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AM26C31: termination resistor design

Gene Qiu
Intellectual 1720 points
Part Number: AM26C31

Hi Team,

Currently are using AM26C31 for the design. There are several questions about the application of am26c31

1. As shown in the figure below, is the maximum driving capacity of am26c31 20mA? Since in ABS table, the maximum of output Io is 150mA

2. As shown in the figure below, what is the appropriate value of the impedance matching resistance in series on the differential output line? As I notice on datasheet, there is a guideline in 9.2.1 for 80 to 120ohm

3. As shown in the figure below, how to calculate the hardware filter bandwidth when RC is paralleled between differential output lines? If the design bandwidth is 1MHz, how appropriate is RC?


  • 0 in reply to Gene Qiu
    TI__Mastermind 25705 points

    Gene,

    20mA is the current of normal operation, while 150mA is the maximum short current (fault condition). You can refer to this post for the Vod vs. Iout plot.

    The termination resistor value should match the cable impedance to reduce the reflection.

    Since RS-485 signal is differential, you can calculate the RC filtering based on the middle point (virtual ground). If your case, time constant is (390Ohm/2)*(1nF*2)=3.9us (about 40kHz). I think it's a bit large for 1MHz bandwidth.

    Regards,

    Hao

  • 0 in reply to Hao L
    TI__Intellectual 1720 points

    Hi Hao

    Appreciate for your reply. 

    1.For AM26C31 output ability, we can follow below pic?

    2. For termination resistor, datasheet recommend 80-120ohm, in my case, if 1MHz differential signal, is it OK to use 10-30ohm? It would be better if TI had related material to show how to consider the cable impedance 

    3.by what your suggestion "RC filtering based on the middle point (virtual ground)", do we have documents to demostrate this?

    Thank you for your time. 

  • 0 in reply to Gene Qiu
    TI__Mastermind 25705 points

    Gene,

    I'm closing the thread since the discussion has been carried over by email.

    Regards,

    Hao