Figure 39 highlights the basic elements present in the audio signal pathway. The primary elements are: input biasing resistors, electromagnetic interference (EMI) filtering, input buffers, and a difference amplifier. The primary role of an audio line receiver is to convert a differential input signal into a single-ended output signal while rejecting noise that is common to both inputs (common-mode noise). The difference amplifier (which consists of an op amp and four matched 10-kΩ resistors) accomplishes this task. The basic transfer function of the circuit is shown in Equation 1:
The input buffers prevent external resistances (such as those from the PCB, connectors, or cables) from ruining the precise matching of the internal 10-kΩ resistors which would degrade the high common-mode rejection of the difference amplifier. As is typical of many amplifiers, a small bias current flows into or out of the buffer amplifier inputs. This current must flow to a common potential for the buffer to function properly. The input biasing resistors provide an internal pathway for this current to the COM pin. The COM pin can connect to ground in a dual-supply system or the output of the internal supply divider (VMID(OUT)) in single-supply applications. Finally, EMI filtering is added to the input buffers to prevent high-frequency interference signals from propagating through the audio signal pathway.