverting.  George

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of David G. Dixon
Sent: 28 January 2009 07:29
To: 'Scott Nordlund'; synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] Non-inverting summing amplifier

Scott,

You can have a non-inverting summer, provided the voltages have very low
source impedances.  The feedback resistor must be equal to the input
resistors (assuming they are equal) times the number of inputs minus one to
get a sum output, as opposed to an equal feedback resistor in an inverting
summer.  I'm just not sure why you would ever want to take the trouble.
It's far easier to use an inverting summer followed by a unity-gain
inverter.

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 Scott Nordlund
> Sent: Tuesday, January 27, 2009 10:04 PM
> To: synth-diy at dropmix.xs4all.nl
> Subject: RE: [sdiy] Non-inverting summing amplifier
> 
> 
> You don't see a non-inverting summing amplifier because it doesn't really
> exist.  An inverting mixer works because it adds currents- the inverting
> terminal is held at a virtual ground (ideally) so the currents through the
> input resistors are summed together.  A non-inverting amplifier doesn't
> have a summing node- instead the non-inverting input is a high-impedance
> voltage input, and you can't really add voltages by just combining them
> (you can just use a simple resistive mixer, but this isn't necessarily a
> good solution).
> 
> > Date: Wed, 28 Jan 2009 04:41:54 +0000
> > From: elektrodwarf at yahoo.se
> > To: synth-diy at dropmix.xs4all.nl
> > Subject: [sdiy] Non-inverting summing amplifier
> >
> > Hi list
> >
> > Is there a reason that i never see non-inverting summing amplifier, i.e.
> simply a non-inverting amp with several resistor inputs?
> >
> > Karl
> >
> >
> >       __________________________________________________________
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> >
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