medical electronics III

Since the output is proportional to the difference between the two voltages, anything (e.g. noise) which is present on both inputs will be cancelled out. However, a signal (e.g. the EKG) which is different on the two inputs will be amplified, which of course is exactly what we want. The ratio of the gain of the difference gain to the common gain (usually expressed in dB) is called the Common Mode Rejection Ratio (CMRR).
CMRR = Differential Gain Common Gain
An typical differential amplifie has a CMRR of about 30,000. So, supposing we build a circuit with a differential gain of 1,000, this means that the common gain (acting on the noise) will be:
Differential Gain / Common Gain= 30,000
So, Common Gain = Differential Gain / CMRR = 1,000/30,000 = 1/30 or 0.03
In other words, instead of getting amplified, the noise will actually be attentuated 30-fold.


CMRR in dBJust to be awkward, gains and CMRR are usually quoted in dB, so for voltage gains, the equation becomes:
CMRR (dB) = 20 log (Differential Voltage Gain / Common Voltage Gain)
Thus, a typical differential amp will have a CMRR of 20 log 30,000 = 90 dB
How about going the other way? Well, the inverse of a log to base 10 is 10 raised to its power, so:
Voltage Gain ratio = 10CMRR/20
e.g. 90 dB = 1090/20 , i.e. a voltage gain ratio of 104.5 or 30,000

Labels: Common mode rejection