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 frequency---select net as--differential and then select the two output nodes of my differential amplifier---It 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 analysis----apply 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?

Can you understand a definition of THD, SNR, SINAD ?
THD is not ratio of funadmental-component and 3rd-order-harmonic.

Don't rely on Direct Plot Form in Cadence ADE blindly.
Use skill functions directly with understanding THD definition surely.

ICdesignerbeginner said:

1. using PSS analysis
............................................
It 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.

Click to expand...

If you use Shooting-Newton-PSS, no. of harmonic as 10 is enough.
However, if you use HB-PSS, it is too few.

ICdesignerbeginner said:

2. Second option is I am doing transient analysis.
On this transient response of the fully differential output I apply DFT fusing calculator
............................................
I again get vertical lines as harmonics this time I get 53 dB which is the difference between first and third harmonic.

Click to expand...

ICdesignerbeginner said:

3. Third option is I have done transient analysis
----apply 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.

Click to expand...

Don't use function without understanding its definition.

"thd()" returns value as "%".

So 20*log10(10**(-10/20)/100)=-50dB

ICdesignerbeginner said:

I havent placed any port in my circuit

Click to expand...

Why do you require "port" ?

ICdesignerbeginner said:

The only source is the input sources of differential amplifiers.

Click to expand...

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.

ICdesignerbeginner said:

Which one is correct?

Click to expand...

Any of three could be correct, as far as you can understand THD correctly and you can do simulation correctly.

ICdesignerbeginner said:

i ALSO want to calculate SNR.
There is no SNR option in calculator?
Anyother way to find SNR?

Click to expand...

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 Transient-Noise Analysis.

Noise power which is evaluated by conventional Transient Analysis is no more than numerical noise not physical noise.

pancho_hideboo said:

Can you understand a definition of THD, SNR, SINAD ?
THD is not ratio of funadmental-component and 3rd-order-harmonic.

Click to expand...

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.

pancho_hideboo said:

Don't rely on Direct Plot Form in Cadence ADE blindly.
Use skill functions directly with understanding THD definition surely.

If you use Shooting-Newton-PSS, no. of harmonic as 10 is enough.
However, if you use HB-PSS, it is too few.

Click to expand...

In cadence version which I am using dosent have shooting-Newton and HB-PSS option so I am just using PSS analysis

pancho_hideboo said:

Don't use function without understanding its definition.

"thd()" returns value as "%".

So 20*log10(10**(-10/20)/100)=-50dB

Click to expand...

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" ?

pancho_hideboo said:

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.

Click to expand...

yes I have placed the common mode and differential mode.

pancho_hideboo said:

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 Transient-Noise Analysis.

Noise power which is evaluated by conventional Transient Analysis is no more than numerical noise not physical noise.

Click to expand...

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?

Post thread after learning very basic things surely.

ICdesignerbeginner said:

I am not taking ratio I am taking difference between first and third harmonic.

Click to expand...

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

ICdesignerbeginner said:

I have seen paper showing difference between third and first harmonic.

Click to expand...

Surely see "paper".

ICdesignerbeginner said:

In cadence version which I am using dosent have shooting-Newton and HB-PSS option so I am just using PSS analysis

Click to expand...

No, you can not understand anything at all.
Surely see GUI of PSS.
Or see "spectre -h pss".

ICdesignerbeginner said:

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.

Click to expand...

Are you kindergarten pupil ?
Surely see my append.

ICdesignerbeginner said:

How can I use Distortion power can you help me in This?

Click to expand...

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).

ICdesignerbeginner said:

I want to calculate input referred noise?

Click to expand...

I can not understand what you want to mean at all.

ICdesignerbeginner said:

To get SNR , I divide my input signal with noise power/input referred noise and get SNR?

Click to expand...

No.
However your input level is linear opeartion region, it is close to true SNR.

ICdesignerbeginner said:

Is this noise power and spectral noise density the same?

Click to expand...

No.