[sdiy] keeping signals seperate..avoiding noise

harrybissell at wowway.com harrybissell at wowway.com
Mon Nov 17 19:57:34 CET 2008


You are between bases...

The inverting input impedance is set by the input resistor in series...

the non-inverting input impedance is set by the input resistors, usually
a divider (one in series, one to ground).  If the opamp input bias current
can flow in the source... the resistor to ground is not necessary then there
is no voltage drop in the input resistor (if used) and the impedance is
effectively as high as the source impedance.

In effect... if you need a higher input impedance, the non-inverting is
usually more practical.

You could put (let's say) a 100 ohm resistor from the non-inverting input
to ground and the input impedance would be 100 ohms... that's pretty low
imho...

The benefit of lower input impedance is that its harder to couple capacitively
to the input (the stray capacitance is the same but you need more voltage to
drive to the same noise level).  If the noise source is from poor power supply
or grounding practice... the lower impedance will probably not help at all.

Note that a too-low input impedance will load a high impedance source and
probably kill off your frequency response.

H^) harry




On Sat, 15 Nov 2008 18:08:38 +0100, Derek Holzer wrote
> EE folks are welcome to correct me if I'm off base here, but the 
> non-inverting op-amp configuration is usually high impedance, and 
> the inverting setup is low impedance. So two inverting op-amps would 
> give a much lower input impedance than a single non-inverting one. 
> Basically you want to make it easier for the "nice" signal to travel 
> through the circuit via a lower impedance than that presented to the 
> "nasty" signal 
> (love these technical terms!!!).
> 
> best,
> D.
> 
> Dan Snazelle wrote:
> > I dont really understand what you mean about lowering the input impedance.
could you explain a bit how impedance might help?
> > 
> > take a look at the first opamp in this schematic:
> >  
> >
http://www.musicfromouterspace.com/analogsynth/GUITARSYNTHAUG2007/subcommanderschempg1.gif
> > 
> > i used ray's (first only) opamp as a model of how to create a non
inverting input
> 
> -- 
> derek holzer ::: http://www.umatic.nl ::: http://blog.myspace.com/macumbista
> ---Oblique Strategy # 92:
> "Intentions
> -credibility of
> -nobility of
> -humility of"
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Harry Bissell & Nora Abdullah 4eva




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