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Old 10-18-2014, 06:22 PM   #1244 (permalink)
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Originally Posted by e*clipse View Post

Thanks for the suggestions!

Yes, the bottom layer - I'm trying to make this a 2 layer design - is primarily the ground, +5V supply and a few signal lines. The boards output to the mainboard run on the bottom - I'm not happy with that because of the proximity to noisy stuff.

If I make the board a 4 layer board, there will be plenty of room for a ground plane and even a bottom layer heatsink. Maybe it's worth it just to pay the extra $$$.

Regarding the mounting hole - yes; I'm working on that. Right I'm generally unhappy with it because of that spacing issue and because it needs to be a relatively small (3mm) bolt that's made of some non conductive material. The reason for this is the matching hole on the motor controller will have to fit in that space between the controller and driver boards. I want to make sure that the fastener isn't the cause for any shorting situations.

I really like the idea of using the heatsink for a fastener - I'll see what I can do there. A relatively large, thick piece of copper would be great for this. BTW, the thermal loading on the output amplifier is pretty constant. Actually the quiescent current (just running the IC, not amplifying anything) is the primary load.

Two questions regarding grounds:

On this board, there are actually two grounds - One for the +24V supply and one for the +5V supply (with is the majority of the board) Is there an issue with connecting the two grounds? Should I put a resistor or some protection device between them?

If I make a ground plane, out of the ground connections (half of the thick blue traces) and the remaining board space, what's the possibility of this becoming a noise "antenna"? How much space should I leave between signal traces and this ground? (for noise issues).

Thanks again,


Typically the grounds are isolated in the following conditions:

Current measurement;
Galvanic isolation;
Analog and digital logic;

I assume the first is N/A here, so ask yourself if you need electrical isolation. If not, a single ground would be better at shielding noise, that two separate.

The last one is analog and digital logic. I assume most of it is digital, so wont imagine many conflicts there, but review as I am not completely familiar with the design. Generally tough, if this is the case, the positive is isolated instead trough a low value 4.7R resistor or an inductor to cut HF noise, along with some nice, low ESR, ceramic decoupling capacitors.

Isn't the board close to the motor? If so grounding the motor will automatically create a "Faraday cage effect" and better yet if you share both grounds (motor + PCB + Vehicle) unless the board needs electrical isolation, but this is another issue you should avoid, because again, if the GND is shared with the vehicle it will work for you at cleaning noise rather than being an antenna, not to mention the risk for an isolated system if a cable gets caught and makes contact with some path in the vehicle, effectively breaking the isolation barrier.

Ground should be as close as possible to your signals. The larger the gap, the larger the airgap and generally the more coupling (noise) from nearby conductors

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e*clipse (10-19-2014)