Vref for ADC.

KeithSloan · Post by **KeithSloan** » Fri Oct 12, 2007 11:34 am

I would like to measure voltage down to a resolution of millivolts. I therefore assume that I need a Vref of 1 volt. What is the best way of providing the Vref of 1 volt. Is a tap between a couple of resistors from 5volt supply good enough or is something more sophisticated required. How about current? or does one just use high valued resistors.

From reading the datasheets I assume I make Ra3 the Vref and set the appropriate control memory/register

Out of interest how low could one go with Vref. Obviously a 10 bit ADC means the resolution is Vref/1024.

Many Thanks Keith

Post by **Benj** » Fri Oct 12, 2007 11:58 am

Hello Keith

This topic in the Flowcode forum should help.

http://www.matrixmultimedia.com/mmforum ... php?t=3181

As for getting an accurate 1V. It may be easier to use a Zener diode or a regulator to acheive an accurate ref voltage. If you are using a resistor based potential devider then there will definatley be error in the resistance values and possibly error in the 5v regulated voltage.

KeithSloan · Post by **KeithSloan** » Fri Oct 12, 2007 1:55 pm

Thanks for the info.

I am a bit concerned about the bit that says

[NOTE: I wrote all of this without realising you cannot use RA3 as the Vref+ pin, but it's all still relevant to anyone who has Vref+ free. I will give you another answer in another post]

May I assume that this is a Flowcode limitation.

Any ideas how low one could go with Vref?
Can you get 1v Zener diodes? I thought there were limited voltage choices.

Many Thanks Keith

Post by **Steve** » Fri Oct 12, 2007 2:31 pm

The ADC component of Flowcode does not currently allow the use of Vref+ and Vref- pins. However, a simple edit of the FCD file for the chip you intend to use will allow you to use them.

As for the zener, Ben will answer that.

Post by **Benj** » Fri Oct 12, 2007 2:39 pm

I've just had a quick look at zener diodes and the lowest i found was 2.4V. This will still give you just over twice the accuracy that you would have with 5V.

Post by **Benj** » Fri Oct 12, 2007 3:17 pm

If you post of the exact PICmicro that you intend to use then I can provide you with the modification to use on the FCD file.

KeithSloan · Post by **KeithSloan** » Fri Oct 12, 2007 3:54 pm

Thanks

But I don't have use flowcode so not a problem as long as I can use Vref/RA3 from assembler

Keith

Post by **Benj** » Fri Oct 12, 2007 4:51 pm

Ah ok then yes that should work fine.

Let us know if you have any problems.