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| Need help with ground loop |
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| magic:
So you have battery power, regulate 5V from it, send it to an external BT receiver over USB, the receiver feeds analog signal back into your board? I suppose what may be happening is that return current from the receiver develops significant voltage drop between the receiver's ground and your ground. The signal is referenced to the receiver's ground, your ground is different, you have an error. If the current is AC then the error is AC too and you have noise. How to check if that's what happens: tie a short and thick cable between the two grounds bodge a huge electrolytic into the receiver add a huge differential mode choke on the power output to convert audio frequency AC currents into voltage ripple in the receiver's capacitors How to fix it in a customer-friendly way: not sure if at all possible. Maybe increase signal output and attenuate on board for more SNR. How to fix with a board respin: check how the various grounds work in your amp chip. You should probably connect AGND to the input jack, disconnect it from the ground plane and figure out how to ground the rest of the chip amp. This is assuming that all the xyzGND pins aren't shorted internally. |
| laejf:
--- Quote ---So you have battery power, regulate 5V from it, send it to an external BT receiver over USB, the receiver feeds analog signal back into your board? --- End quote --- Yes that's correct. --- Quote ---I suppose what may be happening is that return current from the receiver develops significant voltage drop between the receiver's ground and your ground. The signal is referenced to the receiver's ground, your ground is different, you have an error. If the current is AC then the error is AC too and you have noise. How to check if that's what happens: tie a short and thick cable between the two grounds bodge a huge electrolytic into the receiver add a huge differential mode choke on the power output to convert audio frequency AC currents into voltage ripple in the receiver's capacitors --- End quote --- I tried soldering a thick wire between them but it made no difference. I also tried to feed the BT-module via a separate USB jack, grounded at other ground-points than the jack that is on the board but no luck. A big capacitor didn't help either. I will try a differential mode choke! Although I'm not sure about what you mean by "Convert audio frequency AC currents into voltage ripple"? --- Quote ---How to fix with a board respin: check how the various grounds work in your amp chip. You should probably connect AGND to the input jack, disconnect it from the ground plane and figure out how to ground the rest of the chip amp. --- End quote --- I found a section in the datasheet that said that all the grounds should be connected in the thermal pad as a star point ground. This is something that i missed. If i re-design the board, should I scrap the ground plane completely and connect all grounds to the thermal pad using single wires? Including the BT-module's ground. |
| magic:
Truth be told, now that I did the math, it's quite obvious that "big enough capacitor" is one which has less reactance at audio frequencies than there is resistance in the USB cable and connectors. That would be one really big capacitor :) Screw that, try this. You said that external PSU for the BT makes it go away. So remove your 7805 and connect the external PSU to its GND and OUT pads. Should be quiet, right? |
| DaJMasta:
Differential mode could definitely do something, any picked up noise probably differential mode, but you want to keep the filter cutoff away from the audio band, since the input signal isn't differential. That being said, what about an RC or LC low pass filter on the audio inputs? Won't do anything to protect against like power supply noise, but would effectively eliminate high frequency noise picked up on the input cables. Shielded signal cables or shielding for the amp can be a viable option, too. Still, your best bet is going to be to scope it and see where it's coming from. With the gain options for the chip, you can probably still see noise on the inputs/power rails if it's significant on the output, so time-correlating them should show you where exactly it's coming from. Much more narrower scope (ha) of a problem to troubleshoot when you can identify where the noise is actually coming from. |
| xlnx:
I'd like to add that if the if the ground connection from the USB is your real problem, then you may use a digital isolator device from Analog Devices or Silicon Labs. Search for "USB 1500V" on eBay/AliX to se how it's done. That solution also has an isolated 5V to 5V power module with 1500V isolation to bring power to the isolated side - you may not need that if you can power it from the inside. |
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