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ADS127L11: regarding: Power Rail Decap with -ve supply source for AVSS

Bala sundaram S
Prodigy 10 points

Part Number: ADS127L11

Hi Team,

    We are implementing your 24bit ADC ADS127L11IPWR in one of our design we need some clarification based on the datasheet.

We are providing AVDD +Ve voltage and AVSS -Ve voltage.

as per datasheet section 8.3.6.1 AVDD1 and AVSS For CAPA, AVDD1 & AVDD2 pins it is recommended to use the bypass capacitor (decap) to AVSS pin

image.png

In our application AVSS pin is connected to -ve voltage source. providing decaps to this source may couple noise to this source voltage.

We have referred your application circuit where -Ve voltage is not generated (AVSS was shorted with DGND with 0ohm; AVSS=0V). Hence decaps are provided to AVSS net which is AGND.

our query is

How decaps should be provided for our application (AVSS=-Ve voltage)?

shall we connect the decaps commonly to AGND for both AVDD and AVSS pins. (where AGND=0V)

  • 0
    TI__Mastermind 45135 points

    Hi,

    We have received your post and the investigation will take some time. Thank you for your patience.

  • 0
    TI__Mastermind 44485 points

    Hi 

    The +Ve and -Ve bipolar supplies should have a bypass cap directly between these rails, in addition to bypass caps from +Ve to AGND and -Ve to AGND. 

    If you look at the ADS127L11EVM, we use a 5V regulator to set the voltage between AVDD and AVSS, and then for bipolar operation, we connect AVSS to a regulated -2.5V supply rail.

    The below diagram shows recommended bypass capacitor values and connections when using bipolar (+/-2.5V) supplies.