What should AVDD be when VDD = 3V?

I am checking values of my PCB with an nRF51822. VDD = 3V. AVDD ~= 800mV

I was expecting a the AVDD voltage drop to be much less. What should the AVDD voltage be? What could be causing the drop i am seeing on my DMM?

thank you.

  • Converted comment to answer due to limit on number of characters.

  • Thank you Martin. Um...how do I turn the radio on / off? (ugh - this sounds like a dumb question, my apologies).

  • Hi.

    When the radio is active and the DC/DC converter is enabled then AVDD should measure ~1.8V. Under what circumstances are you measuring? I assume you have a DC/DC converter setup with an inductor and capacitor at AVDD. Is the radio active? The DC/DC converter is turned ON only when the radio is running.

    EDIT: Clarifiaction: From Chapter 9 in the Reference Manual V3.0: "Enabling the DC/DC converter will not turn it on, but set it in a state where it automatically gets turned on when the radio is enabled and goes off again when the radio gets disabled. This is done to avoid wasting power running the DC/DC in between the radio events where current consumption is too low." So there is an important difference between "enabled/disabled" and "turned on/off".

    • When the DC/DC is enabled and turned ON then AVVD should measure ~1.8V.
    • When the DC/DC is enabled and turned OFF then AVDD is undefined. Maybe this is true in your case?
    • When the DC/DC is disabled then AVDD should be equal to VDD.

    As you can see in Figure 9 in Chapter 9 VDD is connected directly to AVDD when the DC/DC converter is disabled and vise versa.

    Try this:

    NRF_POWER->DCDCEN = 1;
    NRF_RADIO->TASKS_TXEN = 1;
    NRF_RADIO->TASKS_START = 1;
    

    This will enable the DC/DC converter and turn on the radio and hence, the DC/DC converter is also ON. With this code I am measuring 1.88V on AVDD on my nRF51 DK kit.

    Now you can try this:

    //NRF_POWER->DCDCEN = 1;
    NRF_RADIO->TASKS_TXEN = 1;
    NRF_RADIO->TASKS_START = 1;
    

    Now the DC/DC converter is disabled and AVDD is directly connected to VDD and is measuring 2.82V.

    Finally try this:

    NRF_POWER->DCDCEN = 1;
    //NRF_RADIO->TASKS_TXEN = 1;
    //NRF_RADIO->TASKS_START = 1;
    

    The DC/DC converter is now enabled, but OFF so my AVDD now measures anywhere between 0V - 1.1V.

    Finally, have a look at this great thread about power management: LDO vs DCDC

  • No need to apologize. There are no dumb questions, only dumb answers. I tried to elaborate a bit in my answer below.

  • Thank you very much. your answer is very helpful. I made a "bigger" mistake. I assumed AVDD was a power source for other things than the nRF8122's radio, i.e.: the other chips on the board (like op amps). This assumption points to not knowing what I am doing. Your answer helped me gain a better understanding so that I can make this work and move past it. Thank you!

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