• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 33 subscribers
  • Views 705 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

AM26C31: common mode and schematics review.

Naser S.
Genius 11605 points
Part Number: AM26C31
Other Parts Discussed in Thread: AM26LV31E

Customer is trying to use AM26C31 to drive a differential input that can handle only 600mv pkpk diff. Since the output swing of this device is too high he is using a series resistor at each leg and another resistor in parallel with the load. He did a simple voltage divider calculation to determine the value of the shunt resistor (R3 in the drawing shown below) but when he looked at Figure 3 of the data sheet he got confused about the common mode voltage of the output.

Because from VOH and VOL the nominal VOCM = (VOH+VOL)/2 but on Figure 3 it seems to indicate that there is a 1.3V offset. So he is not sure if the resistor value he calculated is correct.

His questions are

1- what is the common mode nominal value>

2- If he needs a 600mV diff signal at the input of my device or that the load is the value of R3, shown attached, correct?

3- Do you have a spice model for this device?

Thanks.

  • 0
    TI__Mastermind 25705 points

    Naser,

    Theoretically it should not have common mode offset for RS485 differential signal. You can refer to Figure 3. of this app note (http://www.ti.com/lit/an/slla272c/slla272c.pdf).

    Can you provide how 600mV is calculated? Is it based on minimum 2V Vod? If so, as long as the resistor divider has ratio 3/10, it should work.

    Unfortunately we only have a spice model (http://www.ti.com/lit/zip/sllm071) for AM26LV31E. In general we provide IBIS models for our RS485 transceivers.

    Please let me know if you have more questions.

    Regards,

    Hao