In a ckt consisting of analog and digital devices why are the ground lines different?

 

Well, there can be several reasons. Probably the most common one is that digital Supply Vcc & ground can noisy. Separate from analog to avoid polluting "quite" analog supplies. Imagine a large digital circuit maybe a DSP for example running at high frequency. A lot of supply noise can be generated even with a careful layout. An analog circuit like an audio amplifier or high resolution A/D cannot tolerate this noise coupling into it. A few uV of noise can easily kill several LSB's of a 16+ bit A/D.

I'm sure others can contribute many more examples.

 

I designed and built a high voltage AC supply with LED to show when it was on. My prototype worked OK but in the production unit, the LED flickers off and on. I can't find a problem, but the layout mixes the PWM and digital grounds, which my layout did not do.

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