Cmrr question?

Question

I understand the formulas for the common mode rejection ratio. I know how to calculate it. I'm just having trouble understanding the whole concept of it.

What exactly is a common mode signal and a differential signal?

What exactly does this help with? I understand a 70 -90db is decent rection ratio I just don't understand why.

Please help me see the big picture

Answer 1

Common mode voltage is the voltage relative to common (earth?) ground that is the same on both positive and negative inputs of the differential amplifier.  The differential voltage is the voltage at the positive pin, minus the voltage at the negative pin.

Imagine a 120VAC source, a light bulb, and a small resistor (0.1 Ohms) all in series.  The object is to measure the current through the light bulb by measuring the voltage across the light bulb.  By the power law, for a 100 Watt light bulb, the current is 833 mA, so the voltage across the 0.1 Ohm resistor will be 83.33 mV.  

If you connect the positive lead of the amplifier to the AC source (which is also connected to the resistor), and the negative lead of the amplifier to the other side of the resistor (which is also connected to the light bulb); you will measure a differential signal of 83.33 mV.  BUT, there will be 120 volts of common mode on both pins of the amplifier because the ground connection is on the other side of the light bulb.

Besides the fact that you might blow some circuits up if you actually tried this and didn't know what you were doing, there are some actual circuits that do this sort of thing -- 'high' side current sensors and monitors.

If the amplifier had a CMRR of 70 dB, then that 120 volts would be 'seen' as a differential signal of 38 mV. which is almost 1/2 of the correct signal (83 mV).  

What an error that is!!!

For the common mode error to be 1% of the signal, it would have to be 830 uV and the CMRR would have to be 103 dB.

I have dealt with some very good differential amplifiers with CMRRs in the 120+ dB range, for signals in the 100 uV to 1 mV range, but their common mode voltage RANGE wasn't so great.  I have also dealt with some amplifiers with a common mode RANGE of 200+ volts, but their CMRR was only about 90 dB.  A *really* good amplifier can stand both the high voltage, AND reject all of it.  Those are very very hard to come-by.

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