PSD of sigma delta ADc after decimation

[ali kotb]

Hello
I know how to plot the PSD after the modulator.
but I don't know how to plot it after Decimation filter ?
any advice, tutorials

Regards.
Ali

 

[pancho_hideboo]

It is same as Nyquist ADC.

Assume Delta Sigma ADC of 3bits.
After decimation, bid width is wider, e.g. 16bits.

Then evaluate spectrum of 16bits data.

 

[ali kotb]

my ADC was 1 bit modulator
so I am confused for multibits data (after decimation)
how should the number of o/p bits affect this code that is used for 1 bit data :

OSR=66; Nop=2^13;
w=hann(Nop); v=yout.*w;   % yout = o/p of analog modulator
NumUniquePts = ceil((Nop/(2*OSR)))+1; 
z=(fft(v,Nop))/(Nop*(nlev-1)/4);
b=z(1:Nop/2+1);
xm = abs(b); xm = xm.^2; 
f = linspace(0,0.5,Nop/2+1); %normalized freq.
semilogx(f,10*log10(xm),'b'); grid on;

 

[pancho_hideboo]

my ADC was 1 bit modulator
so I am confused for multibits data (after decimation)

Do you understand decimation filter ?

Most simple decimation filter is cic filter. Frequency characteristic of cic filter is same as moving averaging filter.
https://en.wikipedia.org/wiki/Cascaded_integrator-comb_filter

Output data of your ADC are +1 or -1, since resolution of your ADC is 1bit.

Consider output code 1,1,-1, 1,-1,-1, 1,1,1, -1,-1,-1,....
And assume moving average of 3 points.
Resulting data after cic decimation filter will be, 1/3, -1/3, 1, -1, ...
Data reolution is now 4 values, -1, -1/3, 1/3, 1.

If more points of averging are used, data resolution will be larger.