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I am using ADN4655 component to isolate LVDS SerDes lines that run at 1GHz.

https://www.analog.com/media/en/technical-documentation/data-sheets/ADN4654-4655-4656.pdf

Termination 100Ohms resistors are used at every receiving point (inputs of ADN4655) and Rx of the FPGA (using ODT 100Ohms).

The following diagram explains the setup: enter image description here

The LVDS SerDes lines come from the FPGA to the ADN4655 then to a USB-C port that connects with a cable to another small FPGA. Using the ADN4655, I wanted to know how I can calculate the maximum cable length that can be used.

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The data sheet says this on page 20: -

The ADN4654 provides the galvanic isolation required for robust external ports, and the low jitter and high drive strength of the device allow communication along short cable runs of a few meters.

And given my experience in this sort of area, there's nothing as good as trying out a few lengths of different cable to get a feel for this.

I wanted to know how I can calculate the maximum cable length that can be used.

It's not a reliably calculable thing. For me, I'd be modelling the interface and cable over various lengths and using a simulator then, I'd be testing real cable. I expect if you had really decent and expensive twisted pair with shield you might get 10 metres. This is based on my experience of transmitting 650 Mbps (325 MHz) scrambled data over 30 metres on really decent and expensive twisted pair.

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  • \$\begingroup\$ I did try with this isolator and it worked with me up to 2 meters at 300 MHz and not 1GHz so I wanted to see if there was a way to calculate it and see if the 2 meters makes sense or I have some layout issue. \$\endgroup\$
    – Shannon
    Aug 5, 2022 at 12:01
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    \$\begingroup\$ It's down to cable type i would say. I used pretty expensive twisted pair and you might be using not-so-good twisted pair (or even two coax cables). \$\endgroup\$
    – Andy aka
    Aug 5, 2022 at 13:47

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