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Adc noise when powered by usb


#1

Noise now I have. Using a FEZ Domino I’m digitizing an audio signal from a microphone on AD0. When powered from USB the digitized signal is quite noisy with peak to peak noise spikes of several volts. If I switch the power to an external 9 volt supply the noise reduces to a reasonable level of about 100 millivolts p-p with one periodic spike around every 100 ms.

I will probably be able to live with this behavior as the real data collection will occur when the system is on external battery power, but I’d like to eliminate any noise that can reasonably reduced. Right now I have the other analog inputs floating. Should unused analog inputs be tied to ground or vcc? Any other best practices for reducing noise?

Thanks for any advice.


#2

LOL. ;D I like Yoda.

Indeed you must. :wink:


#3

If you are not using other pis as analog then you may not need to do anything for those pins.


#4

Hi,

You can always coarsly filter the DC lines easily (since it is DC). Just put a big capacitor between +V and ground. I commonly use 1000uF to 3000UF / 16V salvaged from dead PC power (they are the black plastic coated cylinders about the sie of a AA battery ). Put the side marked (-) on the ground.

It has an other advantage, when you switch the power on / off, the Capacitor slowly charges / discharges (during a few milliseconds ). This smoothes out the DC surges for any attached LCD (and backlight).

For advance filtering, you can also add inductors but then you need do some calculation on frequencies… This is another topic :wink: