7.3.3.2.2 Automatic Gain Control (AGC)

An automatic gain control (AGC) circuit is included with the ADC and can be used to maintain nominally constant output signal amplitude when recording speech signals (the AGC can be fully disabled if not needed). This circuitry automatically adjusts the PGA gain when the input signal becomes overly loud or very weak, such as when a person speaking into a microphone moves closer or farther from the microphone. The AGC algorithm has several programmable settings, including target level, attack and decay time constants, noise threshold, and maximum PGA gain applicable that allow the algorithm to be fine-tuned for any particular application. These AGC features are explained in this section, and Figure 18 illustrates their operation. The algorithm uses the absolute average of the signal (which is the average of the absolute value of the signal) as a measure of the nominal amplitude of the output signal.

Figure 18. Typical Operation of the AGC Algorithm During Speech Recording