FFT for ADC output
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From: wks_19 on 15 Jun 2010 07:46 Hi, I'm new to DSP and FFT. My question is how to do FFT for ADC digital output code,as FFT algorithms i've seen in Internet always ask for real and imaginary parts input for the FFT algorithm. So what is the input for FFT, if we want to do it for digital codes? Thanks
From: Jerry Avins on 15 Jun 2010 09:47 On 6/15/2010 7:46 AM, wks_19 wrote: > Hi, > I'm new to DSP and FFT. My question is how to do FFT for ADC digital > output code,as FFT algorithms i've seen in Internet always ask for real and > imaginary parts input for the FFT algorithm. > > So what is the input for FFT, if we want to do it for digital codes? > > Thanks A general imaginary number is x + i*y. The output of an ADC is real, so that y = 0. There are canned FFT routines that assume the input is a pure real. They are a bit simpler to use and run a bit faster than the generalized ones. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
From: wks_19 on 15 Jun 2010 23:32 >On 6/15/2010 7:46 AM, wks_19 wrote: >> Hi, >> I'm new to DSP and FFT. My question is how to do FFT for ADC digital >> output code,as FFT algorithms i've seen in Internet always ask for real and >> imaginary parts input for the FFT algorithm. >> >> So what is the input for FFT, if we want to do it for digital codes? >> >> Thanks > >A general imaginary number is x + i*y. The output of an ADC is real, so >that y = 0. There are canned FFT routines that assume the input is a >pure real. They are a bit simpler to use and run a bit faster than the >generalized ones. > >Jerry >-- >Engineering is the art of making what you want from things you can get. >����������������������������������������������������������������������� > Thanks for the reply. My doubt is that the decimal numbers that we obtain from the digital codes exactly how it can be taken as a Voltage sampled? Is that I've to do some calculation with the ADC's Vlsb and give the obtained voltage levels to FFT algorithm.
From: Jerry Avins on 15 Jun 2010 23:44 On 6/15/2010 11:32 PM, wks_19 wrote: > Thanks for the reply. My doubt is that the decimal numbers that we > obtain from the digital codes exactly how it can be taken as a > Voltage sampled? Is that I've to do some calculation with the ADC's > Vlsb and give the obtained voltage levels to FFT algorithm. You may interpret the numbers any way you like: decimal, hexadecimal, octal, or binary for some. They are different representations of the same thing. As for the scale factor, that depends on the hardware. A full-scale reading mat represent 50 mV ot 50V or any other value in or outside that range. -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
From: wks_19 on 16 Jun 2010 01:41
>On 6/15/2010 11:32 PM, wks_19 wrote: > > > Thanks for the reply. My doubt is that the decimal numbers that we > > obtain from the digital codes exactly how it can be taken as a > > Voltage sampled? Is that I've to do some calculation with the ADC's > > Vlsb and give the obtained voltage levels to FFT algorithm. > >You may interpret the numbers any way you like: decimal, hexadecimal, >octal, or binary for some. They are different representations of the >same thing. As for the scale factor, that depends on the hardware. A >full-scale reading mat represent 50 mV ot 50V or any other value in or >outside that range. >-- >Engineering is the art of making what you want from things you can get. >����������������������������������������������������������������������� > Thanks Jerry for ur reply, I'll elaborate you my issue and i hope that you will provide me a sln. I'm capturing a 8-bit ADC output with a digital comparator and it gives me the decimal numbers for the corresponding digital codes. Is that this decimal number is enough to give directly as an input for the real-part of the FFT and imag. part to zero. Actually, I can't able to understand how mere numbers and not voltage levels are enough to calculate FFT, as many FFT algorithms take sinusoidal sampled voltage levels as their input. |