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CDCM7005 Reference Select

Chris Belmonte
Prodigy 160 points
Other Parts Discussed in Thread: CDCM7005, CDCE62005

CDCM7005 has the ability to automatically decide between its two inputs.  I would like to use it without the bells and whistles as a clock reference decider, ie if clock reference 1 fails clock reference 2 is used automatically.  Ideally the part would choose the functioning reference (as it is designed to do), but instead of powering a VCO the chosen reference would be output directly.  Is this possible?  I called TI tech support and was directed here, any help would be much appreciated!

Also, if there is a different part better suited to this purpose, that would be appreciated!

 

Thanks,

Chris

  • 0
    TI__Expert 4585 points

     

    Hello Chris,

    Unfortunately, without VCO or VCXO, you can not use CDCM7005 with input smart Multiplexer. The reference clock does not go to the outputs directly.

    We don't have any buffer with smart multiplexer. But  you can check CDCE62005 which has same input smart multiplexer as CDCM7005 offers. CDCE62005 has internal VCO.

    Here is the link of CDCE62005:

    http://focus.ti.com/docs/prod/folders/print/cdce62005.html

     

    Thanks,

    Firoj

     

     

  • 0 in reply to Firoj Kabir
    Prodigy 160 points

    Hello Firoj,

     

    Thank you for pointing me in that direction!  Please tell me if I am understanding this chip's operation correctly.

     

    I can set the input mux to "smart multiplexer auto mode" at which time the "smart_mux" output is set to "pri_in" until "pri_in" fails, then "smart_mux" is set to "sec_in".  The output mux can be set to "smart_mux", thereby directly outputting one of the references, "pri_in" or "sec_in", depending on the valid input.

     

    Is that correct?  I can ignore the VCO and use only the smart mux capabilities of CDCE62005?  What kind of power consumption would I see using the chip as purely a smart mux?

     

    Thanks,

    Chris

  • 0 in reply to Chris Belmonte
    TI__Expert 4585 points

     

    Hi Chris,

    Yes, your understanding is correct. However the there are some restrictions using the SMARTMUX output to the output directly. The duty cycle may not be close to 50% unless you use output divider. Please check the duty cycle requirements and I recommend you to evaluate the part using our evaluation module (EVM). Sarah can help you to get the EVM.

    As we mentioned in our datasheet input clock frequencies must be within 20% limit. Please look at the "Smart Multiplexer Auto mode" description in page 41 of the datasheet.

    If you use the PLL mode, the duty cycle issue will no longer be there.

    The power consumption depends on device configuration. Please look at Table 44  (page 66) and this will provide the power estimation based on your configuration.

     

    Thanks,

    Firoj

     

     

     

     

     

  • 0 in reply to Firoj Kabir
    Prodigy 160 points

    Hello Firoj,

    I will use the divider and input double my desired freq to protect the duty cycle, thanks for the tip. 

    I want to use the output to drive a single ended 1.8 V square wave.  Using the CMOS output gives me 2.5 V, which is too high, and using a differential output is another predicament.  Any suggestions as to how I can manipulate the output to meet the desired input?  Preferably without a voltage level translator.

    If a voltage translator is the only option, is there a "rugged" part you can suggest?

    Thanks,

    Chris

  • 0 in reply to Chris Belmonte
    TI__Expert 4585 points

     

    Hi Chris,

    You can use a voltage divider. Here is an example below. The resistor values should be chosen carefully so that they can reduce right amount of the signal swing.

    You can find an example here at page 3 of the below applications note.

    http://focus.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=scaa080a&fileType=pdf

     

    Thanks,

    Firoj