AVR causing glitches on Vcc and GND

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I'm building a mixed analog and digital circuit on a breadboard, using an AVR and a DAC. I have separate analog and digital ground planes connected in only one place, with plenty of 0.1uF and 0.01uF bypass capacitors.

However, it looks like the AVR is causing spikes on the supply and ground lines when certain pins change. The spikes are leaking into the analog ground and are quite noticeable in my output.

How can I either 1) prevent the digital part of my circuit from causing spikes on the supply and ground lines, or 2) completely isolate the spikes from the analog portion of my circuit? I put a 0.1uF bypass capacitor between the AVR's Vcc and GND pins, but that didn't seem to do much.

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Quote:
digital circuit on a breadboard
That sounds like troubles. :) IF you need to use breadboard then you will need to makes sure that the "ground" is pretty large and not just a flimsy track. Maybe use a few tracks or cover the track with solder to reduce it's resistance.

Also an inductor on the AVCC line is necessary as shown on the data sheet.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Yes, I know digital circuits on breadboards are usually bad news. I'm actually surprised that everything works this well :-p

I'm not using the ADC, so AVCC and VCC are connected together. Do I still need an inductor? I suppose adding one wouldn't hurt.

http://www.atmel.com/dyn/resourc... also looks helpful.

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I always use an inductor and capacitor with the AVcc supply. It's also a good idea to follow Atmel's PCB layout suggestions in that app. note. The AVR ADC seems to be a lot more sensitive to noise than other MCUs that I've used.

Leon

Leon Heller G1HSM