OK, I made a mistake with a minus sign which screwed up the result. Unfortunately, I let the result override my intuition. So ignore that previous post. CMRR is infinity with matching resistors R1R4=R2R3.I get a different result, steveb. I get:
\( A_c=\frac{R1R4-R2R3}{R1R4+R2R3+2R1R3} \)
For the case where the input is floating (Rs=∞), I get:
\( A_d=\frac{R2+R4}{R1+R3}\)
So I agree with the result for Ac with no source resistance, but when I include source resistance I get something a little different as follows.
\( A_c=\frac{R1R4-R2R3}{R1R4+R2R3+2R1R3+Rs(R1+2R3)} \)
Given the mistake I already made, I should double check this. However, I have to get ready for work now, so I'll check it later if you disagree with it.