Analog and digital grounds/supplies

[bigandy]

Good afternoon folks!

I have been trying to figure out the differences between analog and digital grounds, without much luck. I was hoping someone might be able to post a quick explanation as to how and why digital grounds are different to analog grounds.

I'm thinking it is something to do with having seperate references for analog circuitry and digital circuitry, in applications like analogie to digital conversion etc.

I was also wondering about analog and digital supplies (vcc) too. I have seen a few circuits that have vcc and analog vcc coming from the same power line, but only separated by 10uh inductor. I presume this is to filter the analog circuitry from the digital bits.

Anyway, if someone could post a quick explanation, or a link to some decent resources, i would be most grateful!

Cheers
Andy

[Comatose]

There's not really any hard and fast "analog ground" or "digital ground" unless you design them as such. The first thing to note is that there's not really a perfect ground anywhere in your circuit except the one point you call "ground" (negative battery terminal or whatever, its all just a matter of setting a reference point) Depending on what your circuit is doing, you ground network at any point may be up to a few tens of millivolts higher or lower than the actual ground. Digital circuitry is quite noisy. Signals start and stop abruptly. That makes the digital ground plane quite noisy. If you're trying to measure a signal in the millivolt range, and a common ground is jumping all over the place from the digital bits attached to it, you have to do some rather sophisticated band-aiding (err, i mean filtering) to get a good signal. So, in some cases, its easier to simply separate the ground planes. Separate VCC is the same way. They can both derive off the same source, so there it is mostly a matter of filtering. If your circuit draws 2A at 5v, and your analog parts draw 1mA of that, its easier and cheaper to put a nice filter network that supplies 1A than try to filter everything, and having noise injected from turning on MOSFETs or whatever.

So, basically, they're not different from digital grounds, they're just quieter. Whether you need to go to a dual ground scheme depends on the circuit. If you're just running an 8 bit ADC off of a signal with big amplitude and low impedance, then its a waste of your time. If you're running an analog feedback loop which operates on 5 millivolt signals inside a 200A power driver, then yes you'll need them.