[sdiy] AC coupling caps on MS20 clone

[Ingo Debus]

Am 07.08.2010 um 23:10 schrieb David G. Dixon:

> I believe that the impedance at the inverting input of an inverting  
> opamp is
> R1 || R2 (where R1 is the input resistor and R2 is the feedback  
> resistor).

I have often read that the PCB traces at the inverting input (aka  
summing node) shold be kept as short as possible. I think the idea  
is, if the summing node itself is viewed as an additional voltage  
input, it's a very high-gain one (input resistor zero ->  
theoretically infinite gain).
On the other hand, as long as the opamp does its job properly, the  
input impedance of this "input" is zero (hence it's called virtual  
ground). In other words, it's a current input, not a voltage input.

Ingo