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ADS127L01EVM: Digital Pins and Reference Capacitors

luke jet
Intellectual 290 points
Other Parts Discussed in Thread: ADS127L01EVM, REF6025, ADS127L01

A few beginner questions regarding the ADS127L01EVM if you have the time.

-Why are the digital pins (when necessary) not tied to ground with 10k resistors as recommended in the datasheet? Instead they are tied directly to ground. Will I receive better noise performance if I tie my hardware select pins to dvdd and ground with 10k ohm resistors?

-Why do decoupling capacitors for RefP and RefN have a single 0.1 uf decoupling capacitor? The datasheet recommend two, a 10uF and a 0.1uf in parallel. I am curious about this in particular because RefP has a large Cap tied to ground in its connection to REF6025. Does this compensate?

sorry for so many questions.

AND THANK YOU FOR YOUR DETAILED ANSWERS!

  • 0
    TI__Guru 66186 points

    Hi Luke,

    Just to verify - are you referring to the Hardware Mode Pins? i.e. FILTER[1:0], OSR[1:0], HR, FORMAT, FSMODE? These pins are actually tied to DVDD through a 100k resistor pack on the ADS127L01EVM. When the respective switches close, these pins are pulled low.

    You bring up a good point about the reference capacitors. It is certainly necessary to have a larger bulk capacitor on the REFP inputs (at least 10uF -> 47uF) so that this node remains as stable as possible. The smaller 0.1uF cap directly across the reference inputs helps to supply some of the instantaneous current demanded by the reference switch-capacitor sampling network. However, depending on how far away the larger bulk capacitor is placed from the ADS127L01 REFP pin, it may be advisable to add a second capacitor in parallel to the 0.1uF (say 1uF). This can help a lot with settling at faster modulator frequencies and improve distortion performance.

    Best Regards,

  • 0 in reply to Ryan Andrews
    Intellectual 290 points
    Hey Ryan,

    Regarding the pins, this could be a misunderstanding on my part due to misreading the datasheet or being a novice. However, I am referring to both Hardware Mode Pins and pin8 INTLDO

    the datasheet specifies:
    Regarding INTLDO: "LVDD voltage selection pin (pull high to AVDD or low to AGND through 10-kΩ resistor)"

    Regarding Hardware Mode Pins: "Pull the hardware mode pins high to DVDD or low to DGND through 100-kΩ resistors"

    It seems the resistor pack on the ADS127L01EVM and the jumper INTLDO do not pull low to their respective grounds with 100k or 10k resistors. Is this a case of me misunderstanding the configuration or is there another explanation?

    Thank you for the advice on the capacitors!
  • 0 in reply to luke jet
    TI__Guru 66186 points

    Hi Luke,

    You're not wrong about the EVM configurations - when the switches/jumpers are closed, the respective pins on the ADS127L01 are tied directly to ground and the pull-up resistors are not in the path. The a single pull-up resistor on each pin still works to limit the current drawn from DVDD. Had we included a pull-up and a pull-down resistor on each pin to evaluate each option, we would've needed several more resistors and 3-pin jumpers/double-throw switches to do so and we just wanted to save space. In your final application, you'll likely only need a single pull-up or pull-down resistor on each pin based on the configuration you need. If you're going to use your uP to control these pins, a single pull-up resistor will suffice for each pin.

    Best Regards,

  • 0 in reply to Ryan Andrews
    Intellectual 290 points

    A single resistor can pull up several pins? that will save me a ton of board space. THANK YOU!

  • 0 in reply to luke jet
    TI__Guru 66186 points

    No - one per device pin. I was merely referring to not needing to make provisions for both a pull-up and a pull-down for a given pin if you already know how you want that pin to be configured.

    Regards,

  • 0 in reply to Ryan Andrews
    Intellectual 290 points
    okay, sorry. My background is in geology. Thanks for the correction