common mode rejection: can't build a perfect version of this, basically, noise will be the same on both inputs, so noise gets cancelled
ability of a balanced (aka differential) input to reject the part of the incoming signal which has the same amplitude and phase on both input terminals to the differential amplifier
long runs of wire act like antennas and have internal resistance
2 things will cause you noise, temp and resistance
differential lines allow cancellation of thermal effects and emag effects

1. CMRR: ratio of common mode noise rejected (common mode rejection ratio)
100% noise rejection is not possible
want CMRR to be at infinity
2. common mode noise (interference) voltage at input divided by common mode noise voltage at output
3. (most common) CMRR = 20log(Ad/Acm) (differential gain/common mode gain)
for ICs/SS devices, CMRR ~~ 70dB to 90dB
70dB -> Ad/Acm ~~ 3200
90dB -> Ad/Acm ~~ 32000

for transformer, Ad/Acm ~~ 200000, disadv: heavy, expensive, powerhogs

small signal version of differential amp:
lots of magical math
something about two star equations

the imperfections in the current source determine Rss which determines common mode rejection
CMRR = Ad/Acm = ....ideally infinity, but involves gmRd and Rss