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ADS1248: inductive supply connection questions

Kailyn Chen
Genius 9560 points
Part Number: ADS1248


Hello，

The datasheet described the Do's and Don'ts (continued) at 10.3 chapter, it didn't allow to connect inductor between VCC and GND. Could you please help to describe what is the reason?

One customer used 5V to 3.3V output ,  the output has connected ferrite bead, and found that there is ripple of the 3.3V  which is fed to ADS1248, and it caused the ADS1248 couldn't work normally。If remove the ferrite bead,  the ripple disappear. That is why we asked the ADS1248 datasheet about the Do's and Don'ts (continued) at 10.3 chapter description. 

 

Best regards

kailyn 

  • +1
    TI__Guru** 113765 points

    Hi Kailyn,

    The IC designer designs the ADC so that the AVDD and DVDD supply source uses a low impedance source with only a small PCB inductance.  The reason for a low impedance supply source requirement is due to the internal startup and switching modes for the highly integrated ADC.  Any sharp current demands by the ADC requires available charge to not be impeded.  Inductance by nature is going to 'choke' fast edge rates of current.  A ferrite, although not as bad as a standard inductor, does have sufficient inductance to limit required charge to startup the internal oscillator, internal reference and IDAC current to name a few.  This is the primary reason not to use any form of inductance on the supply.

    It may also be possible that an oscillation can start and be sustained due to R-L-C combinations.  An LDO supply source, like what is shown in the schematic, should not require the ferrites as the output should already be quite clean and not need the ferrite to filter noise.  Any noise created past the regulator would most likely be a system issue caused by a lack or poor positioning of bypass capacitors and issues with respect to the PCB ground.

    Best regards,

    Bob B