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DIT4096: Noise induced from TXP/TXN driver outputs

Scott Potosky
Prodigy 10 points
Part Number: DIT4096

Are there any known issues or specific design considerations already posted somewhere for the DIT4096 TXP/TXN output stage?

I have a design implemented that follows data sheet for all connections including VDD/VIO, bypassing and TXP/TXN connection to a scientific conversion xfmr. Everything works great, all signals look good. The problem is fairly significant EMI noise is generated (only) when the DIT4096 TXP/TXN outputs are driving current into (known good) AES receiver. The EMI noise is corrupting board ground/supplies and impacting RF performance of a nearby Cypress CYBT3430326 BT module located on same board. The TXP/TXN pin output signals look good except ~1V overshoot (symmetrical both pos/neg outputs).The AES signals on the wire look good except can see the overshoot and some small cable reflections. The design is running at 48khz sample rate = 6MB/s data rate on the AES (RS422) signal. 

I noticed several similar parts from other mfrs have internal output impedance about 25 ohms for each of the TXP/TXN outputs. The DIT 4096 does not have this internal impedance. Presumably the other mfrs did this to tame the outputs for reduced EMI. Wondering if anyone has experienced similar issues with DIT4096 and what they did to resolve.

Regards, 

Scott Potosky  

  • 0
    TI__Mastermind 23050 points

    Hi Scott,

    I haven't seen this specific issue before but in general you're always going to have some level of EMI associated with a transformer and high frequency switching, but this does seem excessive. As always it is recommended to follow good layout practices, keeping traces as short as possible and components as close to the IC as you can. Are you grounding the secondary of the transformer on the transmit board or floating the sleeve to be grounded at the receive side as shown in the diagram below?

    You may also try adjusting the impedance on the line as it sounds like you have a bit of a matching issue. Increasing the resistance may help limit the current and reduce the size of the spike. Alternatively, you could try adding an RC snubber. This would reduce your amplitude a bit, but should help smooth out the signal.

     

    Best regards,

    Zak