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Noise analysis has mismatch in units, shows A/sqrt(Hz) instead of V/sqrt(Hz)

AS202512062447


There seems to be Unit mismatch while doing noise analysis. I am trying to plot  V/sqrt(Hz) from main form while doing noise analysis for TIA, but it plots pA/sqrt(Hz). So, which unit is correct here? Is there a way to be sure of what the simulator is plotting here. Could you please help me out in finding a quick resolution. Thanks

  • Andrew Beckett

    This suggests that you have specified the input source as a current source (if it was output noise, then this would happen if the output was specified as a probe). How have you set up the noise analysis? (please show the noise analysis form)

    In case that is all normal, which IC subversion are you using and which Spectre subversion are you using?

    Andrew

  • AS202512062447
    in reply to Andrew Beckett

    Hi Andrew,

    Thanks for replying. You are correct, the input here is the current source and the output is being taken at the collector terminal. there is no probe at the output. Logically the plot should have the unit as A/sqrt(Hz), why then the main form shows only V/sqrt(Hz) units? If the direct plot shows a particular unit, shouldn't it plot in the same unit? The version I am using is IC23.1-64B.ISRB.40

  • Andrew Beckett
    in reply to AS202512062447

    This is an error in the units shown on the direct plot form. The plot is correct because you can't get the results in V/sqrt(Hz) if the input is a current.

    There's an old CCR I filed on this (I'd forgotten) #2114281 and a more recent one #3242387 (I've suggested that they are duplicated).

    Andrew

  • AS202512062447
    in reply to Andrew Beckett

    Hi Andrew,

    So just confirming. When we are plotting noise in V^2/Hz from main form, the result then in this case should be in A^2/Hz, given that the input is current source, isn't it? Both V^2/Hz, V/sqrt(Hz) plots should be in units of A^2/Hz and A/sqrt(Hz) respectively, Please clarify.

  • Andrew Beckett
    in reply to AS202512062447

    This is another aspect of the same bug. If you pick V**2/Hz for a noise that is a current, the data itself is in A**2/Hz, but is plotted with the units shown as V**2/Hz (which is wrong ) and labelled as V**2/Hz (also wrong). The actual numbers are correct though. You can see that by comparing against Results->Direct Plot->Squared Input Noise (or by taking the input noise in A/sqrt(Hz) and squaring it). 

    Andrew

  • AS202512062447
    in reply to Andrew Beckett

    Hi Andrew, I checked after squaring. Indeed, the plots are same. Thanks for your support.

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