[sdiy] Non-inverting summing amplifier

[Ian Fritz]

At 06:17 PM 1/28/2009, David G. Dixon wrote:

>Of course, the impedance looking from the voltage source to an opamp's
>inverting input is simply the input resistance.

Yes, but to reemphasize,  only because the (-) input is held at virtual 
ground by the (+) input being grounded and negative feedback being present.


>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).

No, the (-) input's resistance is *infinite*, because no current flows into 
it.  The current all flows around it into the feedback resistor.  This is 
one of the ideal opamp Golden Rules.  (When you talk about a device's input 
impedance, you mean the impedance looking into it from the outside.  This 
has nothing to do with what may be flowing *past* that particular node.)

Ian