Jeff,
You are correct, the rejection becomes infinite as the undesired gain (either the common mode gain or the gain from the supplies to the output) goes to zero. Therefore
PSRR+ =
Adm /
AddPSRR- =
Adm /
AssCMRR =
Adm /
AcmSo what this says is that there are two PSRRs, one for the positive supply and the other for the negative supply. You would not combine them unless you had reason to believe that the interferrence from the supplies would be balanced relative to ground, which seems unlikely.
Referring to Figure 5 in
http://www.designers-guide.com/Analysis/diff.pdf, if an XP analysis were performed with the output being
Vod, then these metrics are calculated using
PSRR+ =
Vid /
VddPSRR- =
Vid /
VssCMRR =
Vid /
Vic(Here I am assuming that
Vdd and
Vss represent the positive and negative power supplies and that
Vxx represents the gain from source
xx to the output).
Erik was on the right track with his formulas, but he gave the gain from the supplies to the output, not the rejection ratio.
- MM -