how to simulate the lineartiy of amplifier in cadence?

That's a strange result - you decrease the amplitude and you get worse THD?!? If you extrapolate this then with 0 amplitude you'll have really bad THD. Did you mean 100mV instead of 100uV?

i really mean 100uv, 10uv is even worse..

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thd(VT("/vout+") 2m 8m 1024 1000 )
i use the above calculation in candence, 10ms is transient simulation time, i pick up from 2m to 8m , and 1024 samling points, 1000 is fundamental frequencym, anything wrong? i see that when input is 0. the THD equals to almost 1000...

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besides, i have a 0 hz 1.65DC in Vout, that does not affect right?

Do you mean you have signal at the input but nothing (0Hz) at the output ???
What is your supply voltage?

my upply voltage is 3.3v, i am using fully differential amplifier, and the DC of Vout is 1.65,half VDD..i mean, this has nothing to do with THD right?

Half the supply at the output is generally OK. The DC common-mode voltage at the output can mess-up with your THD if it is not correct - for example such that it puts either the pull-up PMOS transistors or the pull-down NMOS transistors in bad mode of operation.
Can you somehow save your output sine wave in a text file as (time,voltage) pairs and attach it here? As far as I know there is an option in cadence to sample a waveform with certain step and save only those sampled points.

dear sir:
recently I have learned to design a gm-c filter, i wonder how to simulate the transconducor in cadence, will you please teach me the necessary simulation, someone told me to complete the dc sweep to get the linerity, but how to do it?
what do i need to complete the design specification?

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checkmate said:

Total Harmonic Distiortion (THD) is often used as the measure of linearity.

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the THD is used for the measure of linearity to transconducor??
how to measure the linearity of transconducor?

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erikl said:

checkmate said:

Total Harmonic Distiortion (THD) is often used as the measure of linearity.

Click to expand...

And additionally you could simulate / measure

• gain vs. input voltage (its large signal dependency)
• gain vs. input common voltage range
• gain vs. frequency

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how to complete the above three simulation you mentioned in cadence?
the input voltage is whether the dc bias voltage or the differential input voltage?

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LvW said:

chichi said:

as asked in the topic, i have no idea how to define the linearity of amplifier and how to simulate it ?i am doing a Gm-C filter in my master thesis, the linearity of which is very important for me to know...I have summit thesis soon, i hope someone could help me with that, thanks a lot.

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When linearity is "very important" for your filter, I suppose you have something like a specification (or upper limit) for it, don't you ? Because "as linear as possible" is not a sufficient requirement.
Therefore, in what terms is linearity specified for the filter?

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which specification do you think is important to the transconducor ?

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chichi said:

it´s just the professor has been telling me that gm c filter has a really bad linearity, which degrade the whole system a lot, so i need to find out this parameter, therewith i determine whether it fits my design or not..e.g the input signal ranges from 50uv to 5mv, so he told me that ,my filter master has a linearity of 40DB=5mv/50uV=100. so i am asking how can i know that my filter linearity is better than 40DB or not?

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checkmate said:

Total Harmonic Distiortion (THD) is often used as the measure of linearity.[/quote
hi , do u know how to find out Total harmonic Distortion by simulation?

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I meet the similar question , i have learned to design a gm-c filter,
how to measure the linearity of the transconducor in cadence or pspice?

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I am study gm-c filter for several months and read some book and papers, in my experience, about how to simulate the linearity of the gm in cadence, you can do it use pss simulation, the input is a port or a sine wave, the output you just put a dc source with the voltage just the offset of you output, the you can plot the gm VS. Pin or gm vs.Va, you can see gm will drop when the Pin is larger, when gm drop 10% then it will cause 1dB distortion.

Hello,
I am also trying to compute the THD for an OTA with Spectre.
The posts are all very useful for THD simulation. It helped me a lot!

Could you please tell me what "number of samples" means. Could you please give me a numerical example.

Thanks

how to simulate the VHD+N?

Hi folks,

after I have read the threads above I am not sure what the initial task was. On the one THD is used to describe the non linearity distortion, thus the effect of non linearity of the transfer curve to distort the output signal depending on amplitude of the input signal and on the other hand the non linearity itself (dVout/dVin vs Vin). The non-linearity can be descriped by static parameters like INL (integral non linearity) or DNL. The more interesting parameter for good filter designs is THD (or similar parameters who describes the same) which analysis the distortion of the output signal by different amplitudes and over the useful spectrum.

Feedback would be nice that we can find a way to your perfect thesis