Low pass filter between Vcc and AVcc?

Forum 2005-2010 (read only) Hardware Development
1

Yet another question related to my embedded Arduino-compatible project.

In the datasheet for the ATMega328P, I just saw this:

AVCC is the supply voltage pin for the A/D Converter, PC3:0, and ADC7:6. It should be externally
connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC
through a low-pass filter. Note that PC6..4 use digital supply voltage, VCC.

From what I see, none of the off-the-shelf Arduino clones or copies implement this, even though indeed the A/D converter is "used" on many of them.

Is it worthwhile to implement this? If so, suggestions for how to do so?

2

Doing some more "light reading" on the AVR Hardware Design Considerations document:

It strikes me that most Arduino-compatible devices ignore most of the advice in these docs. ;D

As a mostly-ignorant self-taught hobbyist, should I take this as a sign that the suggestions in these docs aren't really that critical?

3

It strikes me that most Arduino-compatible devices ignore most of the advice in these docs

Not all of them :wink: If you look in my ZB1, NB1 or NB1A datasheet you
will see the circuitry I use for filtering The NB1A datasheet is at --- http://wiblocks.luciani.org/NB1/NB1A-index.html

Most Arduino applications do not require ADC precision so are fine without
the filter. Also most users probably aren't considering a variety of other
things that affect ADC speed and accuracy (e.g. source impedance).

A couple other thinks to look for ---

  • I also use a protection diode on the reset pin. The reset pin does not
    have an internal protection.
  • Capacitor selection for power components. The dielectric can have
    a major effect on stability.
  • Heatsinking of SMD components. Without sufficient copper area
    and copper thickness underneath power components you will not
    get good thermal performance.

(* jcl *)

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blog: http://luciani.org