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Regarding the termination of LTC6955

I looked at the AC-Coupled Differential CML or LVDS (fIN <5GHz) in the datasheet.

Q1:It depends on the transmission line,but the resistance value here is not 160Ω, but 100Ω?

Q2:,the internal differential impedance of the LTC6955 is 250Ω, but it is C-coupled,

I think the ideal connection for AC coupling is as follows, but is it correct?

I look forward to your reply.
Thank you.



add Q1
[edited by: UBOTI at 1:02 AM (GMT -5) on 18 Nov 2021]
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  • Hi,  In the AC model, the series capacitance is roughly seen as 0 ohm resistance. The internal 250 ohms resistance and the external 160 ohm resistance can be treated as parallel resistors, creating a differential 100 ohm resistance 160*250/(160_250)=97 ohms (roughly 100 ohms).   The external resistor is required on the driver side for LVDS for a proper DC return path.  CML would work with the 160 ohm on either side, but if you are starting and stopping the CML signal its best to have it on the driver side.   The datasheet figure is the one to use.  We've tested and simulated all the figures fairly rigorously.  Let me know if this makes sense.  Thanks Chris

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