ST32L476 use of VREF+

[scott239955_stm1_stmicro]

Posted on February 02, 2016 at 17:01

Thanks for any help. I'm confused on how to connect VREF+. I see that it can be used as an reference voltage output, and I now have it as No Connect. VDDA is connected to its pin, which seems to provide the voltage reference needed for the ADC.

So, my question: Is this enough for the ADC to work (VREF+ No Connect)? We're about to roll with boards, and I'd rather not do an ECO at this point, but I'd also rather have it work :)

[waclawek.jan]

Posted on February 02, 2016 at 17:29

Even if you intend to use the internal voltage reference, the datasheet requires you to connect a typ.1uF capacitor (see ''VREFBUF characteristics'').

You should've read also AN4555.

JW

[scott239955_stm1_stmicro]

Posted on February 02, 2016 at 18:39

Thanks JW.

And if I want to use VDDA as the reference voltage (and not use the internal), do you think I can just tie VREF+ to VDDA? What would the consequences be -- some potential noise on VREF? Pardon my ignorance, this is my first foray into digital. 

Cheers,

Scott

[waclawek.jan]

Posted on February 02, 2016 at 22:40

> And if I want to use VDDA as the reference voltage (and not use the internal), do you think I can just tie VREF+ to VDDA?

Of course you can - it's what ST does on smaller packages, anyway, inside the package.

> What would the consequences be -- some potential noise on VREF?

Noise and fluctuations (which at the end of the day is the same, at different frequencies). Absolute precision can be checked/compensated using the internal reference.

VDDA does not source heavy logic - that's what VDD is for - so it may be almost as good as a separate external reference, if VDDA is properly designed for. Don't forget about quite grounds, too, especially VSSA. Have you consulted that appnote?

Also be prepared for quite a significant noise in the ADC itself. Read thoroughly the values given in the datasheet, these add up to the proportion of VREF noise.

> Pardon my ignorance, this is my first foray into digital.

But, isn't ADC as analog as it gets, in the realm of microcontrollers? ;)

Jan

[scott239955_stm1_stmicro]

Posted on February 05, 2016 at 22:15

Funny!

Thanks for the help Jan.