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[SOLVED] ADC and noise in electronic systems

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arnaud405

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Hi all,
first let me introduce myself.
I am a French student working for my internship on noise in electronic circuits.
I have to redesign an ADC because it is perturbated by noise. A lot of filters are used on the existing circuit, but some strange things happen and keeo fuzzy in my mind.
2 ADCs are used with the same input. The good signal is included but hidden because of the noise.
The first thing is: on the figure attached, what is this curve at the beginnig? it looks like a capacitor discharge. How to solve this problem?
Secondly, I get an other curve coming later, and shifting over the time. I do not understand at all the origin of this problem.
Do you have any idea? The best would be to get a horizontal line...
In addition, what kind of general advice you can give me concerning noise in electronic systems? grounding, type of components to use...

Lastly, I will incorporate a digital circuit into the already existing analog one for the data acquisition. What kind of precautions do I have to take for the combination of digital/analogic circuits together?
Thank you very much for your help,
Regards,
Arnaud Untitled.gif
 

not-a-moderator

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mtwieg

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First of all you should understand that most of what you seen isn't going to be "noise," but rather conducted or radiated interference signals generated from other parts of the circuit, or the circuit's environment. Those signals are completely different from actual noise, and you have to understand the difference if you want to fix them.

Your waveform doesn't include any kind of information on the time or voltage scaling, or what the signal is expected to look like, so it's impossible to really say what's happening, but it's possible that it's charge injection like not-a-moderator suggests. If so, then it's not "noise," it's a coherent signal which can be suppressed.
 

arnaud405

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Thank you for your answer. The x scale is the time in seconds. The resolution used for this sampling is 14 bits. Here is the schematic of the pre amplifier used.preamp.png
What kind of experiment can I do to temporary isolate these radiatons? link to the ground some components? shield some components?

- - - Updated - - -

What do you think about my last question about digital and analog circuits on the same PCB? which particular precautions do I have to take?
 

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