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SN55LVDS31/SN55LVDS32 question

Mike Valvo
Intellectual 470 points
Other Parts Discussed in Thread: SN55LVDS31, SN55LVDS32

hello,  I have a LVDS driver (SN55LVDS31) talking to a LVDS receiver (SN55LVDS32).  when I disable the driver, the driver outputs go into high impedance and the receiver pulls the inputs high through internal pullup resistors.  while this happens, the receiver output has a burst of noise.  what is the recommended way to disable the driver without having a burst of noise at the receiver output?

thanks

  • 0
    TI__Guru** 102705 points

    Hello Mike,

    This problem could be caused by the large shift in common mode that the receiver encounters when the inputs are pulled to Vcc as the driver outputs go into a high impedance state. The section about "Terminated Failsafe" in this app note describes the problem:

    LVDS Note.pdf

    While the driver is enabled, the transmission lines will be moving around a common mode near 1.2V but when the receiver pulls its inputs to Vcc, there will be a large (~2V) jump of the common mode. This is a likely source for the noise you are seeing. Refer to the diagram below:

    This can be counteracted by implementing external failsafe biasing as described in the app note:

    Basically, the external biasing will keep the common mode voltage at the receiver output close to the common mode during driven operation in order to avoid the large jump. This is shown in figure 2 as well. 

  • 0 in reply to Casey McCrea
    Intellectual 470 points
    one side question: what if the receiver inputs are not terminated? would adding R1 and R3 still be the way to go to keep the receiver output from producing noise?
  • 0 in reply to Mike Valvo
    TI__Guru** 102705 points

    Mike,

    I would not recommend operating this scheme without the termination resistor. This will ensure that the driver is seeing its desired load and help clear up signal integrity. 

    Best Regards,

    Casey McCrea

  • 0 in reply to Casey McCrea
    Intellectual 470 points

    I have a multi-drop LVDS bus, so there is only 1 terminating resistor.  so now I have 2 questions:

    1.  what can I do for the other receivers that don't have a terminating resistor?

    2.  there are 4 receiver channels per chip and I may only be using 1 of them.  what should I do for the unused receiver inputs of this receiver?  That app note says to leave them open, but I've seen lots of noise at the output when they are left open.   

  • 0 in reply to Mike Valvo
    TI__Guru** 102705 points

    Mike,

    It is not recommended that LVDS is used for a multi-drop scheme as these devices are designed for point-to-point applications. For a multi-drop scheme, I would suggest using M-LVDS or LVDM devices. He is a guide with some useful information about setting up multi-drop networks with M-LVDS devices:

    slla108a.pdf

    Regards,

    Casey McCrea

  • 0 in reply to Casey McCrea
    Intellectual 470 points
    this is from the SN55LVDS31 datasheet:

    "The intended application of these devices and signaling technique is both point-to-point and multidrop (one driver
    and multiple receivers) data transmission over controlled impedance media of approximately 100 Ω"

    let me ask another side question: in the case of multi-drop, is it essential that the driver be on one end of the bus, the termination resistor be at the other end, and the receivers be anywhere in between, or can the driver be somewhere in the middle of the bus?
  • 0 in reply to Mike Valvo
    TI__Guru** 102705 points
    Mike,

    While MLVDS is the recommended device for multi-drop applications, you are correct that this part can be used. As for the termination, best practice is to put the driver on one end of the bus and the termination on the other side. It is possible to place the driver somewhere in the middle of the bus then you would need to terminate the two bus ends. From a signal integrity standpoint, this should be ok depending on the transmission distance, number of nodes, etc, however this also means the driver will see half as large of a load which is certainly not desirable. For this reason I would say having the driver on one side of the bus and termination on the other is your best option.

    Regards,
    Casey McCrea