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    output swing simulation

    Hi,
    I designed one telescopic opamp and i want to measure the output swing. I always think that we take the max voltage which makes the upper PMOS in triode and lower voltage which brings NMOS in triode would be the output swing. Am I right?
    Also how to find it from simulation?

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    Re: How to simulate the output swing

    connect ur op as an unit gain buffer, then sweep the input from 0 to VDD, then u can find the output range , this range consist of linear region & saturation region,
    calculate the linear output range , that's what u want.



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    How to simulate the output swing

    do fft analyse to the output signal , and get the THD value.



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    Re: How to simulate the output swing

    The last answer -- what does it mean?! A big bug?...



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    Re: How to simulate the output swing

    best to calculate on paper for practice
    and look / workout the correct equassions bit by bit

    sims are ok but not to be trusted atall

    most are only 50% correct

    paper and pencils are your tools here for sure

    then and only then simulation
    now given a set of rules by your clear workings and efforts in maths.... on paper



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    Re: How to simulate the output swing

    hi xusoso
    can you tell me what does this THD tell me the output swing. I am using cadence. can you please write in detail what i am looking for in THD simulation and how to do it.
    thanks in advance

    Added after 1 minutes:

    Btrend,
    I am using the fully differential amp, and my output CM is not equal to input CM so i think i cant use it in unity gain buffer. can you tell me anyway to work around this problem?



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    How to simulate the output swing

    I think xusoso means the upper limit of the SNDR
    for your Amp. For example, if you Amp's output
    can only tolerate 1% THD, then your maximum
    output swing will be the one with 1% THD, even
    your output can swing rail-to-rail.



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    Re: How to simulate the output swing

    I am using the fully differential amp, and my output CM is not equal to input CM so i think i cant use it in unity gain buffer. can you tell me anyway to work around this problem?
    Since differential amp should equipped with CMFB, u had better turn on CMFB to do the unit gain simulation. CMFB should cancel the difference between different CM level, meanwhile, ur input should be swept fully differential too.



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    Re: How to simulate the output swing

    Quote Originally Posted by gte582w
    I think xusoso means the upper limit of the SNDR
    for your Amp. For example, if you Amp's output
    can only tolerate 1% THD, then your maximum
    output swing will be the one with 1% THD, even
    your output can swing rail-to-rail.
    yes, sometimes the linearity of op is very important , for example in sigma-delta modulator. So, you should think of the output swing ,but also you should ensure the good linearity of op in the voltage of you concerned. Thus FFT analysis is a good chiose.



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    How to simulate the output swing

    The easiest way is to do input dc sweep. Then you may see the output range of your amp.

    If you want a linear amp, do THD is a good way to check the linearity of your circuit.

    THD in Cadence: do "tran", get the output waveform, use "thd" function" to get the value.



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    How to simulate the output swing

    There is a big difference between the output swing and the input swing. By checking with unity gain connection, BOTH are being evaluated.

    In other words, even if the output can swing rail-to-rail, the output may be limited by the input common-mode range in follower connection.

    The output swing is limited in a telescopic OA by where the devices leave saturation and go into triode, as the original poster said.

    Check it in simulation by sweeping the input voltage and looking at the output voltage range. For low distortion, output must be well within these limits.



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