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ADS1298 Power, ground, and decoupling concerns

Matthew Pocius
Prodigy 20 points
Other Parts Discussed in Thread: ADS1298

The datasheet for the ADS1298 is somewhat unclear regarding the proper layout procedure for the supplies to the chip. While it recommends looking at the EVM as an example for proper star grounding layout, the EVM datasheet lacks images of the top circuit layer, and also is extremely ambiguous regarding the placement capacitors in its schematic.

With regarding to the star connections, is the ADS1298 datasheet suggesting that I connect all of the AVDD pins, for example, together at one node, and then connect the AVDD1 pin to that node as well?

Finally, with the bypass capacitors, should they both be located near the star connection point for AVDD and DVDD, respectively? And how will designing this on a 2-layer board without a discrete ground plane effect my layout decisions?

Thanks in advance.

  • 0
    TI__Guru**** 180961 points

    Hi Matthew,

    We'll see what can be done to clear up the data sheet in the layout section.  The EVM actually uses the BGA version of the package and that did come through a little fuzzy in the Users Guide.  I've attached the Gerber files for the board, so hopefully this will give you a clearer picture. 

    AVdd pins A6, B6 and C6 are bussed together to a bypass cap and an internal AVdd plane.  The AVdd1 pin A7 is routed separately, also to a bypass cap and picks up the AVdd bus at the same via as pins A6, B6, C6.

    Trying to route the BGA device on two layers may not be possible - let us look at that a little closer.  Using the TQFP package, you may be able to route on two layers using a ground pour perhaps.

    ADS1X98_GERB_A.ZIP
  • 0 in reply to Tom Hendrick
    Prodigy 20 points

    First, I believe the archive you attached is empty or corrupt, as the link produces an empty zip file in Firefox, and nothing at all in internet explorer.

     

    Thank you for the comments on the AVdd pins. That is definitely helpful.

     

    Hopefully samples of TQFP packaged ADS1298 will be available soon; until then, I will have to design our first prototype using a 4 layer (which is not to say we won't remain with it once other packages are available).

  • 0
    TI__Guru* 97775 points

    Matthew,


    I'm attaching a set of PDFs of the ADS1298EVM that show the layout a bit better than the pictures shown in the EVM User Guide. In these, you should be able to zoom in and get a bit better clarity on the lines and placement of the devices and layout.

    In this layout it appears that the pins are generally collected together into one node and brought out.

    If you do decide to run with a two layer board, you will need to be careful with the supply lines and try to get the supply bypass close to the device. If you don't then supply noise is likely to degrade the performance. This could come in the form of some PSR error on the inputs, or supply noise coupling into the inputs or the reference. How much extra noise or degradation of performance depends on the layout, but a couple extra bits of noise is not unlikely.


    Joseph Wu

    ADS1298LayoutColor.pdf
  • 0 in reply to Tom Hendrick
    Prodigy 105 points

    Hi Tom, 

    You mentioned the TQFP package for the ADS1298, when is this device with that package going to be available to buy from the page? 

    Thank you very much.

    Sebastián D' Alessandro