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Jonathan Harris

ADC Noise: More on the Analog Inputs

Jonathan Harris
jonharris0
jonharris0
1/24/2014 9:12:07 AM
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Blogger
Re: PCB layout impact
Hi Jayaraman, there isn't really an alternative action.  I realize that maybe I should clarify evena  bit further.  Symmetry must be maintained between the positve and negative analog inputs of each channel to the ADC.  The positive and negative of each input should be as symmetric as possible in layout. Assyemmtry will degrade even order distortion.  I was not referring to symmetry in layout between two or more channels in the ADC.  Of course, it is also good to keep symmetry between the channels but it will not affect the even order distortion of a given channel, it will just have an effect on the timing between the channels (delta would be based on the prop delay delta between the channels).  Hope this helps clarify a bit.

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JAYARAMAN KIRUTHI VASAN
JAYARAMAN KIRUTHI VASAN
1/24/2014 8:21:52 AM
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Master
Re: PCB layout impact
Jonathan Thx for the clarification. It would be great if you can discuss alternative action in case pcb symmetry in this regard is not possible.

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JAYARAMAN KIRUTHI VASAN
JAYARAMAN KIRUTHI VASAN
1/24/2014 8:21:44 AM
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Master
Re: PCB layout impact
Jonathan Thx for the clarification. It would be great if you can discuss alternative action in case pcb symmetry in this regard if not possible.

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jonharris0
jonharris0
1/24/2014 8:04:19 AM
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Blogger
Re: PCB layout impact
Hi Jayaraman, that is exactly the assymmetry that I was referring to.  It is very important to keep the layout of the analog inputs to the ADC as symmetric as possible.  Thanks for the comment!

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JAYARAMAN KIRUTHI VASAN
JAYARAMAN KIRUTHI VASAN
1/23/2014 11:58:08 PM
User Rank
Master
PCB layout impact
Jonathan,

Nice post.

"Any asymmetry in the analog input network will lead to an increase in the even order distortion of the ADC."

I am curious to know the efffect of any asymmetry introduced by the PCB layout of the analog input network.

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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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