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Original question:

ADS1285: ADS1285

ADS1285: AVSS bypass capacitor requirement

LeonardEllis
Guru 71516 points
Part Number: ADS1285

Datasheet section 9 describes power supplies and the need for bypass caps between AVDD1 and AVDD2 and AGND.  For single-supply operation, AVSS is connected to AGND.  However, for dual-supply operation, AVSS is -2.5v but there is no recommendation for a bypass cap.  Is a bypass cap recommended for AVSS power supply?

  • 0
    TI__Mastermind 37970 points

    Hello Leonard,

    When using dual-supply configuration, you will want to keep the 1uF+0.1uF caps between AVDD1 and AVSS, and add an additional 1uF+0.1uF between AVSS and Ground.  The EVM user's guide includes schematics that show the preferred bypass connection scheme.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    TI__Guru 71516 points

    Thanks for the feedback, Keith.  I'm still wondering why we would connect a bypass/filter between power supply rails instead of to GND?  With single power supply, AVDD1 filters noise to GND, but with the dual supply format, it is now filtered to AVSS, inducing the noise onto the AVSS rail. 

    On the EVM schematic, I see there is a 1uF cap between AVSS and AGND and if this is an acceptable arrangement, shouldn't there also be a 0.1uF cap in parallel to help filter the high frequency noise? 

    ~Leonard   

  • +1 in reply to LeonardEllis
    TI__Mastermind 37970 points

    Hi Leonard,

    Internal to the ADS1285, the AVDD1 supply is relative to AVSS, and all high frequency requirements for this supply are relative to AVSS.  You will get best noise performance by including capacitors directly between AVDD1 and AVSS in this case.  The device will still perform well if AVDD1 and AVSS are only bypassed to analog ground, but best performance is achieved with bypass capacitors directly between AVDD1 and AVSS.

    Regarding the 1uF+0.1uF between AVSS and analog GND, it depends on the capacitor and package size.  For most modern ceramic capacitors, the ESR and ESL are very low, and a single 1uF ceramic capacitor is sufficient.  However, designers will still include the smaller 0.1uF to allow for more variation in the 1uF capacitor used.  Plus, old habits are hard to break.  Before modern multilayer ceramics, the 0.1uF was always necessary, but with modern ceramic capacitors, this is not always the case.

    Regards,
    Keith