FM4 ADC Usage Hardware Consideration - KBA227596

Version 3

    Author: TakashiM_61           Version: **

     

    Question:

    What is the hardware consideration to use ADC in FM4 device?

     

    Answer:
    In order to use ADC properly, the reference voltage (Analog Voltage Reference High (AVRH) /Analog Voltage Reference Low (AVRL)) are important. It is necessary to suppress the reference voltage swing and avoid noise on power and ground lines as much as possible.

    From schematic/circuit observation, the following points must be considered:

    • AVRH / AVRL
      • AVRH-C Bypass capacitor “0.1 μF” is connected between AVRH and AVRL lines.
      • AVRH-R Register “3.3Ω” is populated on AVRH line. This is to make high frequency noise flows in the route of by AVRH-C Bypass capacitor.
    • AVCC / AVSS
      • AVCC-C Bypass capacitor “0.1 μF” is connected between AVCC and AVSS lines.
      • AVCC-R Register “10Ω ” is populated on AVCC line. This is to make high frequency noise flows in the route of by AVCC-C Bypass capacitor.

     

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    From Layout observation, AVRH-R, AVRH-C, AVCC-R and AVCC-C should be populated close to each pin without any vias to avoid redundant line length between the pin and register/capacitor:

    • AVRH pin to AVRH-R
    • AVRH pin to AVRH-C
    • AVRL pin to AVRH-C
    • AVSS pin to AVCC-C
    • AVCC pin to AVCC-C
    • AVCC

     

    The line between the AVRL pin to AVRH-C should be separated from the line between AVSS pin and AVSS-C to avoid common impedance.

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