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Cutoff freq & ADC Resolution

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hw_junkie

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I am trying to figure out how to calculate the maximum cutoff frequency of a low pass filter such that it doesn't affect resolution of ADC. The goal of the filter is to attenuate the 2nd harmonic using 3 poles.

What does (ADC Full Scale/0.5LSB) = ___ give me?

I am missing something & I can't for the life of me figure out what it is.

Please Help!
 

The filter will not affect the resolution of the ADC.
I am afraid the explanation of your problem is not very clear...
 

Okay, let me elaborate..
I require that the filter designed needs to be interfaced with an ADC that requires a resolution of 1.6mV/LSB. The 16-bit ADC has a full scale of 5V.
if I have a 2nd harmonic of 4.8kHz that needs to be attenuated, and the filter is a 3 pole butterworth filter, what must be the max cutoff frequency such that the resolution reqt can be satisfied?

I hope this makes it clearer - if there is any information missing, that's probably my missing link..

thanks
 

First, if the full scale of your 16-bit adc is 5V, the LSB is actually 76uV (5/2^16)... Again, the filter will not affect the resolution of the ADC but, of course, it will affect the amplitude of the input signal and therefore your SNR.

One of the missing pieces is how much you want to attenuate the 4.8kHz and why do you need to do it with a 3rd order butterworth filter.
 

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