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When we say capacitance is important in serial communication, whether we think of parasitic capacitance? So the lower parasitic capacitance in cable, less is voltage drop and the longer distance we can pass over? Or we are talking about some other capacitance?

I assumed that because we have two parallel conductors close together and air between them, so it acts like capacitor, am I right?

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I think you answered the question already :) Capacitances will influence your communication system in two ways:

  • Parasitic capacitances: Influences the frequency behaviour(more specifically, the cutoff frequency) of your communication system and if you are unlucky it will do this in the bandwidth of your system, so your signal will be partially filtered/weakened.

  • Capacitance of the transmission line: In high frequency communication, the data line and ground will form a transmission line, which indeed, as you stated, has a per length capacitance, but also inductance, resistance and conductance. These determine the characteristic impedance. Mismatch between the characteristic impedance of the communication line and impedance at the load, will cause reflections that distort the data.

So yes, The per length capacitance influences the characteristic impedance and thus signal integrity. Parasitic capacitance in other places of the circuit in the communication system will influence the cutoff frequency of the system and can weaken the data signal if this frequency is within your bandwidth.

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Parasitic capacitance is not directly related to voltage drop, but rather to interference that occurs due to oscillations caused by the capacitance between a data line and nearby electronic components (wire, PCB tracks, etc). This capacitance can create impedance to electrical signals, affecting any data sent over the line.

At higher transmission frequencies, this becomes much more of an issue as the quicker timings of the data transmission can be more easily influenced by the impedance, resulting in more data corruption or missed bits/bytes.

You can read more here or with a simple Google search.

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