A Clean Audio Installation Guide™

4.0 COMMON MODE REJECTION (CMR)
The desirable feature of a balanced or differential input is its ability to reject a signal which, referenced to "ground," has the same amplitude and phase on both inputs. This feature is known as common mode rejection (CMR). Whether the input stage is an active or transformer type, the amount of CMR that is needed is directly dependent on the amplitude of the common mode signal. If, for example, the power line related voltage difference between two chassis was measured, and found to be 200 mV (a long run in a network facility, perhaps), this is -11.76 dBu; and if you wanted the "no signal" output of your input stage to be pure noise of, say, -90 dBu, then the amount of CMR that you need is the difference between these two amplitudes - that is, 78.24 dB at the frequencies of interest.

You need to know, however, that not all differential input stages are created equal! The amount of CMR that is built into a circuit is a large variable. To achieve the degree of common mode rejection to which most op-amps are capable requires some very careful design, P.C. layout, and adjustment. Low frequency CMR is achieved by making the resistive portion of the diff-amp gain determining network a balanced bridge. Some manufacturers use 5% resistors to form the differential input stage; more use 1% metal film resistors but do not use trimmers to precisely balance the bridge. The better manufacturers use both 1% resistors and a stable trim resistor to achieve high degrees of CMR. (See figure 3)

The average CMR that can be expected with 5% resistors is approx. 26 dB and with 1% resistors is approx. 40 dB, whereas a carefully trimmed input stage is capable of better than 100 dB of CMR at low frequencies.

To achieve any kind of high frequency common mode rejection requires stray capacitances be trimmed out as well. Practically speaking, 75 dB of CMR is achievable out to 20 kHz by adding a capacitive trim, and even greater CMR is achievable (provided the op-amp is capable) when very careful RF style P.C. layout is practiced. The precision balance of the resistor capacitor bridge around the op-amp makes the common mode signal exactly equal at the amplifier's inputs, and that common mode signal is then rejected by the well balanced differential input stage within the op-amp. Unity gain buffers (not shown) in front of the differential amplifier provide the significant added benefit of equal input impedances.

Likewise, with transformer inputs, a high degree of CMR (sometimes called "longitudinal balance" after the days when the input stage transformer center taps were grounded) is necessary to reach the level of performance that we desire. While it is possible to achieve 90 dB of CMR in a transformer, most transformers on the market won't begin to come close to that figure. The input transformers made by Jensen Transformers and Reichenbach Engineering are notable exceptions, with typical 100 Hz CMR of 105 dB, 1 kHz CMR of 85 dB, and 10 kHz CMR of 65 dB. This kind of CMR performance requires extremely careful manufacturing procedures.

In terms of input types, it is generally preferable to use an active input stage (provided it has been correctly designed) rather than a transformer, because of its lower cost, generally wider bandwidth, freedom from low frequency distortion, hum susceptibility, and sometimes frequency response errors and ringing. If, however, the common mode signal cannot be reduced below two or three volts, as is often the case with Telco feeds, then a transformer input should be used because of its almost unlimited (1600 volt) common mode input voltage range capability. 2 to 3 volts is the practical common mode limit for active inputs. However, with some fancy amplifier tricks, an active input stage can handle up to 200 or 300 volts common mode input.

Go to: Section 5.0