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About M-LVDS

Other Parts Discussed in Thread: SN65MLVD204, DS91D176, DS91C176

I am now designing M-LVDS circuit in my design.

I have two questions.

1. PCB Trace impedance

     I will use 100 ohm termination resistor at both ends of M-LVDS lines. What shoudl be the trace impedance?

            -Differential impedance : 50ohm (Line to Ground impedance : 100 ohm)

              Am I correct?

2. Input threshold of Type 2 M-LVDS receiver

     I am considerign M-LVDS as a drop replacement of RS485 communication. Do I need bias resistor for fail-safe?

     RS485 Receiver (like SN75176) has input threshold ( < -0.2V, or >0.2V) but, Type 2 M-LVDS Receiver( like SN65MLVD204) has input threshold (<0.05V, >0.15V), so in case there is no node to transmit, the receiver output of RS485 with bias resistor will be high, but the receiver output of M-LVDS without bias resistor will be low.

     the output of receiver is connected to UART RX in MCU. so, low input in idle bus will cause wrong UART function. 

     Do I need same bias resistor in M-LVDS bus (as replacement of RS485), and if yes, what resistor value shoud be used?

Thank you for your help and advice in advance.

Best Regards

Hak-Jin Jeong

  • Hi Hak-Jin

    We are looking into your questions, our Application engineer will contact you on Sept 2 (Sept 1 is a US holiday).

    Regards,

    TK Chin

  • 1. PCB trace impedance:  MLVDS drivers and receivers are designed to work on traces with 100 ohm differential impedance.  Typically the traces are placed close together to form a pair.  This coupling has some effect on the differential impedance. 

    Example Without Coupling: 2 x 50 ohm coaxial cables.  Each cable is 50 ohms impedance to GND.  Differential Impedance across the cable inputs or outputs is 100 ohms.

    Example With Coupling: 2 x 60 ohm microstrip traces.  Each trace is 60 ohms impedance to GND. When the traces are a wide distance apart, the differential impedance will be ~ 120 ohms.  When these traces are placed close together the coupling between the traces lowers the differential impedance to ~ 100 ohms.  There are many software tools which can do this calculation automatically.

    2.    Type 2 MLVDS receivers have an offset input threshold.  When no signal is applied to the input, the output is in a valid and defined output state.  For this reason no bias resistors would be needed.  Also, the MLVDS outputs define the common mode voltage on the bus further eliminating the need to externally bias the interface.

    Regards,

    Lee

  • Thank you for your reply.

    I got it about impedance.

    Let me ask a question again for more clarity.

    If I have RS485 communication circuit(SN75176 and UART), can I just replace SN75176 with type 2 M-LVDS transceiver?  Will it be working well functionally?(Ignore distance,etc)

    My concern is "type 2 M-LVDS transceiver has logic 0 output to UART when unconnected, so UART communication may not be working well. (UART RX needs logic 1 input when idle.)"

    1) Just replace type 2 transceiver

    2) Or use type 1 transceiver with proper biasing resistors

    3) in this case, what value of resistors should be?

    Best Regards

    Hak-Jin Jeong

  • The MLVDS outputs do not meet the minimum RS485 output level.  In order to replace RS485, both sides of the interface would need to be MLVDS.

    SN75176 - VOD (minimum) = 2V into 100 ohms, 1.5V into 54 ohms

    DS91D176 - VOD (minimum) = 480mv into 50 ohms

    Over a short distance - without any cable loss, this type of interface would likely work ok.  If you are concerned about the idle state output level, I would keep the resistors and use a type 1 MLVDS input (DS91C176).

    Regards,

    Lee