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Visitor
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Registered: ‎01-17-2018

## ZYNQ LVDS Differential Input Voltage

ZYNQ datasheet list the Differential Input Voltage of LVDS is 100mV-600mV。I have a case need use vsc8558，which Differential output Voltage is 700mV-1200mV, beyond 100mV-600mV。So VSC8558 can be used here？what should I do for this？

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Moderator
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Registered: ‎07-23-2015

## Re: ZYNQ LVDS Differential Input Voltage

@xiaoxiao221111 I didn't look at the part datasheet you using (needs login access), but from the spec you mention, I believe they are  specifying the Differential Output voltage in True differential form.

What we specify in datasheet is Vp-Vn or how much  P or N needs to be above the other to be detected as a Logic 0 or Logic 1.

For e.g. P needs to be 100mv - 600 mv above N to be a Logic 1. This value is Single Ended Peak -Peak voltage swing.

Some vendors specify their swing in Differential Peak -Peak which is 2 x (Single ended Peak-Peak). I suspect VSC8558 is specifying the same i.e 700mv - 1200mv Differential peak -Peak whose single ended peak-peak will be 350mv - 600mv which is within range of the Zynq LVDS input.

You need to check the Common Mode too to make sure it is within range.

As @tenzinc mentioned, running an IBIS simulation would be a good idea in case you are not sure of their specification.

- Giri
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10 Replies
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Scholar
3,617 Views
Registered: ‎02-01-2013

## Re: ZYNQ LVDS Differential Input Voltage

You should be able to reduce the voltage swing at the load with a voltage divider--e.g., 25-50-25.  Aligning the common-mode voltage would be another issue.

Or you can use a better fitting differential standard at the receiver...

-Joe G.

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Moderator
3,611 Views
Registered: ‎09-18-2014

## Re: ZYNQ LVDS Differential Input Voltage

xiaoxiao221111,

What is the output common mode specification for the VSC8558? 480mV min and 610mV max? You can go beyond Vidiff max granted it is still below the input bank voltage VCCO + 0.2V. Assuming the above Vocm figures are correct, on the max input voltage side, you look to be in good shape with 610mv+ 600mV = 1.21V. What I was concerned about is the other end of the voltage spectrum. 480mV-600mV = -120mV so just enough to be within our Vin min at -0.2V specification. Now, these are just eyeballing figures from the datasheet. I would highly recommend backing these up with a good simulation covering all max/min process corners and the full topology of the connection. Especially the low voltage side to make sure even in the worst case conditions that low side of your IO is above 0.2V. Almost all ic vendors readily provide IO simulations models. Most popularly the IBIS models. I suggest leveraging them to your benefit if you have access to simulation tools to do so.

Regards,

T

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Moderator
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Registered: ‎07-23-2015

## Re: ZYNQ LVDS Differential Input Voltage

@xiaoxiao221111 I didn't look at the part datasheet you using (needs login access), but from the spec you mention, I believe they are  specifying the Differential Output voltage in True differential form.

What we specify in datasheet is Vp-Vn or how much  P or N needs to be above the other to be detected as a Logic 0 or Logic 1.

For e.g. P needs to be 100mv - 600 mv above N to be a Logic 1. This value is Single Ended Peak -Peak voltage swing.

Some vendors specify their swing in Differential Peak -Peak which is 2 x (Single ended Peak-Peak). I suspect VSC8558 is specifying the same i.e 700mv - 1200mv Differential peak -Peak whose single ended peak-peak will be 350mv - 600mv which is within range of the Zynq LVDS input.

You need to check the Common Mode too to make sure it is within range.

As @tenzinc mentioned, running an IBIS simulation would be a good idea in case you are not sure of their specification.

- Giri
--------------------------------------------------------------------------------------------------------------------
There's no such thing as a stupid question. Feel free to ask but do a quick search to make sure it ain't already answered.
Keep conversing, give Kudos and Accept Solution when you get one.
-----------------------------------------------------------------------------------------------------------------------
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Visitor
3,546 Views
Registered: ‎01-17-2018

## Re: ZYNQ LVDS Differential Input Voltage

I have two more questions about LVDS ,can you give me a help,thanks.

Q1: ZYNQ datasheet list the differential input voltage is 350mV-600mV, this parameter is single ended peak-peak?

Q2: Dose the DC bias need to be added at the receiver (xc7z100)?  When the LVDS standard (xc7z100) is used here，what should I pay attention to？

Moderator
3,537 Views
Registered: ‎09-18-2014

## Re: ZYNQ LVDS Differential Input Voltage

xiaoxaio221111,

1) Yes, our Vidiff spec for LVDS single ended peak to peak.

2) No, you just use the DIFF_TERM. If you can't use DIFF_TERM than implement a differential resistor on board. If you are AC coupling then you may need to use DC biasing otherwise I don't see a real need for it. In any case, simulate to make sure the effectiveness of the termination schemes are up to par or within the requirements for your design and it's specifications.

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Visitor
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Registered: ‎01-17-2018

## Re: ZYNQ LVDS Differential Input Voltage

@tenzincas you have mentionde" If you are AC coupling then you may need to use DC biasing otherwise I don't see a real need for it",   I will use AC couping at RX，Does DC bias need to be used?  Is it integrated with DC bias in the XC7Z100?

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Moderator
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Registered: ‎07-23-2015

## Re: ZYNQ LVDS Differential Input Voltage

I will use AC couping at RX，Does DC bias need to be used?

Yes, you will need to use DC Bias.

Is it integrated with DC bias in the XC7Z100?

No, it isn't. Only the 100-ohm Differential Termination is integrated and can be enabled by using DIFF_TERM.

Below is Figure 1-72 from UG471

Rdiff can be replaced by using DIFF_TERM.

I would also recommend you go through this AR: https://www.xilinx.com/support/answers/43989.html

- Giri
--------------------------------------------------------------------------------------------------------------------
There's no such thing as a stupid question. Feel free to ask but do a quick search to make sure it ain't already answered.
Keep conversing, give Kudos and Accept Solution when you get one.
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Visitor
3,446 Views
Registered: ‎01-17-2018

## Re: ZYNQ LVDS Differential Input Voltage

I have another question,can you give me a help?

when I connect SGMII  to GTX Transceivers, we can choose LVDS or Serdes, need Use AC coupling capacitors for connection to transmitter , Does DC bias need to be added?

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Moderator
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Registered: ‎04-18-2011

## Re: ZYNQ LVDS Differential Input Voltage

@xiaoxiao221111

you would be better off Posing the MGT related question on the serial transceivers board.

Keith

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Moderator
2,098 Views
Registered: ‎09-18-2014

## Re: ZYNQ LVDS Differential Input Voltage

xiaoxiao221111,

It looks like you have not read our community forum guidelines. Please do so before you use our forums so we can help you in the most efficient manner possible. It looks like your initial inquiry was resolved here. Please select one of the responses as the answer so other folks who have similar questions can also glean knowledge from this post. In regards to your MGT question, that is completely unrelated to LVDS... We have a separate community forum for MGTs. Please create a new post on that forum so MGT experts can see your inquiry and best help you! Thanks!

Community Guidelines:

https://forums.xilinx.com/t5/custom/page/page-id/community_guidelines

MGT Forum:

https://forums.xilinx.com/t5/Serial-Transceivers/bd-p/transceivers

Regards,

T

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