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THVD1550: Using THVD1550DR as alternate to DS3695AM/NOPB

Chaithra R
Prodigy 40 points
Part Number: THVD1550
Other Parts Discussed in Thread: THVD1520, , THVD1500

We have used DS3695AM/NOPB in our design. As we received PDN# 20230217002.3 saying DS3695AM/NOPB is discontinued and THVD1550DR is pin to pin alternate for this. We tried using THVD1550DR in our design. But we have observed some noise in the differential output rise and fall time waveforms. Below is the waveform for rise and fall time of differential output with IC THVD1550DR . Why I see this waveform in the rise and fall time? whereas while using DS3695AM/NOPB rise and fall time is smooth.

Below is the rise and fall waveforms of differential output with IC DS3695AM/NOPB

I have removed put THVD1550DR in the board by removing DS3695AM/NOPB.

Please help me to identify why there is noise present while using THVD1550DR. 

  • 0
    Guru 267980 points

    Can you show the waveform at the D input? Its edges might be too slow.

    The THVD1550 is designed for 50 Mbps. If you do not actually need more than 10/1/0.5 Mbps, use the THVD1520/05/00 instead.

  • 0 in reply to Clemens Ladisch
    Prodigy 40 points

    D inputs is with baud rate of 19200 i.e. 19.2kHz. Below is the waveform:

    Using THVD1550 for slower speed signals will cause problems like this?

    And we are looking for pin to pin matching alternate for DS3695AM/NOPB.

  • 0 in reply to Chaithra R
    Guru 267980 points

    I do not know where those oscillations come from, but fast edges can lead to EMI/EMC problems.

    19200 baud is 0.02 Mbps. You should use the THVD1500.

  • 0 in reply to Chaithra R
    TI__Mastermind 46327 points

    Hi Chaithra,

    So as Clemens has alluded too - using parts that have a much higher speed rating than what you mean can cause issues. It is not guaranteed that you will see problems - but with faster transition times you have more of the signals energy in higher frequency bands - which can result in signal integrity issues that were not present when the signal being transmitted has less energy in higher frequency bands. This can cause issues with emissions but also highlight impedance mismatches that were not noticeable with a slower device. 

    Since you do not need 50Mbps the THVD1500DR would work in this application - the main difference between the THVD1550 and THVD1500 is speed - where the THVD1500 is only rated for a max of 500kbps (250kHz) as its outputs are slew rate limited where the THVD1550's are not. 

    If you can share the RS-485 portion of the schematic, how many nodes, and how long the bus is I can double check to make sure there is nothing that could be causing issues - if you cannot share I understand and I do feel pretty confident the issue is most likely the THVD1550 being too fast but using something slower should help. 

    Please let me know if you have any other questions and I will see what I can do!

    Best,

    Parker Dodson