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| Help for LM1875 PCB layout |
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| ozlow.own:
Hi all! :) I'm trying to design a PCB for a LM1875 amplifier (the circuit won't drive a speaker, but a reverb tank: hence the non-standard choice of values some components). SCHEMATICS: I've read the LM1875 datasheet and I'm worried about the layout. Seems that it needs a carefully designed layout. It's like the 4th PCB I'm designing in my life and I've never had my PCB public or someone helping me, so I never received any feedback about it. TOP: BOT: I'd like to know if the circuit can be prone to oscillation, capacitive coupling and problems like that with this layout. I tried to run separate traces for power ground, compensation, output and signal ground (actually the ground for low level signals is completely separated on this PCB, I will join the two grounds in star ground point in the PSU PCB). The curved traces are just to try something new :D If you need more clear pics of the PCB (3D view or some other way to see better the PCB) just ask. |
| JS:
It shouldn't oscillate if that's your concern. Maybe taking the inverting input components closer to the pin could help with that, they are a bit further away... There are some other considerations. Star ground is for electricians, not for PCB routers. It's been used as a way to cheat and get away with poor designs but not the right way to do it. For signal integrity you want a single ground (maybe two for noisy stuff like digital, lights and relay switching etc.) but one analog ground, which should follow the signal thinking it as a differential signal. I should have a drawing around there to explain this more clear, I'll try to find it. Think it this way. Most of the current goes to the load by the output pin of the output stage, that current comes from rails, that rails needs to close the current path to ground from the power supply (decoupling caps for fast response) that ground needs to close the current path to the load. That last stage acts as a load for the previous stage, and do on till youtube.com get to the input of your circuit, at which point you should treat your input signal as differential even if it comes unbalanced and referenced to ground. In that case you use the ground of the source as reference for the signal. Let's say your input circuit is an inverting stage, the non inverting input should go to the ground of the input and from there some resistorand to your local ground to ensure dc bias to your input stage even if the source is ac coupled or not connected. If the input circuit is non inverting you need to watch the path for the current in the feedback network JS |
| ozlow.own:
Thank you for answer. As i said, i never got feedbacks about my works so, being a newbie, i need every kind of advice...all considerations are welcome! I don't think i got all you said |O I've always read that for minimizing noise in sensitive audio circuits (i.e. mic preamplifiers with a lot of gain) i should go for a star ground topology. Here's what i mean: I've planned to wire all the grounds separately to a common large pad on the PSU board (the dotted line you see from the PSU board to the main board is the "GND_S", separate from "power" ground). The reasons I've always read associated with this practice are: 1) ground loops - as far as we have one ground connection for every board it's impossible to have a ground loop, as long as you connect the boards with just the signal wire; 2) high current return: PCB traces got a resistance value. If we've got high current flowing into the ground trace, for Ohm's law we "raise" the potential from our ground reference point. We could get rid of this second motivation just running two separate grounds: one for power and high current devices (and digital/impulsive/noisy stuff) and one for all the other components. I probably understand the reason behind treating a signal as a differential signal with his ground - it's about common mode interference, right? So I should take the ground coming from the input jack and routing it together with the input signal (and then join it with my ground PSU of course)? |
| JS:
Ground loops is a no no, of course, hence the resistor I told between the input ground and the circuit ground. Now, vumeter should be treated as noisy stuff, as switching and digital. Power output no, it's not noisy, it's signally! Ground there should be trated as precious signal, the previous stage referenced to it, it doesn't matter if the ground there isn't the same ground than at the power supply, that's just a concept, you need your consecutive stages to agree in the gound, so each stage should take the ground reference from the same place the previous stage output, then that stage from the previous one and so on. Probably the most adequate point to connect all the grounds to the power supply is close to the last stage bypass caps. It's very very important to understand and consider opamps as 5 terminal devices, not 3 terminal devices that need power, as current at the output will always come from one of the power rails, then come back from the load to ground and through the bypass cap to the rail again, this loop should be keept small and closed, taking the ground to the PS and back is bad, same for inter stage grounds. JS |
| ozlow.own:
OK! A 10 to 100 ohm between audio input and output ground and PSU ground seems to be the way to go. This will solve the ground loop problems that will occure whenever I'll connect the unit with a earth-connected device, preventing the ground loop (actually limiting the current in the ground loop, from what i understood). Yep, the actual star ground will be at (-) of the last bypass capacitors. I wired the LM1875 "output" ground separately because of the stability chapter in the amp datasheet (for high currents). I actually planned my ground layout from this: http://www.circuitbasics.com/design-hi-fi-audio-amplifier-lm3886#Designing-the-Ground-Layout |
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