Go Down

Topic: Does Aref and Avcc need voltage if ADC won't be used? (Read 1 time) previous topic - next topic

Loudhvx

Mar 10, 2011, 12:41 am Last Edit: Mar 10, 2011, 12:42 am by Loudhvx Reason: 1
This came up in another thread, but it's really a different topic and I didn't want the other thread to wander off topic.

In the standalone tutorial http://www.arduino.cc/en/Main/Standalone , it has the Aref pin hardwired to 5v.  However, in this tutorial about analog reference http://arduino.cc/en/Reference/AnalogReference, there is a warning against this.  

If the default reference of 5v is to be used for AD conversion, isn't the 5v reference already there with nothing connected to the Aref pin? If so, then you'd really never need external 5v on the Aref pin, correct?


Graynomad

#1
Mar 10, 2011, 01:36 am Last Edit: Mar 10, 2011, 01:39 am by Graynomad Reason: 1
AREF should be decoupled to GND if not used as an external reference. Tying it to a hard voltage may cause a problem if the internal references are used (which is what 99% of people do).

Quote
In the standalone tutorial

I would say that's wrong, if you tie to 5v and then select 1.1v internal you have a short between 5 and 1.1v. Not good.

______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

westfw

AVCC should always be connected to the power supply.

Loudhvx

Alright, thanks.

So Aref should be, at minimum, separated by a resistor (5k or higher).

If Avcc should always have power, should it always basically be connected to Vcc? Or is there ever a time it would want it's own power (different voltage or noise immunity)?

Graynomad

Quote
should it always basically be connected to Vcc?

That's the norm. If you're picky about noise you can connect through a coil to provide an LC network.

Using a resistor would drop a small amount of Vs (you'd have to know how much current the ADC draws), I doubt it would matter but I can't see much point.

Maybe you should tell us why you want to fly in the face of convention  :)

Quote
Or is there ever a time it would want it's own power (different voltage or noise immunity)?

I've thought about doing that, using a precision regulator or even a voltage reference if the power required is small enough. However you have to be careful that VCC and AVCC are always within 300mV of each other. I'm not sure what happens if that aren't but I suspect it's not good.

______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

westfw

Quote
is there ever a time it would want it's own power (different voltage or noise immunity)?

It is suggested (Section 23.6.2 "analog Noise Canceling Techniques") that there be an inductor between Avcc and digital Vcc to help filter out the digital noise.

Loudhvx


Quote
is there ever a time it would want it's own power (different voltage or noise immunity)?

It is suggested (Section 23.6.2 "analog Noise Canceling Techniques") that there be an inductor between Avcc and digital Vcc to help filter out the digital noise.

Ahh. Thank you.

Loudhvx


Quote
should it always basically be connected to Vcc?

That's the norm. If you're picky about noise you can connect through a coil to provide an LC network.

Using a resistor would drop a small amount of Vs (you'd have to know how much current the ADC draws), I doubt it would matter but I can't see much point.

Maybe you should tell us why you want to fly in the face of convention  :)

I mentioned the resistor for the Aref pin, not the Avcc pin.

Go Up