Not hard. Measure Er on a big hunk of copperclad. Now cut the test pad and hook up the AADE meter. Ground one side and use a needle probe on the other. Place the probe close to the pad, zero the c-meter, then touch the pad. I've done this on small pads, and it seems legit to me.
As long as the c-meter reads about the same fairly far from the pad as it does just before contact, which it usually does, this should be OK.
John
With pads of 0.062 on a side i would only treat that as an approximation. At that size the fringing from the probes themselves is a significant error source.
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I see a couple things here...
First is that, as I understand it, John is first zeroing out the capacitance from the probe to the ground plane...or at least to some combination of the plane and the pad. I would think that would mitigate the effect of the capacitance from the probe itself in the final measurement configuration. To make and even more accurate measurement, we could put a coaxial shield around the probe, but then the question becomes, what potential should that shield have? If it's grounded (the same potential as the ground plane), then the pad has additional capacitance to it; if it's the same potential as the probe, then it will disturb the field around the pad when measuring the pad's capacitance. It seems to me that zeroing the measurement with the probe close to the pad is a reasonable compromise.
Second, what I really had in mind when I posted the question in the first place is the capacitance not just of the pad but of the pad and the part that's soldered to it. Of course that gets much more complicated than just a planar pad against a ground plane. It's unrealistic to think I'll end up with a perfect model of the parasitic effects in my circuit, but I'd at least like to get an idea of what to do to compensate for them. I do know that they are not insignificant, and that I will need to compensate. Unfortunately, the pad capacitance to ground is not the only parasitic effect involved.
Cheers, Tom
Regular, cheap surface-mount resistors make good terminations at least to 5 GHz. They tend to be a tad inductive, so two 100 ohm resistors, in a V or T splay at the end of a 50-ohm trace, is even better.
You can also AC couple a signal well into the GHz. Make a small gap in a 50 ohm trace and bridge it with 2 or 3 0603 or 0402 caps, regular cheap ceramics.
Tiny surface-mount parts can do things that used to be done only with distributed (transmission-line based) circuits. It's a lot easier to tune a lumped circuit.
John
I have almost never had capacitance problems between tracks on my projects in 25 years.
I have seen problems with ribbon cables and crosstalk.
If I was having problems with a circuit I would be looking elsewhere like logic races, decoupling or software not coping with glitches.
"Tom Bruhns" schrieb im Newsbeitrag news: snipped-for-privacy@n76g2000hsh.googlegroups.com...
Hello Tom,
I successfully tried with the concept of fringe-capacitance as used in SPICE for Mosfets.
C = C0 + Cfringe
C0 = e0*er*Ap/d
Cfringe = Cf' * Pp
Ap = area of pad d = distance of pad to the plane er = 4.5 for FR4 e0 = 8.85e-12A/m Cf' = fringe_capacitance/m Pp = perimeter of pad
From my calculations: Cf' = 24pF/m, er = 4.5
You can get this number indirectly from your transmission line calculator. Choose Microstrip:
Calculate the usual C' (capacitance/meter) for a trace with the pad-width with your impedance calculator. . Calculate capacitance of a trace with the length of your pad C'' = C'*pad_length
Calculate pad capacitance C0 C0 = e0*er*Ap/d
Fringe capacitance per meter Cf' = (C''-C0)/pad_length/2 (2 because of two sides)
Total capacitance of pad:
C = C0 + Cf
C = C0 + Cf'*Pp
Cpad = C0 + 24pF/m * Pp
Pp is the perimeter of your pad in meter.
The good news is that it's roughly 24pF/m with low sensitivity regarding the ratio of pad dimensions to the plane distance. This means you can use this constant for your multilayer board too.
Best regards, Helmut
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