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Transient noise accuracy in AMS

Hello,

I am simulating a model for an audio system in AMS. The model includes digital and analog parts. There are no noise sources in the circuit, all ideal resistors have their 'generate noise' set to 'no'.

When I enable transient noise, the SNR is reduced by ~10dB. Again there are no noise contributors in the circuit so I should see no change to my noise floor (which is set by quantization noise).

I tried tightening Reltol and also switching from fast APS to APS and to Spectre. It seems that I get back the 10dB when I use APS (not fast) and Reltol tightened to 1E-6.

Does anybody know why is it behaving like this? Is it something related to AMS?

My setup for transient noise is: Fmax=100MHz, Fmin=1, Seed=13, Noise Update = 'step'.

The actual time step is much smaller than 1/Fmax.

Thank you,

Tomer

Andrew Beckett over 11 years ago

How are you measuring the SNR? I presume you're doing some kind of DFT (maybe with the dft function)? Are you setting strobeperiod to ensure that the simulator solves on the precise sample points for the dft function (which always uses a power of two points, even if you don't specify a power of two points).
I'd need to understand how you are measuring things and controlling the simulator to know what is going on.
Regards,
Andrew
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avocado over 11 years ago

Hi Andrew,
Yes I set my strobeperiod to 125E-9. I use external post-processing DFT in Matlab to calculate the SNR.
Tomer
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Andrew Beckett over 11 years ago

Tomer,
please contact customer support at http://support.cadence.com - we'll need to see your data. I can't think of an obvious reason.
Andrew
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