Electronics > Beginners
Help for LM1875 PCB layout
JS:
Again, no.
Star grounds is a safety concept used by electricians, in electronics isn't any good, just brute force to make it work without giving s***t about it, not an optimal solution.
First you are not making a 40W amp, still good grounding should be done. Then, what that article says is that high current ground path shouldn't cross low current ground path. That means a high current path shouldn't share conductors with low current path, but the high signal current is closed at the output stage bypass caps, not at the power supply.
If ground goes to the power supply with a wire and come back with a separate wire for high current and low current you are actually sharing more path than connecting them together at the pcb, as all the coming back from power supply to pcb at the high current side has high current for whatever your bypass didn't cover and that polutes more than a short trace putting together both at the pcb.
You will have to wait the drawing, I've been away from the pc since this topic started, if I can't find it I can make it again so to explain, and save it better this time as it's a discussion that always pops up.
JS
ozlow.own:
I'll wait for the pic then, I'm definitely not on it. Thank you for patience!
Relayer:
Hello ozlow.own,
You really need to read the data sheet of the LM1875.
Here's a portion I picked out for you:
When designing a different layout, it is important to
return the load ground, the output compensation ground, and the low level (feedback and input) grounds to the
circuit board ground point through separate paths. Otherwise, large currents flowing along a ground conductor
will generate voltages on the conductor which can effectively act as signals at the input, resulting in high
frequency oscillation or excessive distortion. It is advisable to keep the output compensation components and the
0.1 μF supply decoupling capacitors as close as possible to the LM1875 to reduce the effects of PCB trace
resistance and inductance. For the same reason, the ground return paths for these components should be as
short as possible.
You have to remember one thing, this IC has been around for quite some time, therefore
people who have produced this amplifier using this IC, have had excellent results without
the need for modern ways of producing it (i.e. Old school ways).
I hope the above helps.
Regards,
Relayer
JS:
Well, here it is!
I use the image on this post https://groupdiy.com/index.php?topic=59590.msg756894#msg756894
You could check the explanation there and ask again, I mention 8 bypass caps, because the discussed design used two output opamps as a differential drive, so paralleling 2 caps for each position, 2 opamps, 2 rails add up to 8 caps, call it two in a simpler design as the pict. Seen it from now, 6 should be fine as the bigger ones could be shared and further away from the opamp, but still between output and load as I'll explain.
There it is an inverting stage, but look how it looks like a differential amplifier, if rG1 and rG2 were actual resistors and not parasitics of the PCB. If this is the last stage the PS ground should be connected to the bypass caps and rG1 to the a previous stage's bypass caps. Bypass caps might bring noisy ideas to your mind but they are the ones that carry the signal current from rails to ground, so think them as signal decoupling caps, as ac coupled current signals, going from opamp "vcc/vee output" to the load or receiver reference ground.
If it were a non inverting amplifier, swap the input with the rG1-rG2 node, leaving the non inverting input to the input as it should and the resistor from the inverting input to the rG1-rG2 node.
With this you should see how the ground follows the signal, if your ground is messy you might get crossed resistances along those 3 and even some between this circuit and adjacent ones. Then, how to integrate this to a ground plane is kind of tricky, but think in component placement so it behaves like this, leaving the non inverting input closer to the input and the bypass caps between the opamp and the output, close to the opamp, and close to the output signal trace. As I explained in the original post this is how you squeeze the best THD+N figures of the given opamp/circuit.
JS
PS: along this topic I messed with decoupling caps and bypass caps, calling decoupling to what it should been bypass, so did the guy who wrote the datasheet. Decoupling os for AC coupled signals, bypass is from rails to ground, close to the opamp as a dedicated current path.
ozlow.own:
Hi Relayer!
Thank you for your contribution. I've actually seen some existent layout for the LM1875 looking for a guideline: many of them use a ground plane or a "star ground" wiring. Seems that the latter one (and also the first one, as seen as can easily mess with high current return path) are not really ok, as pointed out by JS.
JS, thank you for your pic! I'll have to slowly read your topic on GroupDIY. Then I'll be back for some other questions for sure.
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