Can I input to Arduino analog or digital from a separate DC supply with common ground and following boards parameters 5v or 3.3v? As in using voltage divider with thermistor 3.3vdc and reading center on A0 on Arduino board or a digital TTL circuit with 5v logic output to digital pin on board. What is the best practice? Thanks!

 

As long as you have common ground, I do not see any problems.. but again, I'm just a stranger on internet forum...

 

Can I input to Arduino analog or digital from a separate DC supply with common ground and following boards parameters 5v or 3.3v? As in using voltage divider with thermistor 3.3vdc and reading center on A0 on Arduino board or a digital TTL circuit with 5v logic output to digital pin on board. What is the best practice? Thanks!

With same supply voltage on both; you only need a solid common ground.

Feeding 5V outputs to 3.3V inputs can be clamped using a Zener and current limiting resistor. That can be tricky with fast signals though.

stepping 3.3V outputs up for 5V inputs usually involves a common emitter transistor and pull up resistor (LL MOSFETs often work better), but that inverts the signal - by the time you've added a second transistor to invert it back again, you might find an off the shelf level converter chip takes less board space.

 

The first thing you should care is that what voltage apply to Arduino which is 5V or 3.3V, if the apply voltage is 3.3V then the input voltage can't be over 3.3V, so you have to use the voltage divider to attenuates the input voltage Vmax=3.3V.

The resistors of voltage divider will be like this:
The scale of 3.3V to 5V is S3.3v=3.3V/5V=0.66=66%
So 3.3V(66%) can be choose 330Ω+330Ω to get 660Ω, and the 1.7V(34%) can be choose 330Ω+10Ω to get 340Ω.

If the Arduino is apply +5V then no needs the voltage divider unless the input voltage is over 5V.