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ADS1602 Clock Input

Eric VanWyk1
Prodigy 30 points
Other Parts Discussed in Thread: ADS1602, CDCLVC1104

I'm looking to use 4 ADS1602 driven from a single clock source.  There are a few aspects of the clock input that I'm not clear on, and I was hoping you could recommend an oscillator / associated circuitry for this parallel use case.

Can you please verify the part number that the reference design uses?  It is indicated as VF900537-40.000MHz, and as part of the VF3 family.  However, the part number does not match the family, and I was unable to find a VF9 family.  Additionally, I was not able to find a supplier source for the provided part number.

I will also need to support selecting between an onboard oscillator and a time base provided via SMA.  Can you please clarify the input structure / thresholds of the clk input on the ADS1602?

Thank you!

 - Eric VanWyk

  • 0
    TI__Guru 57326 points

    Hi Eric,

    Thanks for your question!

    I'm not able to find exactly the same oscillator used on the ADS1602EVM either, but you should be ok with any low-jitter, 40MHz oscillator. Crystal oscillators are preferred over other sources.

    Depending on the input signal amplitude and frequency, there is a maximum allowable clock source jitter listed in Table 1 of the ADS1602 datasheet. Too much clock jitter will degrade the best achievable SNR.

    What else about the clock input structure did you need clarification on?

    Regards, 

     

  • 0 in reply to Ryan Andrews
    Prodigy 30 points

    Thanks, Ryan

    I'm coming at this from an RF background, so I might be overthinking things.  I think it was the resistor network on the EVM that threw me - it looked like it was biasing and terminating an AC coupled signal.

    Is the '1602 expecting a rail-to-rail squarewave, a clipped sine, or a small amplitude sine?

    Do I need termination?

    Can I just wire the 4 '1602s clock inputs together, or do I need an explicit divider?

    Thanks

     - Eric

  • 0 in reply to Eric VanWyk1
    TI__Guru 57326 points

    Hi Eric,

    The clock input to the ADS1602 should be as clean as possible, but I'm not sure it's fair to say that the device is expecting a particular clock input waveform. From what I understand (and I'm not a clock expert), the waveform of an oscillator output is more dependent on the clock frequency - low frequency clocks can output a good square wave, but higher frequency clocks tend to look more like clipped sine waves or even just pure sine waves. A 40MHz crystal oscillator will work fine for the ADS1602. The part number for the oscillator used on the EVM should be XOSM-573BE 40M.

    The load on a clock source will always determine if you need to buffer it or not.  Since you are planning on using 4 devices, I would suspect you will need a clock buffer.  You can take a look at http://www.ti.com/lsds/ti/clock-and-timing/fanout-buffers-products.page for the selection guide (maybe something like CDCLVC1104 will suffice).

    Longer clock traces may need to be terminated, but this is not a requirement for the ADS1602. Perhaps you can leave a resistor pad on the board as a placeholder.

    Best Regards,