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Topic: low-pass filter at AVcc (Read 8618 times) previous topic - next topic

erocha.ssa

Hi,

Let me make that clear: I am not an electrical engineer, but I am a curious person. So,

I was wondering why I need to put a low-pass filter at AVcc so that I can use the ADC (the AVR's manuals say that). I mean, wouldn't low frequencies affect the ADC measurements too? Wouldn't a solid voltage at AVcc be better? So, as I said, I am not an engineer, but I'd really appreciate if someone could give me some pointer on that.

Thanks in advance,
Eduardo

CrossRoads

The low pass filter is to keep any noise on the VCC line from going into the AVCC pin if wired up per figure 23-9 on page 258.
This helps keep any switching noise that is picked up out of the conversion process to yield better results.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

erocha.ssa

So, I suppose switching only generates high frequencies, right?

Btw, figure 23-9 on page 258 of what manual?

Eduardo

CrossRoads

23-9 of the ATMega328 data sheet.
Summary and complete datasheet available from atmel
http://www.atmel.com/dyn/products/product_docs.asp?category_id=163&family_id=607&subfamily_id=760&part_id=4720


Yes, switching generates a lot of high frequencies. The inductor only wants to let DC thru, so it blocks the high frequencies, and the capacitor lets the lower frequencies go to ground as it wants to let AC thru.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

graynomad

#4
Feb 09, 2011, 02:12 am Last Edit: Feb 09, 2011, 04:42 am by Graynomad Reason: 1
I saw a comment on AVR Freaks a while back to the affect that the filter caused more problems than it fixed. What they meant I don't know.

Certainly Atmel recommend using the filter, OTOH I hardly ever see a design with it, most people just decouple AVCC.

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

CrossRoads

Do most folks connect AVCC to VCC? Or just go with the internal 1.1V reference?
I've only done digital with the Atmega's since I started using them in August.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Coding Badly

Quote
23.2 Overview
... The voltage reference may be externally decoupled at the AREF pin by a capacitor for better noise performance ...


That's all I've ever needed.  But I don't have a lot of digital noise to deal with (just the processor).

Or, I've used the ~1.1V internal reference.

CrossRoads

Ok, I think I had myself confused earlier.
The Duemilanove schematic clearly shows AVcc being connected to Vcc (the Promini shows this also), and a decoupling cap. ARef is connected to an IO pin and has a decoupling cap.
So I guess if AVcc is connected directly to Vcc, it would be difficult for us as users to get a 10uH inductor installed per the ATMega datasheet.

Not sure now where I was going with that, so I am not sure if I satisfied myself with anything or not!
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

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