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How to interface 3.3V LVCMOS XO output to CDCLVD1208 input?

John11513
Expert 5620 points
Other Parts Discussed in Thread: CDCLVD1208

Hi,

My customer want to fanout 50MHz XO output to multiple loads through CDCLVD1208. However, their 50MHz XO is 3.3V LVCMOS output, while CDCLVD1208 can only accept 2.5V LVCMOS input. How should they do? I am considering below interface ckt, could you please help me review if there is any problem with it?

The consideration of this design is to attenuate 3.3V LVCMOS XO output down to below 2.5V througn serial resistor (R2) and 49.9ohm shunt resistor (R1) to accomodate input requirement of CDCLVD1208. Meanwhile the R1 resistor also act as termiator and input bias. What I am not sure is that the AC load for XO output will be arround 100ohm, I am wondered if it will result any problem for XO due to so heavy load.

Thank you,

John

  • 0
    TI__Mastermind 24205 points

    Hi John,

    The input to INP would need to be attenuated. One way to do this would be to DC couple the output of XO to INP using a resistor divider (33 ohms in series with 49.9 ohms to ground). The swing at INP will be 3.3*(49.9/(33+Ron)) V. Assuming Ron of ~17 ohms, this should result in ~1.65V at INP. For the unused input pair, the biasing network can be 100K to VDD (2.5V) and 100K to ground. This will result in a bias voltage of ~1.25V (which complies with the datasheet range 1.1V-1.5V). The only catch with DC coupling is that the XO driver should be able to source the current (~33mA) to drive the 50 ohm load.

    The other way to do this would be to AC couple the output of XO to INP using 33 ohms in series with 100nF AC coupling capacitor and 49.9 ohms to ground close to the receiver (INP pin). The swing at INP will be 3.3*(49.9/(33+Ron)) V. Assuming Ron of ~17 ohms, this should result in ~1.65V at INP. For the unused input pair, the biasing network can be 100K to VDD (2.5V) and 100K to ground. This will result in a bias voltage of ~1.25V (which complies with the datasheet range 1.1V-1.5V)

    Regards

    Arvind

  • 0 in reply to Arvind Sridhar
    TI__Expert 5620 points

    Hi Arvind,

    Thank you for reply. I still have 3 questions for your answer:

    1) As I known, a LVCMOS XO typically cannot drive 100ohm load, it is typically specified at 8mA load. So I do not think DC coupling can work.

    2) For AC coupling, I think your propsal is incorrect. AC coupling XO output to a grounded 49.9ohm will result negative signal on INP which is unacceptable. So you should not connect termination resistor to ground, instead, it should be connect to a bias voltage. My question is even though the DC load can be reduced with AC coupling, however, its AC load is still heavy, I am not sure if a typical LVCMOS out can support so heavy (~100ohm) AC load?

    3) This part has generated a 1.25V bias voltage and output through pin AC_REF, why you do not recommend to use this internal bias but suggest to generate externally?

    Thank you,

    John

  • 0 in reply to John11513
    TI__Mastermind 24205 points

    Hi John,

    1) Do you have a datasheet for the XO? Are you referring perhaps to the VOH/VOL specs that are specified at +/-8mA? I think the real question is if the LVCMOS driver for the XO can drive 50 ohms or source ~33mA (1.65V/50). The output amplitude in this case will be ~1.65V. There are quite a number of oscillators that should be able to source this current at the output stage.

    2) You are correct. I overlooked the fact that the biasing is not internal. It should be able to work with the schematic shown below.

    3) You can use the internally generated reference Vac_ref

    Let me know if this works for you.

    Regards

    Arvind

  • 0 in reply to Arvind Sridhar
    TI__Expert 5620 points

    Hi Arvind,

    Thank you. Here is the datasheet.

    Regards,

    0815.vcc1.pdf

  • 0 in reply to John11513
    TI__Mastermind 24205 points

    Hi John,

    I have contacted a few folks from Vectron regarding the drive capability of this oscillator. I hope to hear from them soon. I will keep you posted.

    Regards

    Arvind

  • 0 in reply to Arvind Sridhar
    TI__Mastermind 24205 points

    Hi John,

    Please see reply from Vectorn factory apps below. There should be no issue driving the vectron oscillator output into 50 ohms (dc or ac coupled).

    Regards

    Arvind

    Good afternoon Arvind,

    As the representative for Vectron in the Pacific Northwest; Cain-Sweet, Co. has been copied on your web request question noted below.

     Here is the response that the factory provided.

     "There should be no problem driving the VCC1 into a 50 ohm termination”

    Please know that Cain-Sweet, Co. is here as a service to you.  We have access to price and availability information and will happily assist with your order placement and any expedite needs that may arise.  In addition, our experienced engineers are readily available if technical assistance is required. 

    Have a great day!

    Best regards,

    Tina Grohn / Sales Coordinator

    Cain-Sweet, Co.  (Washington Office)

    Email:  tina@cainsweet.com