I bought the following ADC module and I want to use it with Arduino:
http://www.analog.com/media/en/reference-design-documentation/reference-designs/CN0335.pdf
It states that the supply should be 3.3V. I have checked the individual components and all of them can handle 5V.
Can I supply 5V into the module and will it still convert -10V to +10V analog to digital?
Or can I supply 3.3V and for SPI communication use 5V?
Good question.
The input voltage is the voltage for the ADuM5401 isolator.
That input voltage can be 3.3V and 5V.
When you choose 5V, you have still a choice to let the isolated part of the circuit run at 3.3V or 5V. Every part will also run at 5V as you already figured out.
Are you using a 5V Arduino board ?
I suggest to use 5V for the ADuM5401, and let the isolated part of the circuit run at 3.3V like the Circuit Note CN-0335.
I assume that there will be some kind of signal adaption in the isolator, so that the 3.3V isolated signals will match the 5V SPI signals of the Arduino. I don't understand the datasheet of the isolator very well, it should be in the section "ELECTRICAL CHARACTERISTICS 5V PRIMARY INPUT SUPPLY/3.3V SECONDARY ISOLATED SUPPLY".
This module has the ADuM5401 Vsel grounded on the circuit board, so will it only provide 3.3V to the isolated converter section (as it must). As Peter_n said, the ADuM5401 can take 5V in and provide 3.3V out.
Is it ok, however, to provide 5V to the module via J1? There isn't any issue with the ADuM5401 and the isolated converter section, but does anything else on the J1 side care? The application note doesn't show the actual schematic, and I get a server error when I request the "Design and Integration Files". >:(
From looking at the product photo, it appears that there are two ceramic decoupling caps, the ADuM5401, and a LED+resistor power indicator. I would guess that it's ok to provide 5V, although the LED might burn out (the 75Ω current-limiting resistor is sized for 3.3V). I would be surprised if the decoupling caps are rated less than 6V, so they should be ok. But that's just a guess...
Does anybody else have access to the "Design and Integration Files" from this AD page? Otherwise, you could try an email question.
Cheers,
/dev
I can download it and see the schematic.
The input voltage has a 100nF 16V and 10uF 10V capacitors. That's all.
Great, it should be ok to provide 5V to the module instead of 3.3V, with the exception of letting the magic smoke out of the LED power indicator. 
If you're up to SMD soldering, you could replace the 75Ω resistor with a 220Ω or 330Ω resistor, or even remove the LED and resistor for less current consumption.
It appears that the DC-DC converter efficiency for 5V in/3.3V out is slightly less, but the isolated ADC section is very low power, about 3mA. Actually, the ADuM5401 can provide more current with a 5V input, up to 90mA instead of 50mA at 3.3V input.
BTW, it does do the logical level conversion internally for the SPI signals: 5V on the Arduino side to/from 3.3V on the isolated ADC side.
Cheers,
/dev
Perfect. Thanks for help. I will test it out today 