RadioDoc
Sortofadog
Premium,ExMod 2000-03
join:2000-05-11
Chicago, IL
 ·AT&T Midwest
| Actually you have it right.
"Common mode" refers to equal in-phase signals on both sides of a balanced line. It is one of the design basics of unshielded twisted pair...the twists keep the common mode signals equal. The input to a perfect op-amp only reacts to the difference in voltage between it's two input pins (inverting and noninverting). Theoretically you could put 1000 volts of common mode RF across them and it would be ignored since it would be equal and in phase. Reality intrudes into Electronic Utopia though since the inputs do have voltage limits and are not perfectly linear or identical.
Transformers, which are what existed before op amps of any flavor, are what balanced (audio) lines were originally designed to work into. Those definitely only respond to the differences between their two input wires. With proper equipment on each end you can get 90-100 dB signal-to-noise on a reasonable unloaded phone pair used for a 15 KHz audio circuit. We used to do it all the time feeding program audio to transmitter sites, where the RF is 1,000 times more intense than at any customer location you might encounter.
As for the name change. ROFL. I have a client who says I need a black pointy hat with gold stars on it, and a wand. 
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"Nothing fuels a good flirtation like need and anger and desperation"-Aimee Mann |
