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Hi Team,
I am trying to help my customer with the following question. Can you offer any advice?
"In my design I have a high voltage flyback transformer that will generate most of the noise. I will sync switching frequency with internal logic, so I could at least time correlated measurements with switching frequency.
With SAR, I could do conversion just before the mosfet is on and noise is minimal, but Sigma-Delta has a different kind of operation, it’s integral over time.
What would be the correct strategy to synchronize ADC with Flyback switching frequency ? My flyback will be running at 200-500kHz and it will downclocked from 8MHz with correct phase shift in multiple numbers, so edges would not align in time."
Thank you,
Daniel
Hi Daniel,
The best choice would be to use an LDO on the output of the transformer to clean up the signal and reject some of the higher frequency content.
Here is some other content to pass along to the customer to help with this issue:
Let me know if they have any additional questions
-Bryan
Hi Bryan,
Thanks. Yes, I've already suggested an LDO or even a buck with integrated ferrite beads. I've sent them our ebook and app note on digital filters. Is there anything else that can be done for sigma-delta? Their question was rather focused on synchronization with the DC/DC converter. This a noise critical application as the customer is developing spectroscopy equipment.
Best regards,
Daniel
Hi Daniel,
If this is a noise critical application, an LDO should be a no-brainer.
Otherwise, they can try to follow some of the suggestions in the content I linked to in my last post. Specifically they want to make sure to that the switching frequency is far away from modulator frequency (fMOD) such that the digital filter removes some of the noise power introduced by the DC/DC.
I am not sure what he means about synchronization. The delta-sigma ADC continuously converts during the conversion period, so unless they plan on using a much higher-speed ADC to try to finish conversions within the DC/DC switching frequency period, I don't really know what they are after (though this would increase the noise).
Again, they really should just use an LDO.
-Bryan