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When using a FFT should I convert the AtoD samples into a Voltage? 
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#1
Sep1313, 01:04 AM

P: 272

when using the FFT for DSP should I convert the A to D samples back into a Voltages ?
so A to D sample = sample voltage * ( bits / max Voltages) so sample Voltage = (A to D sample ) *(max Voltages/ bits) that is right, right? anyhow should I keep the samples as digital samples or convert them to analog samples before I run them in to the FFT? why or why not? I kind of think it should matter... but I do not know . I am new to this. I do think that the A to D / mic on a computer might remove all the negative numbers, but i do not know is you would get negative numbers from a microphone... do you ? 


#2
Sep1313, 01:13 AM

Sci Advisor
Thanks
PF Gold
P: 1,908

The units are only a scaling factor ... they don't change the relative values of the amplitudes.
They would be important if you were comparing one spectrum with another: you need to be consistent. 


#3
Sep1313, 06:00 PM

P: 1,084

Do all your computations in the digital domain, and afterwards scale your results to the analog domain if you need to. For example you may need to know the actual energy level of a modulated signal within a bandwidth.
I get in trouble with the 1/N normalization thing all the time. Should I use 1/N for forward, inverse, or 1/sqrt(N) on both. 


#4
Sep1313, 06:12 PM

Mentor
P: 41,107

When using a FFT should I convert the AtoD samples into a Voltage?
If your ADC circuit outputs 2's complement data to represent the AC waveform, you will probably need to do something before the FFT, I would think... 


#5
Sep1313, 08:01 PM

P: 1,084

If you get A/D numbers from a sound card or wave file they are 2's complement positive and negative numbers. Is that what you are doing? Just crunch on them to your hearts content.



#6
Sep1413, 08:54 AM

P: 272

yes, I am getting them from a sound card on my computer ....



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