CMRR IN BALANCED INTERFACES: Part 1
Mar 1, 2003 12:00 PM, BILL WHITLOCK
Professional audio systems have traditionally used only equipment having balanced interfaces. In my humble opinion, only equipment with balanced inputs and outputs should ever be called “professional.” The use of balanced interfaces is an extremely potent technique to prevent noise coupling into signal circuits. It is so powerful that many systems, such as telephone systems, use it instead of shielding as the main noise reduction technique.
BALANCE VERSUS SYMMETRY
The true nature of balanced interfaces is widely misunderstood. For example, each conductor is always equal in voltage but opposite in polarity to the other. The circuit that receives this signal in the mixer is called a differential amplifier, and this opposing polarity of the conductors is essential for its operation. This, like many such explanations in print, describes signal symmetry (equal in voltage but opposite in polarity) but misses the single most important property of a balanced interface.
According to H. Ott in Noise Reduction Techniques in Electronic Systems, a balanced circuit is a “2-conductor circuit in which both conductors and all circuits connected to them have the same impedance with respect to ground and to all other conductors. The purpose of balancing is to make the noise pickup equal in both conductors, in which case it will be a common-mode signal that can be made to cancel out in the load.”
The informative annex of IEC Standard 60268-3 states, “Therefore, only the common-mode impedance balance of the driver, line, and receiver play a role in noise or interference rejection. This noise or interference rejection property is independent of the presence of a desired differential signal. Therefore, it can make no difference whether the desired signal exists entirely on one line, as a greater voltage on one line than the other, or as equal voltages on both of them. Symmetry of the desired signal has advantages, but they concern headroom and crosstalk, not noise or interference rejection.”
A simplified balanced interface is shown in Fig. 1. Theoretically, a balanced interface can reject any interference — whether because of ground voltage differences, magnetic fields, or electric fields — as long as it produces identical voltages on each of the signal lines and the resulting peak voltages don't exceed the capabilities of the receiver. Any voltage that appears on both inputs, because it is common to both inputs, is called a common-mode voltage. A balanced receiver uses a differential device, either a specialized amplifier or a transformer, which inherently responds only to the voltage difference between its inputs. An ideal receiver would have no response to common-mode voltages. In reality, the response is not zero, and the ratio of differential gain to common-mode gain of this device is its common-mode rejection ratio, or CMRR. It is usually expressed in decibels, in which higher numbers mean better rejection.
Note that the common-mode (with respect to ground) output impedances of the driver and input impedances of the receiver effectively form a Wheatstone bridge as shown in Fig. 2. If the bridge is not balanced or nulled, a portion of the ground voltage difference Vcm will be converted to a differential signal on the line. The nulling of the common-mode voltage is critically dependent on the ratio matching of these pairs of driver/receiver common-mode impedances. The nulling is relatively unaffected by impedance across the lines — only the common-mode impedances matter. This bridge is most sensitive to small fractional impedance changes in one of its arms when all arms have the same impedance. It is least sensitive when upper and lower arms have widely differing impedances. Therefore, you can minimize the CMRR degradation in a balanced interface caused by normal component tolerances by making common-mode impedances very low at one end of the line and very high at the other.
The output impedances of virtually all line drivers are determined by series resistors (and often coupling capacitors, too), which typically have ±5 percent tolerances. Because of that, typical drivers can have output impedance imbalances in the vicinity of 10ź. The common-mode input impedances of typical balanced input circuits are in the 10 kź to 50 kź range, making the CMRR of the interface exquisitely sensitive to normal imbalances in driver output impedance. For example, the CMRR of the widely used SSM-2141 balanced line receiver from Analog Devices will degrade some 25 dB with only a 1ź imbalance in the driving source. However, balanced receivers having common-mode input impedances some 1,000 times higher are essentially unaffected by imbalances as high as several hundred ohms. A balanced input using either a quality input transformer or the InGenius IC is virtually immune to this degradation. Either typically achieves 90 to 100 dB of CMRR, virtually unaffected by real-world balanced output imbalances, and typically over 80 dB of CMRR when driven by an unbalanced source.
CMRR MEASUREMENTS VERSUS REALITY
CMRR of balanced inputs have traditionally been measured in ways that ignore the critically important effects of driver and cable impedances. Therefore, actual CMRR achieved in real-world systems is often far less than that touted for the balanced input itself. As shown in Fig. 3, the old IEC method essentially fine-tuned the driving source impedance until it had perfect balance. Other widely used techniques, simply shorting the two inputs together or using high-precision resistors (as in most test instruments), are equally unrealistic measures. I was pleased to have helped persuade the IEC to change its measurement standard for balanced inputs and outputs. The third edition of IEC Standard 60268-3, Sound System Equipment — Part 3: Amplifiers was issued in August 2000.
The new method involves placing a 10ź imbalance, first in one line of the test source and then in the other. The lower of the two calculated CMRR figures is then used.
Of course, people must appreciate that noise rejection in a balanced interface isn't just a function of receiver CMRR. Actual performance in a real system depends on how the driver, cable, and receiver interact. In part 2, I'll explain how the physical construction and shield connections affect interface performance.
Bill Whitlock is president of Jensen Transformers. He has designed audio and video circuits and systems for 30 years. He can be contacted by e-mail at firstname.lastname@example.org.