PCB Layout for Multiple PCBs (Current Loops etc...) - Page 1
Forum section: electronics / projects

author: David
author_rank: Frequent Contributor
timestamp: October 10, 2015, 06:58:04 pm
content: Hi all,I'm currently working on a design and have a dilemma regarding best practice..Looking at the attached image, there are two separate PCBs connected via 3 connectors. CON1 and CON3 have high speed signals on them. Natrually I would put many GND return paths on each connector (at least 1 GND pin per signal). However, what should I do with power that must cross between boards? Do I make a star point by only taking the power through the middle connector (CON2) or have I just produced a current loop for signals passing over CON1/CON3? If so, would it be best to also pass power through all 3 connectors?Thanks for any help,Dave

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 11, 2015, 12:25:54 am
content: For DC, as you might suppose, grounds at the power connector won't matter (except if the boards are ever different, and have options for different pinouts/connectors/assemblies, so that you'd need the common denominator still).For AC, "signal quality" of the power pins still matters, though not in the transmission line sense that applies to the logic signals. You'll inevitably have AC due to varying loads and switching supplies, and this needs to be grounded locally, through the connector. Otherwise you get a much larger ground loop, and EMI (the slot between ground planes becomes a loop antenna, with the AC supply ripple connected directly in series with it!).If you have no sensitive DC signals, then controlling DC ground loops doesn't matter. A few millivolts of drop, here or there, doesn't matter.If you do, you should probably make them differential anyway (i.e., sensing the remote ground level), so the DC measurement isn't subject to local errors (whether due to connector ground loops, drop across the plane, or other means).Tim

author: Rerouter
author_rank: Super Contributor
timestamp: October 11, 2015, 01:46:48 am
content: There is a lot of stuff to keep track of if you want to make something as robust as possible, but its not too hard to learn the rules,First and foremost, think about where your currents will want to flow, If its DC, its in a straight line, if its AC, as the edges get faster proportionally more will want to stay directly under the signal line, And as the currents involved get bigger, you will want to keep them better separated from the rest of the circuit,High speed or high current traces near high impedance circuits should be avoided, it doesn't even have to be a sensitive circuit, if you have the 2 traces close enough for long enough you could flip a digital input to another state when not driven,For power between one module to another, you have more than one choice, First is the normal convention, a ground plane and both connectors tie to it, now depending on the currents involved this will induce a voltage gradient across the board depending on which way current is flowing,Next option is to make a psudo star ground, pick which end is your power source and which is the piggyback to the next, run a separate ground and power trace from the input connector to the piggyback, in my head this would involve the ground skirting the edge of the board and the power directly above it to reduce inductance, You then flood fill the rest of the ground plane, making sure its only on 1 side of the piggyback ground trace, and only connects to the input ground pin, (no other traces should cross the power or ground trace or it will couple back in)There are other things like split planes, but i dont like them,For EMI, just pay attention to where you current is flowing, treat as much as you can like a high speed signal, even your power signals, the ground current wants to follow directly underneath the signal, try and help it do that, if you manage that, then there is less ways noise can get in or out, also look out for when DC currents and AC currents cross, its ok if there small, but when they get big, try and isolate them,