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28th September 2018, 04:28 #1
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THD using pss analysis for a fully differential amplifier
I want to find the THD of fully differential amplifier. I am using cadence virtuoso. I have found three options in simulating the THD in cadence which are:
1. using PSS analysis I select pss analysis then got direct plot  forum forum then select voltage I place no. of harmonics i.e. 10 (I have kept)fundamental frequencyselect net asdifferential and then select the two output nodes of my differential amplifierIt shows it in the form of 10 vertical straight lines because I have set the no. of harmonic as 10. I am taking the difference between the first harmonic and third harmonic which shows 58 dB.
2. Second option is I am doing transient analysis. On this transient response of the fully differential output I apply DFT fusing calculator and then take 20 dB I again get vertical lines as harmonics this time I get 53 dB which is the difference between first and third harmonic.
3. Third option is I have done transient analysisapply THD using calculator take 20 dB This time it doesn't show waveform, it shows in form of numbers 10 dB, which I think is wrong.
I havent placed any port in my circuit The only source is the input sources of differential amplifiers. Which one is correct? Can anyone help.
i ALSO want to calculate SNR. There is no SNR option in calculator? Anyother way to find SNR?Last edited by ICdesignerbeginner; 28th September 2018 at 04:39.

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28th September 2018, 11:53 #2
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Re: THD using pss analysis for a fully differential amplifier
Can you understand a definition of THD, SNR, SINAD ?
THD is not ratio of funadmentalcomponent and 3rdorderharmonic.
Don't rely on Direct Plot Form in Cadence ADE blindly.
Use skill functions directly with understanding THD definition surely.
If you use ShootingNewtonPSS, no. of harmonic as 10 is enough.
However, if you use HBPSS, it is too few.
Don't use function without understanding its definition.
"thd()" returns value as "%".
So 20*log10(10**(10/20)/100)=50dB
Why do you require "port" ?
Do you surely set common mode bias ?
Can you understand the followings ?
There are two modes regarding input drive, common mode and differential mode.
Also there are two modes regarding output, common mode and differential mode.
Any of three could be correct, as far as you can understand THD correctly and you can do simulation correctly.
Simply calculate SNR based on its definition.
Here noise power can be evaluated by PSS/Pnoise.
Distortion's power can be evaluated by PSS.
If you would like to use Transient Analysis, you have to use TransientNoise Analysis.
Noise power which is evaluated by conventional Transient Analysis is no more than numerical noise not physical noise.Last edited by pancho_hideboo; 28th September 2018 at 12:06.

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28th September 2018, 12:42 #3
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Re: THD using pss analysis for a fully differential amplifier
I am not taking ratio I am taking difference between first and third harmonic. I have seen paper showing difference between third and first harmonic.
In cadence version which I am using dosent have shootingNewton and HBPSS option so I am just using PSS analysis
Using calculator THD gives in % but I have taken 20 log with it so its in dB. and thats coming as 10
Please correct me if I am wrong.
Why do you require "port" ?
yes I have placed the common mode and differential mode.
How can I use Distortion power can you help me in This? I want to calculate input referred noise?
To get SNR , I divide my input signal with noise power/input referred noise and get SNR?
Is this noise power and spectral noise density the same?
what does this 0_waveform/n_thd/nil means? what is /nil?Last edited by ICdesignerbeginner; 28th September 2018 at 12:49.

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28th September 2018, 12:55 #4
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Re: THD using pss analysis for a fully differential amplifier
Post thread after learning very basic things surely.
You can not undetstand very basic thing at all.
Difference of dB value is ratio of linear value.
Again, can you understand THD surely ?
https://www.edaboard.com/showthread.php?376046
Surely see "paper".
No, you can not understand anything at all.
Surely see GUI of PSS.
Or see "spectre h pss".
Are you kindergarten pupil ?
Surely see my append.
If you don't include higher order harmonic distortion as noise, you don't evaluate higher order harmonic distortion power.
SNR is defined without distortion in attached figure.
Simply, S/N or S/(N+D).
I can not understand what you want to mean at all.
No.
However your input level is linear opeartion region, it is close to true SNR.
No.Last edited by pancho_hideboo; 28th September 2018 at 13:25.
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