Decoupling and bipolar power supply

rickrick
edited August 2014 in Hardware
Hi,
I have some questions regarding the hardware design:

Why do you use bipolar instead of unipolar power supply? Wouldn't that be much easier? What is the advantage?

Why do you have decoupling between AVDD and AVSS and why is decoupling between AVDD and AGND done twice?
Both is not told in the ADS1299's datasheet.

Thanks in advance for any answers!

Comments

  • macglidemacglide
    Hi,

    I'm working on a 64 Channel Amplifier, based on a ADS1298 and I've got exactly the same two questions. I've got the TI Development Kit and can't see any signal differences between uni- and biopolar supply.

    Thanks
  • AgustinSAgustinS
    I also wanto to know about that.
    Thanks!
  • biomurphbiomurph Brooklyn, NY
    Well, I read somewhere that the bipolar power supply would maximize the performance of the device, and also, having the bipolar supply allows users to attach active electrodes that have bipolar amps on them. 

    It's common, in situations where you want to have a very quiet power supply, to decouple with more than one cap at different sizes. Something about how different size caps respond to different frequencies differently. At any rate, the more decoupling caps, the merrier (quieter).

    Hope that answers your questions
  • JakeStewJakeStew
    Having small decoupling caps will smooth out the high frequency noise the best, while larger caps smooth out the lower frequency noise better.  Many engineers just string out a handful of caps of various sizes to get clean power.

    Otherwise you'd build your board with no decoupling caps, then check the ripple and/or noise frequencies, then pick the right size cap to reduce that frequency ripple, then repeat until the desired cleanliness is achieved.  But small caps are cheap, and you can't always know what noise will be present in your user's application, so it's easier to just throw in a variety of values.
  • AgustinSAgustinS
    Thanks!
    Is it better to place the decoupling caps as near as possible to the chip corresponding pins? This is also applicable to the VCAP1-4 pins of the ADS1299?
  • biomurphbiomurph Brooklyn, NY
    Yes, It is important to keep the decouplers as close as possible to the IC they are supposed to be decoupling. This is best practice in all design engineering application. The VCAP1-4 are specific to the ADS, and should also be super close to the pins they are associated with.

    i designed the board as a 4 layer, with alternating power/gnd layers in order to add more decoupling potential in the board itself.
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