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LMH0384: Termination Resistor

Kazuki Itoh
Genius 11340 points

Part Number: LMH0384
Other Parts Discussed in Thread: LMH0302

Hello team,

Could you please check if termination resistor of LMH0384 side can be removed? My customer concerns that the termination resistor can reduce signal amplitude from 800mV to 533mV.

Regards,

Itoh

  • 0
    TI__Guru* 93570 points

    Greetings,

    1). Please note in normal mode with Rref=750 ohm, LMH0302 output amplitude is about 800mV peak to peak single ended.

    2). SMPTE specification requires compatibility to a return loss template. In order to enable compatibility with this requirement, we need to provide 75-ohm Resistor and the 5.6-nH inductor that you have noted. Meeting this requirement is very important because this indicates how much high frequency content of the signal would be received by the LMH0384 device.

    Regards,, Nasser

  • 0 in reply to Nasser Mohammadi
    Prodigy 10 points

    Dear Nasser

    1).According to the data sheet, with a load of 75Ω, the output level of the LM0302 is 800 mV.

    Regards

    Ikeda (Original questioner)

  • 0 in reply to Nasser Mohammadi
    TI__Genius 11340 points

    Hi Nasser-san,

    Thank you for your support. Mr. Ikeda is my customer.

    As he's image is missing for some reason, let me post it again for him.

    According to the LMH0302 data sheet, the output signal level will be 800 mV with 75 Ω load.(R Ref=750Ω）

    In the connection below, the load is not 75 ohms. Is the input level of LMH0384 800 mV?

    Regards,

    Itoh

  • 0 in reply to Kazuki Itoh
    TI__Guru* 93570 points

    Greetings Itoh-San,

    1). First of all there must be a 75-ohm resistor on LMH0302 load. Otherwise, there would be signal reflection and return loss would not be an optimum or we will fail SMPTE return loss requirement.

    2). Please note a combination of 75-ohm in parallel with 5.6nH and 75-ohm to ground provides as close to 75-ohm impedance across signal spectrum. This way LMH0384 would be able to receive signal frequency content all the way to 1.5GHz at least.

    3). Please note LMH0302 driver is a current mode driver. This makes sure there is 800mV across 75-ohm impedance on input of the LMH0384 device.

    Regards,, nasser

  • 0 in reply to Nasser Mohammadi
    TI__Genius 11340 points

    Hello Nasser-san,

    Please let me and my customer make sure:

    Regards,

    Itoh

  • 0 in reply to Kazuki Itoh
    TI__Guru* 93570 points

    Hi Itoh-San,

    Your understanding is correct. 65 and 91 ohm impedance at 1.5GHz and 3GHz are correct. Please note 6.8nH inductor and 75-ohm Resistor are chosen to optimize SMPTE return loss requirement. These values plus driver and parasitic provide optimum return loss across frequency band in order to meet SMPTE 424 and 259 return loss requirements. Please refer to these documents for detailed requirements for return loss requirement.

    Regards,,, Nasser   

  • 0 in reply to Nasser Mohammadi
    TI__Genius 11340 points

    Thank you so much!

    My customer is happy with your support.

    Regards,

    Itoh