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Crossover PCB in progress, ground plane or star/rail?
Crossover PCB in progress, ground plane or star/rail?
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Old 14th May 2019, 09:59 PM   #11
Mark Tillotson is offline Mark Tillotson
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Make the power traces wide to lower stray inductance and resistance (and much easier to follow them for testing/debugging) I find with a 2-layer board they have to weave between front and back a bit to avoid breaking the ground plane up to much. The most important thing is that the trace between a supply pin and its decoupling capacitor is short and wide, especially with high bandwidth opamps, since the impedance at 10 to 100MHz is all important for its function.
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Old 15th May 2019, 03:00 PM   #12
Lingwendil is offline Lingwendil  United States
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I just redid some of the power traces, and upped them to 1mm width, which should be nice for these opamps (likely OPA2132PA since I have planty on hand) and should be getting close to done.

Any suggestions on opamp bypassing? Since I don't plan to use cranky opamps here I went with the oft-recommended (by datasheets at least) cap from each rail to ground. Should I put in the option to put one from rail-to-rail at each opamp as well?
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Old 15th May 2019, 03:39 PM   #13
Lingwendil is offline Lingwendil  United States
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Current state of the board, still may redo the input traces between the first buffer and the filters, by bringing them topside. Otherwise I'm pretty happy with it.
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Old 15th May 2019, 04:35 PM   #14
SemperFi is offline SemperFi  Wake Island
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Quote:
Originally Posted by Mark Tillotson View Post
One fact its worth knowing about groundplanes in standard 1oz copper pcb is the resistance per-square is about 0.5milliohm, ie 0.0005 ohms. 10mA flowing between two arbitrary places on a large ground plane is going generate something in the order of 5 or 10ÁV. If the signal level is about 1V, that's -100dB down. Something like an active crossover or line preamp isn't going to be troubled by this. A microphone or phono preamp is a different matter.
That be for DC. At higher frequencies than DC you want return paths near signal traces as well as better decoupling of opamps.
Allthough this is low frequency there might be high frequencies near by. Ground plane increases the imunity to HF entering the signal traces.
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Old 15th May 2019, 07:07 PM   #15
ohdsp is offline ohdsp
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Common misconception that circuits only work in the frequency range you want.

If you stick a Wi-Fi router next to your PCB, the PCB traces could unintentionally turn into nice little antennas/transmission lines. Previous EMC testing has shown me any interference above 2GHz easily becomes an issue even on the best PCB layout.

With bypassing requirements; power rail to ground next to each opamp supply pin is good, use 100nF ceramic and 10uF electrolytic or similar. Having capacitors (again 100nF and 10uF for example) every couple of inches between positive and negative supply rails can help. For example you could fit them in next to CB10, CB11, CB13.
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Old 15th May 2019, 07:13 PM   #16
Lingwendil is offline Lingwendil  United States
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I think it would be easy to throw in a pair of small electrolytics at each of the buffers opamps, good idea, thanks for the suggestion
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Old 15th May 2019, 08:24 PM   #17
Lingwendil is offline Lingwendil  United States
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Any suggestions on quality, affordable rail caps in the 10~47uF range for the local bypassing, from digikey? 2.5mm or 5mm lead spacing? I don't do lower volatage often, so I'm having a hard time choosing something suitable here. I'm thinking 16 or 25 volt is plenty, as the intended application will likely call for 12 or 15 volt rails.

Do these 25 volt, 47uF guys look good? Nice long life and decent ratings-
ULD1E470MDD1TD Nichicon | Capacitors | DigiKey
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Old 15th May 2019, 08:31 PM   #18
ohdsp is offline ohdsp
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Spec wise those Nichicons look good. Panasonic FC parts are always a safe bet, supposed to sound good as well...

Example
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Old 15th May 2019, 08:52 PM   #19
Lingwendil is offline Lingwendil  United States
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Both will fit with the parts outline I've chosen anyway, so nice options. Only a penny difference too.

Added some local supply caps that should fit those caps-
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Old 15th May 2019, 08:54 PM   #20
asuslover is offline asuslover  Romania
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Crossover PCB in progress, ground plane or star/rail?
EEU-FM1C680 Panasonic Electronic Components | Capacitors | DigiKey
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