Using capacitors on inputs and outputs, ac coupling simplifies circuit design by removing dc voltages on the transmission line and isolating ground connections between the transmit and receive systems. On the other hand, the presence of those capacitors compromises signal quality. Some systems can accommodate those compromises, while others cannot. Eliminating the cost of the output capacitor, which must be on the order of several hundred microfarads to minimize those compromises, makes dc coupling particularly attractive in high-volume, price-sensitive products. But it loses that attraction when the input signal swings positive and negative. (Think negative video-sync pulses.) That’s because that complication introduces the need for an additional, negative supply to ensure an input common-voltage range that accommodates plus-and-minus signal swings.
How do you size an input capacitor for ac coupling?Video signal bandwidths require capacitors large enough to pass the minimum frequency (Fig. 1). In video, this is most commonly the 50- or 60-Hz vertical frame rate. Given values of 10 kΩ each for R1 and R2, a value of 0.1-µF for input capacitor CIN would place a pole at 318 Hz. For a 6.5-µF capacitor, f0 would be 6 Hz.
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