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Jonathan Harris

ADC Power Supplies: Inputs

Jonathan Harris
jonharris0
jonharris0
6/9/2014 10:30:30 AM
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Blogger
Re: Starting supplies - requirements
Another good point.  I discussed a little about PSRR back in my December blog: http://www.planetanalog.com/author.asp?section_id=3041&doc_id=562140

I'll be continuing to look at power supply schemes in my upcoming blog and try to point out some topologies and strategies to help mitigate any issues from power supply noise.  Stay tuned! :-)

Thanks for both of these great questions/comments!

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jonharris0
jonharris0
6/9/2014 10:26:24 AM
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Blogger
Re: avoiding crosstalk
That is a very good question.  Unfortunately, those are internal design details.  Care is taken inside the ADC to minimize crosstalk and part of this is to separate the supply domains at the pins of the device.  As you have pointed out there are also frequency relationships inside the ADC that must be evaluated...but those are part of the 'secret sauce' inside the device. :-)

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etnapowers
etnapowers
6/3/2014 11:25:53 AM
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Newbie
avoiding crosstalk
"There are various portions of circuitry within each of these blocks that operate at different frequencies, and keeping the supply domains separate helps keep these frequencies from making their way back and forth between the two sections. Having crosstalk between these sections can result in degraded performance, which may manifest itself in the SNR (noise performance) or the SFDR (spurious performance) of the ADC."

 

I wonder if there is a guideline to establish the minimum value of difference between the frequencies that prevents crosstalk, which affects the performances of the ADC.

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RedDerek
RedDerek
6/2/2014 11:18:43 AM
User Rank
Master
Starting supplies - requirements
Best place to start, besides voltage levels, is allowed ripple on the supply. This would then be the PSRR. And thus can affect how the supply is designed - frequency for one. Is is always best to use a high frequency power supply? Challenges here is that the dynamic range of output power is reduced at higher frequencies. (I ask these questions only to get the conversation started.)

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More Blogs from Jonathan Harris
In this blog we’ll take this one step further and look at driving the ADC power supplies directly from a DC/DC converter.
Previously in this blog series we looked at using a DC/DC converter (switching regulator) in combination with an LDO to drive the power supply inputs to an ADC. What we found was that using the DC/DC converter to step down the input voltage for the LDO was a much more efficient way to drive the power supply inputs to an ADC.
We look at using a DC/DC converter along with an LDO to drive the ADC power supply inputs.
I thought it would be good to continue looking at the example I gave in my last blog where we looked using fewer LDOs and combining power supply rails on an ADC while maintaining isolation with ferrite beads.
There are some disadvantages when driving low input supply voltages, where multiple LDOs may be required.
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