One of the most confusing concepts for junior design engineers is the difference between power, digital and analog grounds and how the placement of the analog components could affect the functionality of the system as a whole.

When a junior design engineer in my team complained about the irregularity of the potentiometer reading acquired by the microcontroller, my first suspicion was the grounding of the PCB and the correct placement of the analog components.

I’ll use the simple schematic below for the sake of discussion but the concept discussed is applicable to any similar design. There is a potentiometer connected to the analog pin of the microcontroller and powered by simple circuits of linear voltage regulators.

 

digital1

Now, technically the power ground, digital ground, and analog ground are referenced to the same point. So most inexperienced designers would produce a PCB design like this.

digital2

What happens is the analog component (potentiometer) is being placed close to the power components and digital components. Now, the power ground could sometimes be noisy, especially if the PCB is powered from a switching power supply. And the digital components often contain clocking signals or communication traces and that could easily affect the values of the analog signals.

 

It’s not surprising if you’ll get a fluctuating analog reading, as that was the case of my junior design engineer. Here’s what I suggested as a revision.

digital3

You’ll always want to start your design by placing the noisiest component first and the most sensitive components at the end of the board. While it’s true that the different grounds are ultimately the same reference level, you’ll need to keep those different ground joined at only one point. The reason is you have to take in the various current paths involved in the PCB. Current always find its shortest path and you’ll want to confine them to the specific ground they are designated.

Ultimately, my junior design engineer reported a stable analog reading after revising the said PCB as per this suggestion.

 

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