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TIDA-01023: LMK04828 CLKin0 DC-coupling method

xiao peng ma
Intellectual 640 points
Part Number: TIDA-01023
Other Parts Discussed in Thread: LMK04828, ADC12DJ3200, LMX2594

In TIDA-01023,the master LMK04828 SDCLKout1 should be DC-coupled to the slave LMK04828 CLKin0.

I assume that the slave LMK04828 CLKin0 should be used as SYNC input.

The SYNC input  should always stay low and accept a positive pulse,so it can not be AC-coupled.

Looking at the default configuration of the LMK04828 EVM,the SDCLKout1 is LVPECL16 and there should be a termination resistor(100 ohm) on the slave LMK04828 CLKin0,but I can not find it.

I also can not find some differential DC-coupling method or specification(like internal bias voltage,input impedance) on the LMK04828 datasheet.

That makes me confused.

Another qusetion is that the ADC12JD3200 SYSREF input should always stay low and accept a positive pulse too,like SYNC input ,but TI recommends AC coupling for SYSREF input .

So I assume that the internal structure will keep SYSREF input  low when there is no driver.

Or I should enable SYSREF receiver before SYSREF event and disable SYSREF receiver after SYSREF event.

Thanks.

  • 0
    TI__Mastermind 28960 points

    Hello,

    Technically this is not disclosed anywhere, but we have observed that LMK04828 HSDS 8mA output common mode and swing matches very well with the CLKinX common mode and swing requirements in bipolar mode. So in this exceptional case, DC coupling the differential input should work. We do not have this use case characterized in the datasheet, so this behavior is not guaranteed. But it is very unlikely that anything in the future will prevent this use case.

    I agree, the SDCLKout1 100Ω termination is missing. It should be included, preferably close to the input pins.

    As for the SYNC input staying low, there are two ways around this: First, DC coupling can ensure that the output state of SDCLKout1/3 is always seen as low when not in use. Second, software control can be used to reroute the SYSREF input path, so even if CLKin0 changes state, nothing will happen.

    Recently, the ADC group has been recommending the second solution (enable receiver before event, disable receiver after event) along with AC coupling.

    Regards,

  • 0 in reply to Derek Payne
    Intellectual 640 points

    Thank you very much,Derek.

    I also have noticed that the slave LMK04828 SDCLKout9 is AC-coupled to the LMK04828 CLKin0 on the ADC12DJ3200 EVM.

    The LMK04828 CLKin0 on the ADC12DJ3200 EVM is a Single-ended input,so I guess that it can not be DC-coupled the slave LMK04828 SDCLKout9.

    I have only found that there is one termination resistor on the ADC12DJ3200 EVM,so I assume that the output type of the slave LMK04828 SDCLKout9 is HSDS.

    Is it correct?

  • 0 in reply to xiao peng ma
    TI__Mastermind 28960 points

    I think the suggested modifications in the ADC12DJ3200EVM User's Guide Appendix B.2 have been performed, bypassing the onboard LMX device for external input. This passes the signal through a balun to a 100-Ω terminated AC couple input. The leftover 270-Ω resistor R71 may be for biasing LVPECL. I will try to get someone from the ADC group to comment.

  • 0 in reply to Derek Payne
    Intellectual 640 points

    Thank you very much,Derek.

    It looks like that it is my mistake.

    LMX2594 SYSREFout is AC-coupled to the LMK04828 CLKin0 on the ADC12DJ3200 EVM and  the slave LMK04828 SDCLKout9 is connected to TSW14J56  board.