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SN65LV1023A: Termination resistors of multiple serializer and single deserializer on the same differential bus.

Part Number: SN65LV1023A
Other Parts Discussed in Thread: SN65LV1224B,

Hi, I'm wonyoung.

I want to know how to calculate termination resistor value of our LVDS configuration.

We have a LVDS backplane, which consist of multiple serializer(SN65LV1023A, 2 to 7), and single deserializer(SN65LV1224B).(It's simillar configuration to SLLS621E, figure 24)

In the datasheet of the chip(SLLS621E, Figure 24). It saids, I have to match termination resistor value to total loaded impedance.

In my case, I have many number of serializer and It's variable. In this case, How can I calculate the value of termination resistor? Our PCB trace impedance is 100 ohm

And there are any information of output / input impedance of SN65LV1224/SN65LB1023.

Best regards

Wonyoung.

  • I recommend to place a termination resistor at each end of the LVDS backplane equal to the PCB impedance ~ 100 Ohms.

    Since there are multiple loads on this bus, the maximum frequency of operation is likely to be much less than the serializer - deserializer pair all by itself.

    Regards,

    Lee

  • Thanks for answering!

    I have additional two questions on your answer.

    <Question #1>

    Is there any problems, if we use six loads(serializer) and single deserializer on bus, with 500Mbps(50MHz * 10 bit) ?

    I am afraid of our operation frequency. because TI's LVDS Owner's Manual(4th Edition) said the maximum operation frequency of M-LVDS is 250Mbps on page 10 & page 96.

    <Question #2>

    In datasheet of sn65lv1224(document number SLLS621E), Figure.24 describe a "multipoint configuration".

    In this case, 54 ohm is just example?

    In our case with six loads(serializer) and single deserializer, should we use exactly same resistor value(54ohm)?

    Or should we calculate our termination resistor value, which considered with number of loads?

    <Question #3>

    In 'Multidrop configuration', should we use 50ohm or calculate our termination resistor value?

    Our Multidrop configuration consist of single serializer and five deserializers.

    Best Regards,

    Choi

  • Hi Choi,

    Q1:  Six loads seems like a difficult task at 500 Mbps.  Have you thought of using IBIS to simulate the interconnection?  The stub length to each load will ultimately determine the maximum datarate possible on the interface.

    Q2:  54 Ohms is an example.  I would start with terminations that match the transmission line impedance.  Typically 100 Ohms at each end.

    Q3:  50 Ohms is an example.  I would start with a termination that matches the transmission line impedance.

    Regards,

    Lee