I'm building a project that is going onto a project board. I've prototyped the project on an Adrduino Duemilanove and am now in the process of moving the chips to the project board for permanent installation.
I'm following the great instructions from Physical Computing at ITC for building an Arduino on a bread board.
The catch is, apparently if you're going to use ADC (which I am), you need to hook Pin 20 (AVcc) to power through a low pass circuit. And the instructions above doesn't provide such guidance on how to do that.
Can anybody point me to how to build such a circuit? Is it easy to do?
How necessary is the low pass? I've got a separate regulated power supply providing 20A of 5v and 6A of 12v DC for the project. Can I get by without the low pass??
Time is of the essence unfortunately as the project is due in a few days. Any help is appreciated. Thanks.
You only need a low-pass filter circuit if there's significant analog noise on the power supply rails. You can check this with an oscilloscope if you have one. To make a filter, put a small resistor in series with the power (from Vcc to AVcc) and a capacitor from AVcc to Ground. A resistor of maybe 10 Ohms would be a good starting point and 10 microFarads for the capacitor, with maybe a 0.1 uF ceramic capacitor in parallel. But you may need to experiment to find a good combination of values. Alternatively, you can put an inductor in series with the power supply, but once again the value of that component is hard to predict.
Most designs just wire the VCC to the AVCC and mount a .1 mfd cap from ground and the Avdd terminal, as close to the AVCC pin as possible.
This seems to be good enough for the 10 bit A/D resolution and accuracy specifications for the AVR analog section. If you were dealing with a 12 or 14 bit or better A/D convertor then more attention to the Avcc and Agnd might be justified.