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Bypass capacitor for analog line

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estratos

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Hi!

I have a simple circuit formed by a microcontroller reading an analog voltage. This voltage is suffering from noise due to the microcontroller high speeds I guess. The issue is that when I touch the analog line with my screw drive (or any other metalic tool) the signal gets stabilized and the uC reads it correctly. I know that I should have added a low pass filter in order to avoid this kind of problems but I didn't :cry:. However, there is some place to add a bypass capacitor between the analog line and ground. I've tried several values for this capacitor (10nF, 100nF, 33pF) without sucess. I just guessed that my body acted as a capacitor connected to ground so I tried to reproduce this effect.

Any idea other than adding a RC filter?

Thanks,

Daniel.
 

well normally a cap across the voltage you are reading will do it....but like u said it doesnt.

you know ADC modules in pics have sample and hold caps in them...and you need to let that ttime to charge up.

i trust you have a cap across the microcontroller supplu right next the micro?

did you ensure you have minimised all current loop areas?

have you scoped the noisey node...can you describe what you see........the size of the noise etc...frequency, etc
 

Thanks for your reply

My circuit is a 2-side PCB, being the bottom side a whole ground plane. There is also a 100nF bypass capacitor near the microcontroller.

The noise is a 50 mV peak to peak signal, ~1us and repeating randomly. I've attached a screenshot of the bitscope screen.

Maybe it's a matter of tunning the correct value for the capacitor...

Thanks again,

Daniel.
 

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