Second Order Delta Sigma Modulator SNR calculation

[Reza.mrk77]

Hi,
I am trying to simulate the second order sigma-delta modulator in MATLAB. According to the books, the in-band noise is calculated from:
n = 10*log10((0.25/12)*((pi^4)/5)*(1/OSR)^5)
(My quantization step is 0.5 and so the 0.25 is its squared)
Above value is around -44 dB.
I am simulating this second order modulator for input range from -50 dB up to -6 dB(FS) but the simulation result does not match with the hand calculations.
this is my code:

%second order delta modulation with 4 level
clear
clc
l = 1;
for A = 0.004:0.001:1
fs = 84;
t = 0:1/fs:5 - 1/fs;
fb = 5;
x = A*sin(2*pi*t);  %input signal
z = 0;
OSR = fs/(2*fb);  
for m=1:1:length(x)
    u1(m) = x(m) - z;
    if m ~=1 v1(m) = u1(m) + v1(m-1);
    else v1(m) = 0;
    end
    u2(m) = v1(m) - z;
    if m ~= 1 v2(m) = u2(m)+v2(m-1);
    else v2(m) = 0;
    end
    if v2(m) < -0.5
        y(m) = -1;
        z = -1;
    end
    if (v2(m) >= -0.5 && v2(m) < 0)
        y(m) = -0.5;
        z = -0.5;
    end
    if (v2(m) >= 0 && v2(m) < 0.5)
        y(m) = 0;
        z = 0;
    end
    if (v2(m) >= 0.5 && v2(m) < 1)
        y(m) = 0.5;
        z = 0.5;
    end
    if v2(m) >= 1 
        y(m) = 1;
        z = 1;
    end 
end
% plot(x)
% hold
% stairs(y)
y_fft = abs(fft(y))/(m);
%plot(10*log10(y_fft))
[~, sigBin] = max(abs(y_fft));
sigBin = 6;
sigPower = y_fft(sigBin)*conj(y_fft(sigBin));
noisePower = sum(y_fft(1:sigBin-1).*conj(y_fft(1:sigBin-1)));
SNR = 10*log10(sigPower/noisePower);
predSNR = 10*log10(sigPower)-10*log10((0.25/12)*((pi^4)/5)*(1/OSR)^5);
SNR1(l) = SNR;
predSNR1(l) = predSNR;
X2(l) = A;
X1(l) = 10*log10(sigPower);
l = l + 1;
end
plot(X1, SNR1)
hold on
plot(X1, predSNR1)

------------------------------------------
and the SNR plotted:

As you see the simulation SNR is around 25 dB higher than the calculated SNR.
What is the problem?
Thanks.