SN65LVDT250: No LVDS termination

David Nelson

Part Number: SN65LVDT250

Tool/software:

We designed a board that uses the SN65LVDT250 LVDS buffer. Unfortunately the far end receiver mistakenly does not have a termination resistor (missing the 100ohm resistor). What are the implications of not having the termination resistor? I know it will affect signal integrity and this varies based on the interconnect. Will the SN65LVDT250 still function and drive it's output properly? I am assuming the output will be rail to rail instead of 350mV as it should be. Is this correct? I also assume we would need to operate the interface at a much slower speed?

1 month ago

0 Brian Zhou 1 month ago

TI__Guru*** 145650 points

Hi David:

Do you mean input or output of SN65LVDT250 ? There is 110 ohm termination at the SN65LVDT250 input.

Regards

Brian

0 David Nelson 1 month ago

Prodigy 10 points

The output of the SN65LVDT250. We are connecting this output to a downstream buffer that does not have in-built termination.

0 Brian Zhou 1 month ago in reply to David Nelson

TI__Guru*** 145650 points

Hi David:

Without 100 ohm termination for LVDS input, transmission line impedance will be mismatch which could cause signal integrity issue.

Regards

Brian

0 David Nelson 1 month ago in reply to Brian Zhou

Prodigy 10 points

Thanks for the reply. I know signal integrity will be an issue. I was asking more on the functionality and what the output voltages from the SN65LVDT250 would be without termination. Does the SN65LVDT250 still output 350mV without the termination or will it swing rail to rail (3.3V to GND)? Maybe the output voltage is undefined without a termination resistor?

0 Vishesh Pithadiya 1 month ago in reply to David Nelson

TI__Mastermind 38965 points

Hi,

Looking into this now

+1 Vishesh Pithadiya 1 month ago in reply to Vishesh Pithadiya

TI__Mastermind 38965 points

Hi David,

We have not tested this behavior before, so we do not have any concrete data to support out claims.

LVDS output circuitry is a constant current mode driver meaning that an output impedence is required to operate properly. Under the assumption that the output impedence is an open, then the swing will most likely be from rail to rail. This will also depend on the common mode of the signal. There will most likely be some waveform clipping and major reflections.

As I have mentioned we have not tested this before as LVDS is defined to require a coupled output impedence. This estimation is all we can offer here.

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SN65LVDT250: No LVDS termination