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
![SNR _Post2.PNG SNR _Post2.PNG](https://www.edaboard.com/data/attachments/53/53789-a67a10a54c0542085ab3ffb3a60fe247.jpg)
While when i simulate using 8192 FFT points, the output is as follows yielding SNR = 58 dB !!!!
![SNR _Post.PNG SNR _Post.PNG](https://www.edaboard.com/data/attachments/53/53788-7e99db5d0c8dffd4b3619be29c8f4991.jpg)
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 ) ?
![SNR _Post3.PNG SNR _Post3.PNG](https://www.edaboard.com/data/attachments/53/53792-eda28efbed4bc2b3c25d454745a35b3f.jpg)
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
![SNR _Post2.PNG SNR _Post2.PNG](https://www.edaboard.com/data/attachments/53/53789-a67a10a54c0542085ab3ffb3a60fe247.jpg)
While when i simulate using 8192 FFT points, the output is as follows yielding SNR = 58 dB !!!!
![SNR _Post.PNG SNR _Post.PNG](https://www.edaboard.com/data/attachments/53/53788-7e99db5d0c8dffd4b3619be29c8f4991.jpg)
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 ) ?
![SNR _Post3.PNG SNR _Post3.PNG](https://www.edaboard.com/data/attachments/53/53792-eda28efbed4bc2b3c25d454745a35b3f.jpg)
Thanks in advance
Last edited: