254 Chapter 8
8.3.3 Balancing Requirements
8.3.3.1 Input Impedances
The norm in modem pro-audio equipment is 10 k Ω across the line. This is commonly
known as a “ bridging load. ” It is also the differential input impedance.
The common mode impedance, what any unwanted, induced noise signals will see, is
often (but not always) half of this, for example, 5 k Ω in this case.
Considering the hum/RF noise rejection capability of an effective balanced input, input
impedances much higher than 10 k Ω , say, 500 Ω , would seem feasible and useful in
professional systems. However, if the input resistance is developed by the ubiquitous
input bias path resistors connected from each input to the 0-V rail, then there are limits to
the usable resistance, before the input stage’s output offset voltage becomes unacceptably
high. Although low Voos op-amps exist, a number of otherwise good ICs for audio have
execrable DC characteristics, as IC designers do not appear to comprehend that good
DC performance is a most helpful feature for high performance audio. In this case, input
impedances above 15 to 100 k Ω are found to be impractical, depending on bias current.
A galvanically fl oating input (i.e., the primary of a suitably wired transformer) has no
connection to signal 0 V (as it has no bias currents), so there can be a very high common-
mode impedance, say, l M or more, up to modest RF. This aids rejection.
Conversely, differential impedances of less than l Ok increase the infl uence of such
random, external factors as mismatched cable core-to-shield capacitances.
8.3.4 Introducing Common Mode Rejection
Common mode rejection is an equipment and system specifi cation that describes how
well unwanted common mode signals, mainly hum and RF interference, are counteracted
when using balanced connections.
8.3.4.1 Minimum Requirements
At the very least, all the equipment in a system must have a balanced input (alias a
“ differential receiver ” ). CMR can be improved and made more rugged when balanced
inputs are used in conjunction with balanced outputs (alias “ differential transmitters ” ),
but this is not essential.