SN65MLVD203: Using dummy loads in unloaded slots on the backplane for uniform and consistent loading of the bus

Dennis McGlumphy

Part Number: SN65MLVD203
Other Parts Discussed in Thread: SN65MLVD202A

I am designing multipoint backplane for a centralized control system. When a slot is not filled with a control module, the slot must be filled with a protective cover. If I fill the unpopulated slot with a dummy load that is equivalent to a MLVDS transceiver will this be better than leaving it blank insofar as signal integrity is concerned? I am not concerned with the minor power consumption. Data rates will be less than 100Mbps and the slot spacing may be between 12mm and 15mm.

over 6 years ago

0 Yaser Ibrahim over 6 years ago

TI__Genius 10250 points

Hello,

Yes; I believe that adding dummy loads would help signal integrity. Also, it would be best if the dummy loads have the same stub length as that of the modules. The more uniform the loading is the better.

Regards,
Yaser

0 Dennis McGlumphy over 6 years ago in reply to Yaser Ibrahim

Expert 1390 points

I am planning to use the SN65MLVD202A in full duplex mode. I see that the input capacitance is 5pF on each input and 3pF across the inputs. Please see the attached schematic. Is this what the dummy load should look like?

0 Yaser Ibrahim over 6 years ago in reply to Dennis McGlumphy

TI__Genius 10250 points

Hello Dennis,

I don't see the attachment. Can you please resend?

Regards,
Yaser

0 Dennis McGlumphy over 6 years ago in reply to Yaser Ibrahim

Expert 1390 points

Dear Yaser

I attached a pdf file below. If you can not read it, send me you e-mail address so I can attach it that way. Thank you for your time.

dummy load.pdf

0 Yaser Ibrahim over 6 years ago in reply to Dennis McGlumphy

TI__Genius 10250 points

Hi Dennis,

You shouldn’t have the 100 Ω termination (terminations only at both ends of the bus or backplane, not in the intermediate slots). Instead you should have a resistance that emulates the input impedance of the an M-LVDS receiver, which is in hundreds of kΩ (worst case 120 kΩ on each input and 137 kΩ differential). Also, the capacitance values are 3 and 2.5 pF (use Receiver electrical characteristics, since this is full duplex).

I guess ideally you would measure your actual plug-in board loading and emulate it. But the above should probably be good enough.

Regards,
Yaser

Interface

Interface forum

SN65MLVD203: Using dummy loads in unloaded slots on the backplane for uniform and consistent loading of the bus