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Jonathan Harris

LVDS Is Dead? Long Live LVDS & JESD204B

Jonathan Harris
jonharris0
jonharris0
7/1/2013 3:45:35 PM
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Re: LVDS in Automotive
@eafpres Great comments.  Thanks for the kind words as well.  I am not as familiar with the automotive market, so thanks for the insight.  With high speed converters, the push is for a faster interface which I imagine eventually will make its way into automotive applications.  Indeed, EMI is something to consider regardless of the interface and the interface alone does not make the system immune.  It is imperative to have good design practices and be aware of return current paths to help with EMI.  Thanks again!

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eafpres1
eafpres1
6/26/2013 3:39:03 PM
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LVDS in Automotive
Hi Jonathan--your blog today caused me to read the prior blog again which caused me to read this one again and I thought I would mention that LVDS is used in Automotive, where EMI is a big issue, as well as safety and reliability.  Data rates are often low (but getting higher all the time).

I note your company makes parts for this market, like:

ADN 4667 LVDS driver

This came to mind becuase I just mentioned this in a blog over on the sibling site The Connecting Edge:

High Speed Data in Cars

There I was looking at different technologies and LVDS has the advantage of better immunity and supporting fairly high data rates in some instances.

Thanks for all your articles, very helpful.

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jonharris0
jonharris0
5/23/2013 8:51:05 AM
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Re: higher speeds and higher integration push up noise
Thanks Brad, indeed, it was an edit that I overlooked.  It should have read low voltage differential signaling instead.

Jeurgen, thanks for pointing out the error.

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Brad Albing
Brad Albing
5/19/2013 5:26:07 PM
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Re: higher speeds and higher integration push up noise
Juergen - don't blame Jonathan for that. Someone else along the way had a brain cramp and mistyped.

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Juergen1
Juergen1
5/17/2013 3:47:30 PM
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Re: higher speeds and higher integration push up noise
Dear Jonathan,

it's called Low Voltage Differential Signaling and not Low Voltage Differential Switching. Anyhow thanks for the alternative!

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jonharris0
jonharris0
5/13/2013 9:03:42 AM
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Re: higher speeds and higher integration push up noise
Hi, these are are good questions.  The physical layer specification for 204B only calls out 20cm of routing and one connector and does not include provisions to try and increase the length.  Converter and FPGA vendors can use techniques such as variable driver strengths (varying the output level) as well as pre-emphasis or de-emphasis to help mitigate the issues seen with poor interconnect and possibly for longer interconnect, but the specification does not govern these per se.  Instead, it governs measurements like the eye diagram.

Differential traces by nature are much more immune to cross talk than singled ended, but this is always a concern regardless of the interface being used as would be EMI (which 204B has scrambling capabilities to help with).

As for programmability usually, the high speed serdes drivers are separate drivers because of the design requirements for the high speeds associated with these signals.

You can also go to http://ez.analog.com/ and ask questions from a community of engineers.

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green_is_now
green_is_now
5/8/2013 8:35:51 PM
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Re: higher speeds and higher integration push up noise
Thanks Jonathan for the clarification.

I did not appriciate that this is recommended for short runs only.

But it does reinforce my hunch/statement that it would cause problems if long runs are used.

To add to the discussion can you comment on using guard rings and/or layer shielding in terms of high density packing and if this can extend the length of the runs?

 What is the limiting factor in the short run recommendation?

Is it crosstalk coupling to other JESD204B traces?

Radiation and EMI issues raise their ugly heads?

Are IC venders putting both on for I/O flexibility?

Are they making I/O pins programmable for both approaches for designer flexibility?

As an example one could use all JESD204B for side by side runs between chips and program the side on the wrong side of the IC's to be LVDS?

 

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jonharris0
jonharris0
5/8/2013 5:02:38 PM
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Re: higher speeds and higher integration push up noise
Great points.  Thanks for the comment. The 204B interface is mainly intended for short runs.  The physical interface spec calls out up to 20cm and one connector so it is not for long runs.  Typically designs usually place the FPGA/ASIC near the converter.  There are cases where this isn't true, but largely it is preferable to keep the two close even if 204B is not the protocol being use.

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green_is_now
green_is_now
5/8/2013 3:08:55 PM
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higher speeds and higher integration push up noise
You forgot to mention that the advantages of JEDEC standard do not solve the exponentail noise generation as densities increase.

 think you are mostly right for short distances in hte relm of the IC that has been designed with noise margins.

But for longer runs and other IC's and other types of circuits in the mix this is not so easy to predict.

Short fast serial runs to close IC probably ok.

for other longer runs once outside of imediate area of driver IC a LVDS translation from serial input to // output will be needed.

But that drives up costs and complexity.

 

Shrinking the IC and having to add another to compensate not so elegant a solution.

 

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