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Original question:

SDI Interface- Single ended and differential

LMH0303: The possibility of the invalid input signal

Hao Wang14
Expert 4415 points
Part Number: LMH0303
Other Parts Discussed in Thread: LMH0302

Hi Team,

When the resistors in the red box are not connected, the AC coupled input signal can be detected, why can the  AC input signal be detected by doing so.

However, when the resistors are connected, the DC coupled input signal with 1.6V common mode voltage can not be detected, what's the possible reasons?

One more question about output impedance, is the75homs resistor needed when the PCB trace is well controlled to 75homs?

Thanks and Best Regards!

Hao

  • 0
    TI__Guru* 93670 points

    Hi Hao,

    1). 49.9-ohm Resistors provide required termination. For device proper operation we must use these Resistors. Device has not been characterized without these components and operation is not guaranteed. 

    2). Common mode voltage and peak to peak input signal amplitude could be incompatible with the LMH0302 device. Suggestion is to use AC coupling so the device will be immune from differences in common mode voltage.

    3). Yes these are needed. These provide 75-ohm driver impedance that would match 75-ohm trace impedance.

    Regards,, Nasser