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Project Help and Ideas » External Voltage Reference

July 13, 2010
by akschu
akschu's Avatar

Nerdkit users,

Anyone know how to wire up an external voltage reference and which one to use? I need to sample voltage on a ADC pin but I need it to be VERY accurate. Using the voltage regulator for the voltage reference isn't going to cut it. Can I use this device to provide a 5v reference:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=REF195GPZ-ND

If so, how would I program for it and how would I wire it up?

Thanks, schu

July 13, 2010
by hevans
(NerdKits Staff)

hevans's Avatar

Hi akschu,

The chip you linked to is a volage regulator that has the same basic functionality as the 7805 included in the NerdKit. It is just more accurate and has a low drop out voltage, which means it can regulate smaller voltages down to 5V well. Whereas the 7805 requires at least 7V to regulate down to 5V effectively, that chip would be fine with 5.5V and can likely go lower. There is no programming needed for it, you would just wire it up similar to the way you did the 7805 by following the datasheet.

To wire up an external voltage reference to the ADC you need to get your accurate voltage to a AREFF pin of the MCU, this is the reference voltage against which the ADC measures. Before going out and buying a chip like the one you linked to though, you might want to just try putting a low pass filter on the AREFF pin it might be stable enough for you to get the accuracy you need, particularly if nothing is loading the power supply too badly (i.e. you are not running a motor, or blinking 100 LEDs at the same time).

Hope that gets you started.

Humberto

July 13, 2010
by mongo
mongo's Avatar

The reference chip you mentioned should work just fine. If you use it in place of the +5V reference the NK programs already have the facility for, there shouldn't be any actual programming changes. Just a little wiring alteration is all that you will need to accommodate.

July 13, 2010
by akschu
akschu's Avatar

Thanks for the replies.

I really want the voltage to be within +-0.05% accurate which on 4v only allows for +-.002v.

If I can get even more accurate without significant cost or complexity then even better.

It seems to me that the only way to get this level of accuracy is an external voltage reference as the loads on the main voltage regulator would cause it to drift within my tolerances. Is that correct?

The part I linked to above is accurate to +-.01v which is close, so I think that I could go that route and calibrate it in software. Is this a good way to go?

Also, I'm finding my meter is not up to the task, any comments on this meter:

http://www.tequipment.net/Protek608.asp

Seems to be pretty accurate, does true RMS, backlit display, etc. It seems the only way to get better is to start parting with some serious money.

Recap:

  1. Do I need an external reference to get the accuracy I want?
  2. If I get something close, can I calibrate in software?
  3. Is the Protek 608 a decent meter given that it's specs are MUCH better than a fluke that costs 50% more.

Thanks, schu

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