Can anyone tell me:
1) Capacitance between adjacent SO-8 pads.2) Capacitance between different-size pads on opposite sides of PCB, spaced say 5mm apart, with standard 62-thou FR4 material, etc.
Wish Altium Designer could calculate and report to me inter-trace capacitances.
--
Thanks,
- Win
NSC an-1205 says 0.1pF, IIUC. But I think (hope) it's probably a little less. What if there's a 10-mil ground trace between the pads?
--
Thanks,
- Win
3-terminal, against a ground plane?
The gross (big pads, no fringing) capacitance between sides of an
0.062 thick FR4 board is about 15 pF per square inch. Tempco runs maybe +900 PPM/K.-- John Larkin Highland Technology, Inc picosecond timing precision measurement
It depends what's underneath and once that is known I'd use one of the online cheat sheets:
-- Regards, Joerg http://www.analogconsultants.com/
If that is based on the UltraCad stuff, I wouldn't entirely trust it.
The Saturn PCB calculator looks really nice:
One of my guys did check its asymmetric dielectric diff pair against an em simulator, and it was close.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement
So far I have lucked out. At least down to the 100psec pulse widths everything panned out. You guys probably go a lot lower than that. Problem is, I have to fly blind part of the way and have to wait for the results because I don't have one of those 20GHz sampling scopes. Meaning the circuit tests itself bug then, of course, it's quite late in the game. Nail biters like with the New England Patriots.
-- Regards, Joerg http://www.analogconsultants.com/
You can always drop in here and use one of our scopes. We go to 7 GHz realtime and 50 GHz sampling. You would have to buy the beer.
1 ns is where things are getting interesting. 100 ps is where things are starting to get hard.-- John Larkin Highland Technology, Inc picosecond timing precision measurement
Nothing is underneath, bare FR4, no copper. It's a simple case of side-by-side pads.
--
Thanks,
- Win
Want me to measure one for you? My Boonton is on my bench.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement
I thought you'd never ask.
This is a plain double-sided board.
C56 = 0.3 pf. That includes the traces and vias.
C34 = 0.06 pf, just the pads with traces cut away.
Measured in a Boonton 72B, probably not super accurate.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement
I also have instruments that can measure to 0.01pF, but setting the zero value is a bear. Your 0.06pF measurement is encouraging, better than the 0.1pF numbers I've seen. Well, sheesh, I need 0.02 or 0.03 pF. I'm hoping that judicious ground traces can make up the difference. Alternately, I can entirely eliminate some pads on the pcb (and cut device leads). Maybe that's a better approach.
--
Thanks,
- Win
I bet a ground plane would help a lot. I'll try it again on a 4-layer board with L2 ground plane, but it's going to be hard to resolve on the Boonton. It's an analog meter with 1 pF full scale.
I think there is a way to measure 0.02 pF with a scope and a signal generator.
A ground trace between the pins would help, too. But the SO8 package itself will have C between the pins, and inside.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
How about some unplated drilled slots between the pads?
Er of FR4 is ~4 vs. 1 for air.
--sp
-- Best regards, Spehro Pefhany
Am 22.02.2017 um 02:31 schrieb John Larkin:
Maybe he can bring some of his home brewed beer? ;-)
Jorgen
I think not. He likes that dark stuff, and I don't like to drink anything that I can't see through.
We'll have to go to Zeitgeist.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Or grounded plated slots. More capacitance to ground, less pad-to-pad.
Or switch to a single op amp so it doesn't matter as much. ;)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant
A tight row of grounded vias, maybe zigzag, would have about the same effect.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
And make the SO8 pads smaller!
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Thanks. That would also be a good excuse for a visit.
Since I am married I can only brew the occasional dark stuff and that is almost pitch-black. Then there is the Belgian Tripel which looks light-amber but has almost 8 volts. Most other beers I brew are golden-colored and between 4.5% an 6%. Pale Ale, IPA, Cream Ale, Koelsch. Slightly darker but still to your taste would be Irish Red, Autumn Amber and Copper Ale (like a miner's beer).
Looks like the Marco building only a few more steps walking distance to Zeitgeist. Otherwise I couldn't imagine you guys even considering a move.
-- Regards, Joerg http://www.analogconsultants.com/
That, and if cost is not too critical consider board material with a lower dielectric constant.
In the extreme maybe porous silicon dioxide.
-- Regards, Joerg http://www.analogconsultants.com/
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