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joegtp:
I'm trying to build a simple logging multimeter using an Arduino's ADCs. The ADCs are 10 bit, running at 5 volts which would give me about 0.005V accuracy. This is assuming I have exactly 5V AREF. The question is how do I get a better AREF signal? Looking at the 5V power it jumps around quite a bit (relativly) which I assume throws off the readings. Also whats the best way to protect the inputs from the stray high voltage (maybe 10V)? I was thinking a 5V zener.

tyblu:
Those are some loaded questions! There are books dedicated to each... what complexity are you looking for? Maybe a few diodes for clamping.. What about the supply filtering, though? Best to figure out what noise is 'acceptable' to you. If you want less than 5V/2^10=5mV noise, you are entering realm of linear supplies with active filtering stages; good ones have <1mV; ok ones get 10mV. Not that I actually know what I'm talking about, but this is the idea...

EEVblog:

--- Quote from: joegtp on February 01, 2011, 03:53:45 am ---I'm trying to build a simple logging multimeter using an Arduino's ADCs. The ADCs are 10 bit, running at 5 volts which would give me about 0.005V accuracy.

--- End quote ---

No, that's called resolution, not accuracy. The two are not the same. Very commonly confused.

--- Quote ---This is assuming I have exactly 5V AREF. The question is how do I get a better AREF signal? Looking at the 5V power it jumps around quite a bit (relativly) which I assume throws off the readings. Also whats the best way to protect the inputs from the stray high voltage (maybe 10V)? I was thinking a 5V zener.

--- End quote ---

No, zeners are hopeless voltage references.
You need a proper 5V reference chip, but it needs to be powered from more than 5V.
The accuracy and stability of the reference will depend upon how much you pay.

Dave.

joegtp:
Thanks for the replies.

Looking at DigiKey it looks like a TL1431CZT (http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-7665-1-ND) Voltage Ref (±0.4%) would do the trick. I'm guessing I just feed the AREF with that and use the 7805 for everything else.

I was thinking of using the 5V zener to protect my ADCs from unwanted high voltages.

So are there any good places to find information on how a modern auto-ranging multimeter works. For my purposes, the above should work well but I'm curious how the auto ranging, input protection, etc works.

allanw:
A 5V zener will affect your readings since it'll divert noticable current away from the input pin at voltages around 4-5V. You'll need a higher voltage zener which won't provide as much protection.