PCB – part 3 of X, Grounding…

We have this far covered two aspect of signal integrity. Ground bounce and clock distribution.

I think the subject of grounding is worth some attention. I have already talked about the need for ground planes. But how to design ground on a PCB where you have digital ground and analog ground? And does a ground plane solve all grounding issues on my PCB?

First of all, the very most signals on a circuit board are single ended. They all refer to the any of the grounds in the design. Kirchoff’s Law tells us that at any point in a circuit the algebraic sum of the currents is zero. This tells us that all currents flow in circles and, particularly, that the return current must always be considered when analyzing a circuit. The return current in single ended systems is flowing through the ground. Having that said, if the impedance of the ground is too high, such return currents will create voltages that especially in the analog part will degrade the S/N ratio of the analog signals. When designing analog filters, knowledge of return currents can be vital. A least if you are doing a real high performance design. The figure below illustrates two cases of return currents. The decoupling of an IC and the return current of a switching signal.

Grounding

 

So what can we do to minimize the problem with return currents causing noise in our system? There is a lot we can do. The most obvious is of course to use a low impedance ground plane.  This minimize voltages across the ground plane. Partitioning in digital and analog sections will reduce the amount of digital and high noisy return currents in the analog domain. By only having one point of the PCB connected to other parts of the system the risk for currents flowing through the PCB ground caused by external currents is avoided.

The decoupling capacitors actually ensure that switching currents from digital circuits is not flowing in a wide circle back to the regulator. This would cause a very noisy environment for other circuits of the board. Decoupling capacitors should therefore have low impedance at the frequency range is the generated noise and be located as close as possible to the power and ground pin of the IC. If there are several power pins/balls, if they are separated with other connections in between them should have decoupling capacitors.

About the connection between digital ground and analog ground. I many cases it can be a good solution to have one point of connection between the grounds. But remember that this point must be low impedance as this is the only return path for return currents between the analog and digital domains.

/Anders

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