ground plane question

hi everyone. so i'm trying to use a DC 24V 225mA power supply to power a AD595 thermocouple controller. I've been told that i should keep the logic and the power supply ground separated.

but i wanted to run the arduino off of the 24V supply stepped down to 5V using a 7805. is it okay to make the grounds common? or should i have 2 separate ground planes? (don't want to have a current spike at the arduino ground pin

and if 2 ground planes exist, will the analog signal out of the AD595 float significantly? and would the 7805 be tied to the power supply ground, or the arduino ground?

thanks for the help

i was thinking i could stick a 50mA fuse at the ground between high voltage ground and common... is this a good idea?

is it okay to make the grounds common?

It won't work if you don't. Keeping the ground planes seprate is not the same as keeping the grounds seprate. What it means is ensuring that any large ground currents don't flow in the same path as you signals. However they need to be joined at one point.

Generally fuses are a waste of time in electronics.

In it's documentation, Atmel promotes putting up 'trenches' around noisy ground plane areas, such as where the oscillator is located. So, while the ground plane is shared, the area around the oscillator or resonator only connects in a small area near the ground pin next to the 328P. Similarly, for high-resolution ADCs, you apparently must think carefully about how adjacent digital circuits and other elements may be influencing the signal (because the sensitivities are much higher on a nice ADC than the 5mV that the Atmel/Arduino may or may not resolve to).

I'd suggest looking for application sheets on the chip you're interested in or write to the manufacturer re: reference designs that they might be willing to share. For example, many manufacturers will offer eval boards that can be a great place to start / emulate.