Common-Mode Balance

Common-mode balance is a term that applies to a differential transmission system. Common-mode balance is the ratio of common-mode to differential-mode signal amplitudes within the system. A perfectly differential system with no common-mode component is perfectly balanced (i.e., the signals on the two wires are exactly antipodal, or opposite ).

When calculating the common-mode balance ratio, it is common to refer only to the AC component of the common-mode signal. For example, a perfectly complementary pair of TTL 3.3-V signals may share a common-mode DC offset of 1.65 volts , but the common-mode AC voltage (the changing part) might still be very small.

The common-mode balance ratio is often expressed in decibels. A common-mode balance of 1 part in 10,000 ( “80dB) is exceptionally good. Digital logic parts sold as differential transmitters may exhibit common-mode balance of perhaps only “30 dB or even “20 dB.

Some authors express the common-mode balance in terms of the even-mode to odd-mode signal amplitude. The even-to-odd mode amplitude ratio is half the common-to-differential mode amplitude ratio, making it 6dB less impressive-sounding. An older term used to describe the common-mode to differential-mode amplitude ratio is longitudinal balance .

POINT TO REMEMBER

• Common-mode balance is the ratio of common-mode to differential-mode signal amplitudes.