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Pnoise vs Transient noise

Haoyi219

Hello!

I am simulating the noise effect on an opamp output. The output is buffered so I used both pnoise and transient noise to calculate the rising jitter rms of the output signal. My problem is that the pnoise and transient noise give me pretty different results. (pnoise: 2.8ps; trans. noise: 4ps, these values make big difference to my design) Please can you tell me which result should I rely on in such case. My simulation setup, simulation results, and circuit test bench are attached as below. Thank you a lot!

  • How are you determining the jitter from the transient noise as being 4ps? Firstly, to get accuracy you probably need to do something which forces a timestep at the measurement threshold otherwise you might just be getting interpolation error caused by the fact that the time steps are either side of the threshold (I'd do this with a Verilog-A model using @(cross(V(inp)-thresh)) or something like that). Then are you measuring the standard deviation of the jitter?

    I'd expect you to be able to get good correlation between the two though (with a long enough simulation in the transient). I'd also be a bit concerned if your design is dependent upon such a small difference in the jitter though - that sounds a bit marginal to me.

    Andrew.

  • in reply to Andrew Beckett

    Hello Andrew,

    Thank you for the answers. I actually used rms(period_jitter) function to calculate the transient jitter. The maximum time step is set to be 10ps, and the stop time is 4us (around 1200 signal periods). 

    I understand that the transient noise accuracy is affected by time step and stop time, and I am not sure if my settings are good enough to obtain a precise result. So I should assume that the pnoise result is more reliable? 

    Thank you.

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