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Question about the analog low pass filter

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EDA_hg81

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adc ground bounce how to fix

I am working on a 12 bit ADC and the input voltage range is 0V to 5V.
At the front end only simple one pole RC ( 33ohm and 100pF )lows pass filter is implemented.
A 75K sine wave (with 20 mV amplitude) noise was noticed before I switched the cap value from 100 PF to 0.1UF.

Right now the 75K sine wave noise is gone but I still notice a high frequency noise 500K to 1.25MHZ with 20mV amplitude.

I am wondering what the problem is.

Why the low pass filter looks like a high pass filter?

Thanks.
 

create an analog low pass filter

You should be very careful with the way you ground your scope probe. Quite often the noise you see is not real, it's picked up by the probe's ground lead.
I recommend you use one of those short ground probe adapters and measure right across the caps.
 

ground bounce filtering

Thanks. let me try
 

If you don't have a short probe adapter just make sure you don't have any ground loop on your grounding pin for your scope.

Any opening between the ground and the signal pin will will allow an electromagnetic field in the loop which will create a voltage on your scope line looking line noise in your system.
 

    E

    EDA_hg81

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I have measured right on the signal and signal ground but I found out the noise still are same.

Since this PCB is mixed with digital and analog circuits. I think it is ground bounce generate the high frequency noise and this ground bounce made the ground reference of ADC unstable.

If this is true, I can not filter out this high frequency noise anyway.

How I can fix the ground bounce problem with a sample way?

Thanks.
 

Could the noise be coming from the ADC. Some kind of switching current which is converted to 20mV through the 33Ohm and your source impedance. Or was the input open?

What is the part number?
 

    E

    EDA_hg81

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It is AD7998.

Thank your all for helps.
 

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