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Design >> Mixed-Signal Design >> output spectrum of sigma delta modulator
https://designers-guide.org/forum/YaBB.pl?num=1327071430

Message started by singh on Jan 20^th, 2012, 6:57am

Title: output spectrum of sigma delta modulator
Post by singh on Jan 20^th, 2012, 6:57am

Hello, i have already plotted FFT of output of ADC (attached picture 1) but that is after using filter function . i am using ADC architecture attached in the link
http://www.ntbaba.com/wenku/10/00000313.PDF

. there is filter (accumulator + counter). it is initialized before every conversion.
suppose y is filter output and xx(i) contains comparator outputs for n cycles

ADC function
inside adc function
[
for i = 1 to OSR
input = amplitude* sin(2pi*f*i/OSR + phase)

do all ADC stuff
get integrator 2 output;
get comparator values in xx(i) for 128 cycles
end
y = 0;
for i = 1 to n
y = y + xx(i);
end
output_ADC = y;

then i get an FFT in which noise bed is suppressed as shown in figure 1

Now i want to see noise spectrum also befoe applying filter. how can i check my output spectrum if i remove this filter function?

if i remove my filter functionality, I will just have compartor output values and output voltages of the integrators? how i see the spectrum of both signal and noise together as shown in fig 2 ?

Title: Re: output spectrum of sigma delta modulator
Post by singh on Jan 20^th, 2012, 7:00am

i want to see output like attached picture

Title: Re: output spectrum of sigma delta modulator
Post by thomasT on Jan 23^rd, 2012, 5:39am

i also face same problem :-|

singh did u find out?

Title: Re: output spectrum of sigma delta modulator
Post by ywguo on Jan 27^th, 2012, 6:54pm

Sing,

I recalled that it is a MASH structure for you modulator in another post. So the modulator outputs are combined and filtered in one digital block. If you can split the digital block and remove the filter only. You can see the FFT plot of the modulator. I don't have experience of designing MASH and am not sure if it is feasible.

Anyway, why do you need to check the modulator spectrum now that you already have good ADC spectrum.

Best Regards,
Yawei