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ADC12DJ5200RF: sysref

Amy Luo
Expert 6895 points

Part Number: ADC12DJ5200RF
Other Parts Discussed in Thread: TIDA-010128

Hi team,

With reference to the design below, the customer has designed the acquisition system of ADC12DJ5200RF+ FPGA + HMC07044 + LMK2595.

TIDA-010128 ：Scalable 20.8 GSPS reference design for 12 bit digitizers

The ADC works in JMODE = 1 single channel，16 lanes，jesd204b compatible mode. LMK2595 outputs a 2.5GHz frequency clock and a single pulse form of sysref. HMC7044 sends REFCLK and sysref to FPGA, and ensures homology with LMK2595 input clock.

The initial phenomenon is that after sending sysref to ADC and FPGA, sync can be pulled up in test mode (ramp), but it will be pulled down occasionally. If it is set to normal mode, sync will be pulled down periodically. It seems that there is a problem with the sysref for ADC.

Now, the customer directly grounded the sysref pin of the ADC and only sent sysref to the FPGA. Actually, the link can be established. Sync can continue to be pulled up, not pulled down.

Customer wound like to know why? Does ADC not need sysref to synchronize LMFC when it works?

  • 0
    TI__Mastermind 45670 points

    Hi Amy,

    How many pulses is the customer sending from the sysref? It sounds like only one pulse. If that is the case, I would suggest adding more pulses (4 to 8 pulses in a row) or DC couple SYSREF. (I am assuming the sysref is AC coupled).

    Once the LMFC is established the sysref doesn't need to send any more pulse, otherwise the LMFC resets again and again.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Prodigy 40 points

    Hi, Rob

    thanks for your reply.

    But I directly grounded the sysref pin of the ADC and sent sysref only to the FPGA.Then the link was established. It showed that Sync was solely pulled up without being pulled down.

    But the manual says only when the pulse is sent to both ADC and FPGA  and the LMFC is reset will the Sync takes on such status. How to explain this irregularity?

    Or the truth is the link can also be established with sysref pin grounded and the LMFC not reset by sysref pulse?

    Regards,

    Ty

  • 0 in reply to tianyi wei
    TI__Mastermind 45670 points

    Hi Ty,

    Is the sysref to the FPGA AC coupled or DC coupled?

    How are the sysref pins ground from the ADC? Are the sysref pins connected directly to ground or through an AC coupling cap to ground or some other way?

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Prodigy 40 points

    Hi dear,Rob

    The original figure is the following one which I referenced "TIDA-010128 ：Scalable 20.8 GSPS reference design for 12 bit digitizers". But when I sent sysref pulse to ADC and FPGA,the sync signal would be pulled down occasionally.

    And then I change my schematic to the following one that the sysref pin of ADC12DJ5200 is grounded directly by DC coupling:

    so I post my question:

    "But I directly grounded the sysref pin of the ADC and sent sysref only to the FPGA.Then the link was established. It showed that Sync was solely pulled up without being pulled down.

    But the manual says only when the pulse is sent to both ADC and FPGA  and the LMFC is reset will the Sync takes on such status. How to explain this irregularity?

    Or the truth is the link can also be established with sysref pin grounded and the LMFC not reset by sysref pulse?"

    Look forward to your reply.

    Regards,

    Ty

  • 0 in reply to tianyi wei
    TI__Mastermind 45670 points

    Hi Ty,

    See my comments and questions below....

    "But I directly grounded the sysref pin of the ADC and sent sysref only to the FPGA.Then the link was established. It showed that Sync was solely pulled up without being pulled down. RR: okay, understood now., However, you can disable the sysref Rx using Reg 0x29. By default it is disabled, there is no need to pull this to ground and change the circuit.

    But the manual says only when the pulse is sent to both ADC and FPGA  and the LMFC is reset will the Sync takes on such status. How to explain this irregularity? RR: sysref is only needed when two or more devices are going to synchronized, sysref is not required when bringing up the link. Can you tell me the sample rate frequency (2.5GSPS? in JMODE1?) and sysref frequency you are using? 

    Or the truth is the link can also be established with sysref pin grounded and the LMFC not reset by sysref pulse?" RR: no, this is not the truth, please see my comments above.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Prodigy 40 points

    Thanks a lot! Rob.

    Answer for Question1:The sample rate frequency is 10GSPS,in JMODE1,using DDR sample clock method.

    Answer for Question2:And the frequency of sysref is 3.90625MHz.(R*fs/10/F/K/n)

    then I recognize that the sysref signal is use for multi-device with Deterministic Latency.

    and I am confused that if I want to synchronize multi-device with timestamp method, the manual says when N' equals to 12bit, the LSB of the 12bit represents the timestamp bit. The question is that how I recognize which meaning of the LSB, timestamp bit or sample data LSB bit?

    Regards,

    Thanks again,

    Ty

  • 0 in reply to tianyi wei
    TI__Mastermind 45670 points

    Hi Ty,

    See my comments and questions below....

    Answer for Question1:The sample rate frequency is 10GSPS,in JMODE1,using DDR sample clock method.

    Answer for Question2:And the frequency of sysref is 3.90625MHz.(R*fs/10/F/K/n)

    then I recognize that the sysref signal is use for multi-device with Deterministic Latency.

    and I am confused that if I want to synchronize multi-device with timestamp method, the manual says when N' equals to 12bit, the LSB of the 12bit represents the timestamp bit. The question is that how I recognize which meaning of the LSB, timestamp bit or sample data LSB bit?

    RR: bit0 replaces the time stamp, see below in the datasheet...

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Prodigy 10 points

    Hi Bob,

    I have a similar problem.

    "RR: sysref is only needed when two or more devices are going to synchronized, sysref is not required when bringing up the link." "when two or more devices synchronized" means two transmits? Whether sysref is needed when one transmit and one receiver are going to sychronized ?

    regards,

     Hezhou

  • 0 in reply to user5177423
    TI__Mastermind 45670 points

    Hi Hezhou,

    What is your similar problem? Please directly ask your question and describe your issue or problem....I just see a copy of a response to the E2E thread above.

    Regards,

    Rob