[sdiy] Differential amps and noise cancellation in dsPIC 33FJ128GP802 uP's DAC

[Guy McCusker]

> You mean the input impedances of the complete circuit with respect to ground? These are equal.
> As long as the feedback works properly, both the inverting and the noninverting input of the opamp are at the same voltage. Since R5 and R7 are both 10k, a signal source connected to the left side of either R5 or R7 sees the same input impedance.

I don't think that is correct but happy to be put straight if I am
mistaken. My take is as follows.

Ignoring the capacitor to make life easier, a signal connected at R7
sees 20k to ground always.
With the input at R7 grounded, a signal connected at R6 sees 10k to
the virtual ground. I can't see any circumstances under which a signal
at R6 sees 20k to ground.

What am I missing?
Guy.