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LMH0387: Layout design : Relief under LMH0387

Kawai
Guru 19775 points
Part Number: LMH0387
Other Parts Discussed in Thread: LMH0397

Hi Team,

Could you please advise us how to control 75ohm impedance under LMH0387 device ?

Trace impedance from connector to device can be controlled by trace width and the depth to the GND or VCC reference plane.

Since LMH0387 integrates the return loss network, I wanted to know what would be the recommended depth (distance) to reference plane under the device.

Can we just place 75ohm reference plane underneath which designed based on the trace width ? (Reliefs on above layers.)

Another question I have is that closely looking to the LMH0387EVM PCB design, it seems left half uses 75ohm reference and right half uses 50ohm reference.

However, this is not described in the datasheet.

Do you need to place 50 ohm reference plane underneath the device ?

If you do, from where do we need to split the reference plane ?

Best Regards,

Kawai

  • 0
    TI__Guru* 93670 points

    Hi Kawai-San,

    I've been on vacation and just noticed this question.

    1). First of all, there should NOT be any trace under LMH0387 package. This is because there is actually a pcb under the device. When i get back to office, i can send you pcb layout for the LMH0387 EVM and it shows this clearly. Trace - 75-ohm or 50-ohm - is controlled impedance up to the pin of the device. Controlled impedance depends on the pcb board stackup.

    2). Connections to C1/BNC, C2/R2, and C3/R3(C6/C7) - left hand side - all are 75-ohm controlled impedance.

    3). SDI/SDIbar(C4/C5)and SDO/SDObar - right hand side - are 100-ohm differential controlled impedance.

    4). I'll be back in the office next Monday and can check on LMH0387EVM pcb layout to see how far each reference layer comes under the device.

    Regards,,nasser

  • 0 in reply to Nasser Mohammadi
    Guru 19775 points
    Hi Nasser-san,

    Thanks for the support during your vacation.

    Please allow me to discuss with you next week. I would like to clarify how it should be designed.

    I am understanding that newer device LMH0397 should be considered rather than LMH0387, however, I would like to request that description of this PCB design information should be described in the LMH0387 data sheet.

    Best Regards,
    Kawai