#### Shady Ahmed

##### Member level 5

I am designing a 3rd order Sigma Delta ADC, with 1.5 bit comparator, i modeled the system using ideal components and the system worked as expected.

Now, i inserted the comparator i designed (CMOS Comparator), while simulating , i got confusing results

When i simulate and calculate the FFT (or DFT) using 4096 points, the output is as follows yielding SNR = 74.3 dB

While when i simulate using 8192 FFT points, the output is as follows yielding SNR = 58 dB !!!!

How is that possible, has the 8192 points simulation encountered an error (specially that i think there is a memory problem occurs while simulation) ? Can I trust the 4096 points simulation ? specially that it yields a logical value (as expected from the system i am designing)

--- UPDATE

What is more confusing is that i ran the simulation again with the same conditions and everything DIRECTLY after the previous simulation, and it gave the following result !! SNR = 28 dB !! What does that mean ?! i ran the simulation 2 times with the same conditions and different results , does that mean that the first explanation is correct (that the 8192 simulation is erroneous ) ?

Thanks in advance

Now, i inserted the comparator i designed (CMOS Comparator), while simulating , i got confusing results

When i simulate and calculate the FFT (or DFT) using 4096 points, the output is as follows yielding SNR = 74.3 dB

While when i simulate using 8192 FFT points, the output is as follows yielding SNR = 58 dB !!!!

How is that possible, has the 8192 points simulation encountered an error (specially that i think there is a memory problem occurs while simulation) ? Can I trust the 4096 points simulation ? specially that it yields a logical value (as expected from the system i am designing)

--- UPDATE

What is more confusing is that i ran the simulation again with the same conditions and everything DIRECTLY after the previous simulation, and it gave the following result !! SNR = 28 dB !! What does that mean ?! i ran the simulation 2 times with the same conditions and different results , does that mean that the first explanation is correct (that the 8192 simulation is erroneous ) ?

Thanks in advance

Last edited: