0
\$\begingroup\$

I have a quick sanity check question because it seems too good to be true.

If I have a LvPECL signal that I want to receive, I can just use a LVDS receiver (422/485 specifically) right? From reading the specs, LvPECL has a swing of 800 mV while 485 is 200 mV (while 422 is just the swing). Since my receiver is more sensitive, I should be fine?

Thanks!

\$\endgroup\$

1 Answer 1

1
\$\begingroup\$

Firstly LVDS and RS-485/RS-422 are generally different things. While RS-485 is a low voltage differential signalling standard, LVDS as a standard is quite different.


In the case of RS-485/RS-422, yes you can directly connect an LVPECL transmitter to an RS-485/422 receiver (TI App Note AN-972). RS-485 receivers have a very wide common mode offset tolerance that will happily cope with the LVPECL common mode offset. The LVPECL differential voltage is also well within range for an RS-485 receiver.

For your case of LVPECL to RS-485, you will need to use the correct termination for your receiver. Do not use the parallel 120R termination that you would use for RS-485. Instead you should use an LVPECL termination scheme which terminates each line via a 50R resistor to VCC-2V (for 3.3V LVPECL, that would be 1.3V). Alternatively you can use a Y termination such as:

LVPECL Y Termination Image Source

Going the other way (RS-485 Tx to LVPECL Rx) would not be advisable.


In the case of LVDS, the receivers typically require specifically a 1.2V/1.25V common mode offset, and a 400mV differential voltage. An LVPECL transmitter uses a 2V common mode offset which would be out of range for most LVDS receivers, and the 800mV typical differential voltage is possibly also too high.

\$\endgroup\$
1
  • \$\begingroup\$ I believe my 485 RX has a 100 ohm termination at each end? So that means 50 ohms at each port right? so I can just connect it straightforward no? Regarding LVDS, I plan on using the PXIe 6674T from NI. That can accept voltages up to 5V and detect swings as low as 200 mV so that should be fine. Do receivers typically have an upper threshold limit? Every spec I've seen only seems to give minimum threshold. \$\endgroup\$
    – bchang32
    Mar 3, 2021 at 18:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.