Understanding parasitic capacitance on a PCB resistor

[Mohammed Ubaid]

Hello dear all,

While designing PCBs, we model a resistor accounting for parasitic capacitance (in parallel to the resistor) and stray inductance. with the increase in signal frequency, the impedance of the circuit drops due to parasitic capacitance as it bypasses the resistor and after further increase in frequency, stray inductance dominates the resistor and increases the circuit impedance.

The question is, if parasitic capacitance bypasses the resistor leading to drop in circuit impedance, where is it finding a path for current to flow.?
Because on a trace we have a physical resistor, how come a parasitic capacitance can bypass a physical trace..? How does it bypass a resistor.?

 

[Harald Kapp]

The capacitance of a trace along the trace is negligible, unless you go to spuer high frequencies.
The relevant capacitance is between trace and other traces of power supply planes. This is due to the metallic trace and the insulating PCB material and another trace or plane forming a plate capacitor.