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SN65EPT21: SN65EPT21 output oscillation

Vincent Marsaud
Prodigy 70 points

Part Number: SN65EPT21
Other Parts Discussed in Thread: SN65EPT22

Good Morning.

I'm using SN65EPT21 receiver, powered through +3,3V power supply +/- 100mV.

SN65EPT21 inputs are connected to a transformer and middle of the transformer is connected to VBB. A 200 ohm differential resistor is present between D and /D. AC input signal injected through the transformer's primary and SN65EPT21 output is responding well.

But when I have no connexion on the transformer's primary, I can see the output which is oscillating. As It is indicating on the datasheet descritpion : "The device includes circuitry to maintain inputs at Vcc/2 when left open". As the input may have the same voltage level, why output is oscillating ? Does power supply may be the cause of the oscillation ? 

Thanks per advance for your answer.

Sincerely,

Vincent

 

  • 0
    TI__Genius 12796 points

    Hi Vincent, 

    Can you share your schematic showing how the PECL driver and receiver are connected as well as the transformer connections? Also, can you share waveforms for the following signals: driver input, driver output, receiver input, and receiver output?

    Also, why are you using a transformer to AC-couple the PECL signals instead of a series capacitor? I don't think Vbb was intended to be used on inductive loads like a transformer, so I'm assuming some extra capacitance may be needed. 

    Regards,

    Matt 

  • 0 in reply to Matt Smith
    Prodigy 70 points

    Hi Matt.

    I'm using a transformer to isolate the SN65EPT21 inputs from external source. Also, external source includes SN65EPT22 with AC-couple serial capacitors to remove the output signal DC part.

    Signal frequency is 60MHz. 

    As external source is generating differential signal (+/-800mV swings), transformer allows the addition of the AC part to the VBB biased voltage.

    But I agree with you, I suspect an impedance adaptation on the input.

    Sincerely,

    Vincent

  • 0 in reply to Vincent Marsaud
    TI__Guru 62267 points

    Hi Vincent,

    We are currently on holiday. Please help give until next week for some feedback, thanks.

    Best Regards,

    Michael.

  • 0 in reply to Vincent Marsaud
    TI__Genius 12796 points

    Hi Vincent, 

    Isolation can also be achieved with AC-coupling capacitors. I would highly recommend changing your isolation of this bus to capacitive-based instead of magnetic-based since that's how the device is meant to be used. Transformers also have a higher tendency to pick up unwanted EMI. Please refer to the screenshot below from the AC Coupling Between Differential LVPECL, LVDS, HSTL and CML (Rev. C app note. 

    Also, your termination should be 100Ohms, not 200. 

    Regards,

    Matt