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Of VSSA, PLL ground, datasheets, and EVMs

DJH
Intellectual 985 points

Hello all,

Anybody confident enough to explain why VSSA is called 'PLL Ground' and 'Analog Ground' in the DM365 datasheet, yet in both EVMs that I have (Leopard and Spectrum) it's tied directly to digital ground?

For some reason the EVM makers put VSSA in the USB module of the schematic - very confusing!

So where do I take VSSA and, if it is indeed 'PLL Ground', then why is the ground reference for the PLL a digital ground?  I feel like the kid that is laughing at a joke with everyone else, but really wondering what is so funny.  What am I missing?

Thanks,

-David

  • 0
    TI__Expert 5955 points

    Hello David,

    The DM36x PLL is an analog/digital combo Voltage Control Oscillator (VCO) PLL. The PLL has dedicated analog VDDA and VSSA supply pads which are preferred for best jitter performance. That is why the referred PLL ground VSSA is specified as analog ground in the DM365 DS and it should be okay to tie all the grounds together unless there are separate ground planes on the board as you see the SDI EVM and Leopard boards had been done.

    Hope this helps.

    Thanks,

    Tai Nguyen

  • 0 in reply to Tai Nguyen
    Intellectual 985 points

    Hi Tai,

    A couple of items:

    1)  Neither of the EVM's provide an analog supply to VDDA18_pll.  The 1.8V is referenced to digital ground and is the same 1.8V used by the rest of the system.

    2)  Does VSSA have to be at the same potential that  VDDA18_pll is referenced to?

    I do have separate ground planes for analog and digital, although there isn't a lot of circuitry that is referenced to Agnd.  But they are on separate layers, even if the Agnd layer is cut up a little bit for other things.

    What is the recommended configuration for VDDA18_pll and VSSA?  If it's okay to tie all the grounds together, why does the data sheet specifically say to keep VSSA separate?  (It's in section 5.2)

    Thanks Tai,

    -David

  • 0 in reply to DJH
    TI__Expert 5955 points

    Hi David,

    As I mentioned these analog supply pads are for to maximize the jitter performance. It is recommended to separate as analog supplies if jitter is a concern otherwise they can come from the digital supplies. That is what SDI and Leopard had done just to simplify the power rail design.

    Thanks,

    Tai