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ADS8684: AVDD power

AndreyCobra
Prodigy 100 points

Part Number: ADS8684

AVDD power 9 contact and 30 contact, consistently applied through the 1000 Ohm resistor +5 V supply,

and then measured at terminals 9 and 30, the voltage was 1.6 V ,

what could be the problem on 3 chip ADS8684.  

Other connections according to the documentation. 

DVDD - filed 3.3 V.    Filed GND on pins 4,6,8,31,32,28,29.

  • 0
    TI__Mastermind 25070 points
    Hello Andrey,

    The behavior you are seeing is expected from the device due to the 1k resistor placed on the power supply line. You can imagine this resistor, R1, behaving as a voltage divider with the device, R2. In this situation, R1=1k, and R2 = 1.762k, but you have three devices on the line, so adding them in parallel would yield a new value of R2 = 587. These values show that AVDD would infact be around 1.8, near what you are seeing. For simplcity, also note that in this situation, the device would scale its impedance with the value of R1, thus if R1 = 1k *5 = 5k , the device R2 = 1.762k *5 = 8.8k.
    May I ask why the 1000 resistor?

    Regards
    Cynthia
  • 0 in reply to Cynthia
    Prodigy 100 points
    1000 ohms is installed in the example
    www.ti.com/.../TIDA-00310 page 1 resistor FB3,
    I made 3 boards with ADS8684 and everywhere after the resistor is 1.6 V
    Thank you
  • 0 in reply to AndreyCobra
    TI__Mastermind 25070 points

    Hello,

    I see where the misunderstanding has occurred. 

    The component FB3, as well as FB1,2, and L1, L2, are ferrite beads, not resistors. They can be thought more as inductors. 

    It is common to use ferrites for isolation, in this case they are used to provide clean rails to the system, the rail is protected from any surges that may occur on any other power rails that use the same source. 

    Although in the schematic it shows that the ferrite bead is 1000ohm, it does not act as a resistor, this is merely a parameter of the ferrite.  Placing a resistor in place of the ferrite is not an equivalent change. 

    One ferrite is placed at the rail, and all the power supplies are connected to the other side for a clean and protected power rail, as you can see, the "+5V" rail changes names to "+5_ISO_AVDD" on the other side of the ferrite. 

    Regards, Cynthia