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Original question:

ADS1248: Input ground reference

ADS1248: Changing from internal reference to external reference

Saribel Daza
Prodigy 50 points
Part Number: ADS1248

Hi,

Using the ADS1248, I will changing the design to use an external reference instead of the ADC internal reference for both the ADC and transducers (from 2.048V to 3.3V), since the transducers require a minimum reference voltage of 2.5V.  I will be leaving the capacitor across Vrefout and Vrefcom in place.  Should I make any changes to the pull-down resistors connected to REFP0 or REFN0, since I will now be using the external reference voltage for the ADC?  Should I add any capacitor across REFP0 or REFN0?  Any other suggested circuit changes that should be made when changing from the internal reference voltage to an external reference voltage?

Thanks,

Saribel

  • 0
    TI__Mastermind 31655 points

    Hello Saribel,

    Yes, since you are now using the REFN0 and REFP0 pins for your external reference you should remove the pulldown resistors. REFP0 should be connected to your external 3.3V supply, and REFN0 should be connected directly to ground. I recommend adding a 0.1uF capacitor between REFP0 and REFN0 in this case.

    The GPIOs are not enabled by default, but don't forget to change your device configuration from your master if you were originally using these pins as GPIOs.

    Since you are using a single-ended supply, you should also connect AVSS directly to DGND. (Please see the Do's and Don't section of the datasheet: http://www.ti.com/lit/ds/symlink/ads1248.pdf)

    Regards,

  • 0 in reply to Scott Cummins
    Prodigy 50 points

    Thank you for your prompt response.  I updated the redline based on the feedback.  Looking more closely at the requirements for AVDD and DVDD, I noticed that the absolute maximum for both is 5.5V and the recommended maximum is 5.25V.  Per our procedure, we set the 5V power supply to between 5.1V to 5.2V, which is close to the limits and could easily go over 5.25 or 5.5V when adjusting the voltage.  Would it make sense to use the 3.3V instead for AVDD and DVDD? 

    Also, the "triangle" ground shown in the schematic is for the +12V power supply.  Would it make sense to replace the +12V ground with the 3.3V ground?  In our application, the 5V power supply ground is the same as the 3.3V ground. Thanks! -Saribel

  • 0 in reply to Saribel Daza
    TI__Mastermind 31655 points

    Hello Saribel,

    This new schematic looks good.

    How far above 5.5V might the supply go, and for how long? If it's for a short time, you can increase the decoupling capacitance on AVDD and DVDD to allow additional filtering of quick transients. Or some external surge/TVS protection could be used. This is mostly dependent on the length of overvoltage.

    Yes, I would recommend keeping the grounds the same, so it would be fine to use the 3.3V/5V supply ground. I still recommend connecting the AVSS to DGND as close to the ADS1248 as possible. Is there a reason you are isolating the 12V supply ground on your board and it is not connected to the 3.3V/5V supply ground?


    Regards,