[sdiy] Non-inverting summing amplifier

[Ingo Debus]

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