question about ground lines

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

electronist said:

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

Click to expand...

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.