The Designer's Guide Community
Forum
Welcome, Guest. Please Login or Register. Please follow the Forum guidelines.
Aug 18th, 2019, 10:43pm
Pages: 1
Send Topic Print
SNR of Fully Different Opamp with SC CMFB using Spectre and MATLAB (Read 1472 times)
repah
Community Member
***
Offline



Posts: 41

SNR of Fully Different Opamp with SC CMFB using Spectre and MATLAB
Dec 10th, 2017, 9:22pm
 
Hello,

I am simulating a Fully Differential Opamp with Switched Capacitor CMFB.

I have a specified input frequency, say 30MHz for the sinewave input.
I am sampling at 100 MHz.  I want 1024 samples.

I want to find the SNR of this structure based on the sizing of the transmission gates used for the switches.

My question: how to do I calculate the FFT of the output waveform ?

I dont want windowing the discrete-time output signal and want to make sure that the output samples have complete samples.

So based on my example, I pick 30MHz/100MHz * 1024 samples = 307.2.  I choose the closest prime number -> Say, 311.

Then, the input frequency I put into the amplifier will be 311/1024 * 100 MHz = 30.37109375 MHz.

Then I just extract the data using value function in cadence, making sure I wait until the SC CMFB settles and then I output this to matlab and plot.

I use PRETTYFFT (not sure if you've heard of it):
http://www.utdallas.edu/~yxc101000/courses/7327/proj/prettyFFT.m

Is this methodology correct for doing this in Spectre ?

Thank you for any advice or tips.


Back to top
 
 
View Profile   IP Logged
sheldon
Community Fellow
*****
Offline



Posts: 745

Re: SNR of Fully Different Opamp with SC CMFB using Spectre and MATLAB
Reply #1 - Dec 11th, 2017, 6:36pm
 
Repah,

   Are you using Virtuoso or just Spectre?

   The measurements are available when you plot the results using the
ViVA waveform display. Display the reconstructed analog waveform,
select measurements --> spectrum from the pull down menu. Then
setup the measurement conditions.

Comments:
1) In general the methodology should work, but the measurement
   can be directly done in ViVA
2) You should parameterize the test bench so there is no rounding
   error to cause spectral leakage, see the ADC Verification RAK
   on the Cadence support site
3) Unless you are doing a transient noise simulation the SNR is of
   limited utility. Most of the degradation will be due to distortion
   since the quantization limits the noise floor and the only noise
   is the numerical noise of the simulator
4) BTW, the RAK also shows how to use the spectrum function

                                                                     Sheldon
Back to top
 
 
View Profile   IP Logged
repah
Community Member
***
Offline



Posts: 41

Re: SNR of Fully Different Opamp with SC CMFB using Spectre and MATLAB
Reply #2 - Dec 14th, 2017, 3:38am
 
My mistake, I need to find the SNDR.

Thank you.

What is a RAK ?

Back to top
 
 
View Profile   IP Logged
sheldon
Community Fellow
*****
Offline



Posts: 745

Re: SNR of Fully Different Opamp with SC CMFB using Spectre and MATLAB
Reply #3 - Dec 14th, 2017, 3:50pm
 
Repah,

 A RAK is a Rapid Adoption Kit, it is a document that explains how
to use a tool and provides an example. In this case, the RAK explores
how to simulated an ADC.

                                                                         Sheldon
Back to top
 
 
View Profile   IP Logged
Pages: 1
Send Topic Print
Copyright 2002-2019 Designer’s Guide Consulting, Inc. Designer’s Guide® is a registered trademark of Designer’s Guide Consulting, Inc. All rights reserved. Send comments or questions to editor@designers-guide.org. Consider submitting a paper or model.