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For example, if the capacitance formula is applied to the following trace:
4 Layer board signal routing next to ground plane
Board layer thickness: 0.188 mm
Trace Width: 0.75 mm
Trace Length: 7.5 mm
A typical value for ER of FRC4 PCB material is 4.5. Due to the variations of material from which an FRC4 board can be fabricated, this value is not guaranteed, but should be in the range of 4 to 5. The capacitance between these traces would be 1.1 pF. Of course, the antenna effect on a 7.5-mm trace would be devastating, so this example is a bit extreme. Ignoring the antenna effects for now, there are cases in which even a very small parasitic capacitance like 1 pF is unacceptable. It dramatically illustrates the effect of 1 pF capacitance occurring at the inverting input of the op amp. It causes a doubling of the output amplitude near the bandwidth limit of the op amp. This is an invitation to oscillation, especially since the trace is an efficient antenna above 180 MHz.
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