DC coupling allows both AC and DC signals through, while AC coupling accepts only AC signals.
Issue in AC coupling: AC coupling rejects DC component in the signal, making
the average value of the signal to zero.
Case 1: Waveform has 50% duty cycle, the peak value in both positive and negative cycles will be same.
Case 2: Waveform doesn’t have 50% duty cycle.
After AC coupling, (average value is zero)
Area under the positive half cycle = Area under negative half cycle.
But the peak value for positive and negative half cycles will not be same.
Example: Consider a square wave with duty cycle 1/3 and peak-to-peak value 3V.
After AC coupling, the peak values would become 2V, -1V respectively.
If the AC coupled device detects 1.5V as logic high,
-1.5V as logic low, due to the above effect, there would be erroneous detection.