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joujou
Joined: 24 Jun 2008
Posts: 19
Helped: 1
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 23 Sep 2008 9:40 Need Help: Design of 14 bits SC Sigma Delta ADC |
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Hi,
I designed a SC Sigma Delta ADC with its different compenents: TG, feedback TG,OTA, Latch Comparator .
But when I analyzed its spectrum to have 14 bits , I obtained these results:
The first one is a superposition of all spectrum:
**The ENOB of the modulator with a real OTA (13.31 bits) is better than the ENOB of the modulator with an ideal OTA (13.04 bits) : illogical result !!
** All spectrum contain a third order harmonic!!
The second one is a zoom on the bandwith
Someone can help me to analysis these result?
Please Let me Know if you have some comments
Joujou
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tony_taoyh
Joined: 20 Oct 2004
Posts: 136
Helped: 2
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| 23 Sep 2008 16:51 Need Help: Design of 14 bits SC Sigma Delta ADC |
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I think, you need to check the setting of the ideal OTA: the main thing is gain, gm, and slew rate.
The third hamonica is due to the non-linearity. Normally, it is caused by the gain non-linearity and switch's on-resistance non-linearity.
Hope this information helps.
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joujou
Joined: 24 Jun 2008
Posts: 19
Helped: 1
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| 26 Sep 2008 10:16 Re: Need Help: Design of 14 bits SC Sigma Delta ADC |
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Hi tony_taoyh,
Thank you for your help and sorry for my late replay
I think that the probléme is on the transmission Gate because when I test the spectrum of the ideal modulator i used an ideal amplifior
My request now is:
What are the simulations that you should do to confirm the origin of the third order harmonic?
joujou
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tony_taoyh
Joined: 20 Oct 2004
Posts: 136
Helped: 2
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| 29 Sep 2008 2:22 Need Help: Design of 14 bits SC Sigma Delta ADC |
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The simulation skill apply to any circuit:
You can use transient simulation to catch some non-linear problem: for example, the output swing of the OTA, the non-linearity of the ON-resistance. However, be careful on the result: Because, device mismatching is not included for the transient simulation. Some side calculation is required for this one.
Another one is: the noise effect is not included in the transient simulation. Notes: in the latest cadence tool, the vendor try to include it, using some random number generator.
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