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ADS1298: PCB questions (capacitors and power supply)

Pablo Sebastian Parodi
Prodigy 140 points
Part Number: ADS1298

Hello. I'm working in a project with the ADS1298. For supply I use +3.3v for DVDD, +1.5v for AVDD, -1.5v for AVSS...and the reference of both (the analog supply and the digital supply) connected to each other, I mean that the reference of the digital supply (DGND=0V) is connected with the reference of the analog supply (the 0v reference between +1.5v and -1.5v). Am I doing it right?.

Other question is related with de datasheet of the ADS1298. In page 99 you give a layout example in wich you say that this example is correct for either a single analog supply or a bipolar-supply connection. I think that in the case of a bipolar supply, all the capacitors wich you shown between AVDD and AVSS (I say AVSS because you reference that black triangle as AVSS in the top-right corner) should be connected between AVDD and AGND (AGND=DGND=0v) instead of AVSS. The same for the capacitor in the low pass filters in the inputs. Is correct what I'm saying?. That layout results a little confusing when you use a bipolar power supply.

Thank you!.

  • 0
    TI__Guru 66186 points
    Hello Pablo,

    Thanks for your interest in our ADS1298!

    What you described is correct for a bipolar analog supply. The ground of your system should be common between both analog and digital circuitry.

    For the decoupling capacitors, we want you to place them differentially across the supply pins - meaning between AVDD and AVSS and between DVDD and DGND, regardless of the supply voltage. This is to ensure that the voltage potential of the positive supply stays constant relative to the negative supply. As far as the low-pass filters are concerned, you can actually tie the common-mode capacitors to either AVSS or GND since both are "ground" for AC signals.

    Best Regards,
  • 0 in reply to Ryan Andrews
    Prodigy 140 points

    Hello Ryan. Thank you for your reply!.

    So, it's the same to tie that common mode capacitors to AVSS or GND? Is there some kind of difference

    in the signal or the filtering in the low pass filter?.

    The question about that capacitors came to me because I saw the schematics in the page 67 in the "ADS1298ECGFE-PDK User's Guide" (I attached an screenshot here). There you use a bipolar supply and tie the capacitors to AGND (I assume this is the 0v reference).

  • 0 in reply to Pablo Sebastian Parodi
    TI__Guru 66186 points
    Hello Pablo,

    Yes, I believe the low-pass filters will behave the same whether you short the common-mode capacitors to ground or to AVSS. Again - for AC signals, both AGND and AVSS function as "ground." It means to say that as the impedance of the cap decreases with higher frequency, the signal will be further attenuated.

    Best Regards,
  • 0 in reply to Ryan Andrews
    Prodigy 140 points
    Thank you very much Ryan!