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Simulators >> Circuit Simulators >> SNR of Fully Different Opamp with SC CMFB using Spectre and MATLAB
https://designers-guide.org/forum/YaBB.pl?num=1512969753

Message started by repah on Dec 10^th, 2017, 9:22pm

Title: SNR of Fully Different Opamp with SC CMFB using Spectre and MATLAB
Post by repah on Dec 10^th, 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.

Title: Re: SNR of Fully Different Opamp with SC CMFB using Spectre and MATLAB
Post by sheldon on Dec 11^th, 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

Title: Re: SNR of Fully Different Opamp with SC CMFB using Spectre and MATLAB
Post by repah on Dec 14^th, 2017, 3:38am

My mistake, I need to find the SNDR.

Thank you.

What is a RAK ?

Title: Re: SNR of Fully Different Opamp with SC CMFB using Spectre and MATLAB
Post by sheldon on Dec 14^th, 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