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Topic: Whats the capacitance of a sm pad? (Read 586 times) previous topic - next topic

westfw

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there's hardly any spacing between the two. 

Worse than that, since that doesn't look like a very accurate outline for a typical crystal!
Otherwise, the SMD pads should not affect the accuracy in any meaningful way.  (oh, let's plug in numbers!
Assume a pad 2mm on a side, with a separation of 1.6mm from the other plate, and a dielectric constant of 4 for fiberglass PCB material.)
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C = Er * E0 * A/d
   = 4 * 9e-12 * (2e-3 * 2e-3)/1.6e-3
   = 0.1 pF (approx)


Here's the layout from one of the official arduino boards, BTW:

oric_dan

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0.1 pF (approx)

Sounds a lot lower than what I might have expected, but it's a nice formula. I didn't think anything
had stray capacitance that low, :-).

westfw


MarkT

Fringe field effects will increase the value, as will capacitance of the traces going up to the pads, but the general
order of magnitude is correct.  Small things have less capacitance than big things, all else being equal.  Stray
capacitance on interconnect on a VLSI chip is measured in fF (femto farads)!

For comparison a capacitor made from two metal plates sandwiching a credit card would be about 180pF - 0.9mm
thick, assume dielectric constant of 4 (incdentally here the geometry makes fringe-effects negligible).

Most circuit construction techniques mean stray capacitances are a few pF for small parts of the circuit, scaling up
with size.  A ground plane actually reduces the stray capacitance between different signal wires (at the expense
of increasing the capacitance to ground).  For a high-accuracy crystal oscillator you might want to leave a hole
in the groundplane around it to reduce stray capacitances altogether (although for a low-power crystal oscillator
(such as for RTC chip) you want the groundplane to reduce the risk of external interference on the very weak signals.

For a general purpose digital logic clock accuracy isn't of huge concern, so stray capacitance isn't very crucial
here, it will just slightly retune the oscillator.
[ I won't respond to messages, use the forum please ]

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