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Can I use 1 Oscillator to drive 2 ads1255's clkin?

yiyang zou
Prodigy 10 points
Other Parts Discussed in Thread: ADS1255

Dear TI team,

I am designning a PCB with 2 ads1255 on board. I choosed one Oscillator(8 Mhz) which can drive 15pf load, but I can not find any informaton about the input capacitor of the clkin pin of the ads1255 on the datasheet.  So ,I wonder is the Oscillator capable of driving 2 ADS1255?  If it's ok, will a small series machting resistor be good enough for the terminaton? Thanks!

Yiyang 

  • 0
    TI__Guru* 97775 points
    Yiyang,


    Are you using a crystal or a full oscillator circuit? Do you have a make and model for the oscillator that you plan to use?

    If this is a full oscillator circuit, it should be fine. Looking into the XTAL1/CLKIN pin there is a pad, ESD structure and then the input to the circuit that would be used to drive the crystal, if it were used. The total of that capacitance would likely be somewhere in the 0.5 to 2pF range, coming from the pad and some various gate and junction capacitances.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Guru* 97775 points

    Yiyang,


    To be very clear, I do not think that a crystal oscillator will work. I asked for the make and model to make sure of what device you were using to supply the clock.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 90 points
    Hi Joseph,
    Sorry for the late reply. I am not sure what you mean "a full oscillator circuit". Here, I want to use a chip like this one from mouser
    www.mouser.com/.../008-0253-0-258246.pdf
    Will it work? Thanks!
    Yiyang
  • 0 in reply to user4164168
    TI__Guru* 97775 points
    Yiyang,


    This crystal will probably not work. A crystal requires feedback supplied by the ADS1255, so that the crystal can oscillate. In the case of two ADS1255s, Having two different feedback loops may cause problems with the oscillation.

    What you want is an active oscillator circuit that requires a supply and ground pin which includes it's own power and feedback to give a clock output. It will require more current, but it should be able to create a clock signal given a power supply. Look at this datasheet below, and see if there's an appropriate frequency and voltage level:

    www.mouser.com/.../008-0256-0-258244.pdf

    Note that while the input pins of the ADS1255 have only a few pF of capacitance, you'll still need to worry about stray capacitance from the layout of the board. I'd make sure that the oscillator can drive extra capacitance other than just the pins of the devices.


    Joseph Wu