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DDC112: Isolation resistor

Christer Bengtsson
Prodigy 100 points
Part Number: DDC112

I am designing an instrument based on the DDC112 as a student project.

The lower part of figure 29 in the datasheet shows the DVDD power supply derived from the AVDD supply with a < 10 ohm isolation resistor. I would be grateful if someone could explain how the resistor protects the analog supply from noise. The isolation resistor is absent in Application Note MULTI-DDC112 BOARD DESIGN (figure 1a) as well as in User Guide DEM-DDC112U-C EVALUATION FIXTURE (figure 10). 

I also noticed that the Application Note MULTI... (figure 1a) has decoupling capacitors of 1uF at the supply inputs and the User Guide DEM-DDC.. (figure 10) has 10uF + 0.1uF. Why so?

Regards,

Christer

  • 0
    TI__Genius 12025 points
    Hi Christer,

    How are you?
    Thanks for using DDC112 device.
    I will forward your question to the system engineer.
    He will reply to you very soon.

    Thank you again!

    Best regards,
    Chen
  • 0 in reply to Chen Kung
    TI__Genius 12025 points
    Hi Christer,

    How are you?
    since digital circuit portion (powered by DVDD) usually is pretty noisy,
    this 10ohm isolation resistor with 10uF+0.1uF are acting like a low pass filter
    with 1.6kHz cutoff freq to block the noise coming from DVDD (generated by digital signal)
    to AVDD. This assumes the digital signal freq is running much higher than 1.6kHz.

    Thank you!

    Best regards,
    Chen
  • 0 in reply to Chen Kung
    Prodigy 100 points

    Chen,

    Thank you, I understand. I thought there was a more complicated explanation. Maybe I read to much before asking. Still this lowpass filter is absent in the documents I referred to. Obviously that works too, maybe not as good.

    Best regards,
    Christer