In an AC coupled system, the DC
Biasing at the input of the ADC is independent of the DC level of the source.
The ADC input is biased internally by the chip at an optimum DC level
maintaining that ADC digitizes full scale AC input signals.
Coupling capacitors used at the
ADC input form a high-pass filter with the input impedance of the ADC limiting
the low frequency response of the system. For audio applications, the capacitors
are sized such that frequencies as low as 20 Hz can be digitized.
A coupling capacitor must be
charged to a steady state value on power-up. Until the time the capacitor
reaches this value, the input audio signal is not passed properly to the ADC.
Sometimes this situation is heard as a pop in the audio output.
A coupling capacitor needs to behave as a short circuit for all AC signals in
the audio range. In practice the capacitance value and the capacitive impedance
can change with signal amplitude across the capacitor. This value can result in
non-linear behavior and harmonic distortion in ADC output at low
frequencies.
Some capacitors also display
microphony. When experiencing vibration, the capacitor can induce a voltage in
the AC path due to the Piezo Effect.