how to separate power ground and signal ground in PCB

Thread Starter

annapurna

Joined Feb 11, 2016
19
how to separate power ground and signal ground in PCB, my power ground has 30A current return and signal of some mA. i need to refer the signal ground and power ground . how can i do that in PCB? if i want them to connect each other then what way the better to implement?
 

MaxHeadRoom

Joined Jul 18, 2013
28,617
If both have to be referenced to each other, ensure that the power common traces are as short as possible and centralized at one point as close together as possible to ensure all high currents are contained to the power circuit.
If you need to reference the logic common to the power common ensure it is done at one point.
Otherwise system separation is usually ensured with the use of opto couplers etc.
Max.
 

Thread Starter

annapurna

Joined Feb 11, 2016
19
If both have to be referenced to each other, ensure that the power common traces are as short as possible and centralized at one point as close together as possible to ensure all high currents are contained to the power circuit.
If you need to reference the logic common to the power common ensure it is done at one point.
Otherwise system separation is usually ensured with the use of opto couplers etc.
Max.
what if we use inductor between those two grounds to refer each other???
 

Sensacell

Joined Jun 19, 2012
3,432
Bad idea, don't use an inductor.

The entire point of connecting the grounds together is to provide a common, low-impedance reference point.
An inductor would raise the impedance at higher frequencies, not a good idea.
High currents can be problematic, but a bit of thinking about where the voltage drops are is all that's needed to get this right.

Connect the two grounds together at the lowest impedance point in the circuit, probably where the main power comes in. Use a single point.
 

Thread Starter

annapurna

Joined Feb 11, 2016
19
Bad idea, don't use an inductor.

The entire point of connecting the grounds together is to provide a common, low-impedance reference point.
An inductor would raise the impedance at higher frequencies, not a good idea.
High currents can be problematic, but a bit of thinking about where the voltage drops are is all that's needed to get this right.

Connect the two grounds together at the lowest impedance point in the circuit, probably where the main power comes in. Use a single point.
Thankyou,
Infact that's what we want .To eliminate noise,the impedance should be high at high frequencies ....
Am I correct?
 

crutschow

Joined Mar 14, 2008
34,280
Inductors (ferrite beads) can be used if you want to isolate the high frequency noise between two ground planes (such as digital noise from and analog ground, but if your concern is about keeping high power supply currents from affecting the signal, then the inductance may not be helpful.

The main thing is to make a single point ground connection such that none of the high currents go through the signal ground.

If you post a circuit diagram, we could make more specific suggestions.
 
If both have to be referenced to each other, ensure that the power common traces are as short as possible and centralized at one point as close together as possible to ensure all high currents are contained to the power circuit.
If you need to reference the logic common to the power common ensure it is done at one point.
Otherwise system separation is usually ensured with the use of opto couplers etc.
Max.
Hi Max
Correct me if I'm wrong but I think I now understand this correctly...
My example is an automotive application (12v input) with motor drivers, sensors, and a microcontroller (5v).
Best practice would be the following:
  1. In the schematic, establish 2 different grounds. One digital and one for the motors. In eagle or altium these will be 2 different "nets"
  2. In the layout, put the motor drivers down one end of the board, and create a ground plane corresponding to the net used in step 1.
  3. Put the microcontroller, crystal etc up the other end of the board, and create another ground plane corresponding to their net used in step 1
  4. Place the optocouplers on the top side of the board, spanning the gap between the 2 bottom side ground planes.
  5. Join the nets together with a single reasonably sized trace as close as possible to the incoming power connector.
The only part Im not certain of is where to put the 5v power supply circuit for the mirco? In my case I have a buck converter with some caps and a large inductor.
Im assuming the best place would be over top of the motor supply ground, also close to the power connector, that way I can route the 5v output on the top side of the board directly over top of the bottom side trace joining the 2 ground planes.
Ive attached a drawing to illustrate my plan, showing the bottom side ground planes in red.
Am I on the right track?
 

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