Grounding options between three PCB's - Page 1
Forum section: electronics / general

author: wacky_aki
author_rank: Newbie
timestamp: May 05, 2016, 04:45:05 pm
content: I have a PCB (let's call it PCB #1) that has a Li-ion battery as its power source, sourcing 12V to 17V. The power from the battery is used to step down to 6V using a buck converter on PCB #1. A cable connection sends the input power (12V-17V) to the second PCB (let's call it PCB #2). The 12V-17V is regulated to 12V using a non-isolated DC-DC converter. Both the 6V from PCB #1 and the 12V from PCB #2 are sent (using cables) to the third PCB, PCB#3. PCB #2 is attached to the PCB #3 and thus is in close proximity to it.Aside note: PCB #1 and PCB #2 are "set in stone" and cannot be changed.A 5V DAC sends two outputs to the 12V circuitry (used as control signals for two HV DC-DC converter modules).Currently I have a copper pour for each and they are connected at one spot using a net tie near the connector on the Top Layer. The 5V return is shown in the top image as dark green; the 12V return is shown in the bottom image as light green.My question is whether the 5V return and 12V return on PCB #3 should be joined together or should they NOT be connected? I'm worried that by connecting them I would be creating a ground loop.Which one of these options would be the best logical solution?(a) simply join them together and instead of having GND-12V and GND-5V, I'll have GND?(b) join them at one spot using a low-impedance connection, i.e. using a 0 Ohm resistor, net-tie (as I have now), or a jumper?(c) join them at one spot using a resistor? (I have personally never done this but it has been done)(d) join them together, but keep them separate with a slit in the ground pour/plane?(e) should I separate them from each other on PCB #3?Thanks in advance.

author: Neilm
author_rank: Super Contributor
timestamp: May 05, 2016, 06:50:42 pm
content: Personally, I have always found that splitting the 0V causes problems - especially if you are meeting EMC regulations. The only reasons I have ever found for doing so are safety and if measuring low currents (<1nA).Look at the current flows in your circuit. You are indicating that the DACs are going into the 12V circuit, so assuming there is no isolation between them the current will flow from the 5V rail, through the DAC into the 12V circuit. It will then go to the return of that device which will the the 0V(12V). The current will then have to flow through the other two boards and cables to get back to the return for the 5V

author: wacky_aki
author_rank: Newbie
timestamp: May 05, 2016, 07:10:23 pm
content: Yes, you are right - there is no isolation between the 5V DACs and 12V circuitry. However, I did mention that there is a net tie (connection between the two grounds) at one point near the connector (on PCB #3). The return current of the DAC would have to flow through that one point to get back to the return for the 5V.So you are advising against this and saying to simply use one ground?

author: Mr.B
author_rank: Unknown
timestamp: May 05, 2016, 07:25:28 pm
content: I am no expert, but the way I see it is:The current return path for the 5v section of PCB#3 is to PCB#1.The current return path for the 12v section of PCB#3 is to PCB#2.PCB#1 and 2 have their grounds tied together.The circuit has current return path integrity.You have said that the 12v section is HV DC-DC converters.If there is any chance of inducing high frequency noise into the 5v section then I would not join the two grounds on PCB#3.Or perhaps you can join them through an inductor to allow only DC current to pass.

author: tautech
author_rank: Super Contributor
timestamp: May 05, 2016, 07:36:50 pm
content: Be sure to have plenty of local bulk capacitance on the PCB's that are remote from any of the PSU's.That alone will save some headaches.

author: wacky_aki
author_rank: Newbie
timestamp: May 05, 2016, 08:10:49 pm
content: Quote from: Mr.B on May 05, 2016, 07:25:28 pmThe HV DC-DC converters were the reason I connected the two grounds at one point only - at the connector and away from 5V circuitry.Wouldn't the addition of the inductor between the two grounds possibly cause ground bounce?

author: wacky_aki
author_rank: Newbie
timestamp: May 05, 2016, 08:12:10 pm
content: Quote from: tautech on May 05, 2016, 07:36:50 pmI definitely included plenty of bulk capacitance.