ADuCM350 - VBIAS is not equal to 1.1V

Hello,

I am using the AFE to read out a voltage of about 0.9V and I never get the right value.

My conversion code is:

float measuredVoltage = 1.1 + (float)(AFE_Val - 32768) / 18204;

The AFE_Val I get is around 12200.

The problem is that as I should have 1.1V as VBIAS, I measure about 1.75V. I have followed exactly the dev board schematic that means I have 0.47uF connected to VBIAS and 4.7uF connected to VREF.

My VREF voltage is 1.8V and it is correct.

Would you have an idea why I don't get the right result ?

Thanks.

  • 0
    •  Analog Employees 
    on Sep 30, 2020 1:39 AM 1 month ago

    Hi Jimmy,

    Can you check if your VBIAS is shorted to VREF or other parts of the board? Have you tried your code in the EVAL kit first? Your conversion formula is correct so it should be a problem with the hardware.

    regards,

    Mark

  • Hi Mark,

    I have already checked but VBIAS is apparently not shorted to VREF. Once I do the power up and initialization of the AFE, VBIAS has a voltage of about 1.75V and before that it has about 3V (VCCM).

    I have also tried with the EVAL kit and when I measure the VBIAS (at C41), the voltage is well 1.10V. When I perform a single measurement with the function "adi_AFE_AuxChanMeasFixed" at AN_C, I get 0.886V instead of 0.916. At least that's much closer than with my custom board.

    I would agree that it is an hardware issue. I have attached the schematic and maybe you will find a possible mistake.

    Best,

    Jimmy

  • Hi Mark,

    Here is also my config variable:

    /*
     * AFE auxiliary channel measurement of channel C(single measurement)
     */
    const uint32_t seq_afe_auxchanmeasC[] = {
        0x00080096, /*  0 - Safety Word, Command Count = 8, CRC = 0x96                                                                                  */
        0x84007818, /*  1 - AFE_FIFO_CFG: DATA_FIFO_DMA_REQ_EN = 1 DATA_FIFO_SOURCE_SEL = 0b11 CMD_FIFO_DMA_REQ_EN = 1 CMD_FIFO_EN = 1 DATA_FIFO_EN = 1 */
    //    0xA0000000, /*  2 - AFE_ADC_CFG: MUX_SEL = 0 GAIN_OFFS_SEL = 0 ANEXCITESW_EN = 0                                                                */
        0xA000000A,   /* 2 - AFE_ADC_CFG: MUX_SEL = 01010 (AN_C) GAIN_OFFS_SEL = 0 ANEXCITESW_EN = 0     */
        0x0000063E, /*  3 - Wait: 100 us                                                                                                                */
        0x800301B0, /*  4 - AFE_CFG: ADC_CONV_EN = 1 SUPPLY_LPF_EN = 1                                                                                  */
        0x0009087E, /*  5 - Wait: 37 ms                                                                                                                 */
        0x800200B0, /*  6 - AFE_CFG: ADC_CONV_EN = 0 SUPPLY_LPF_EN = 0                                                                                  */
        0xA0000000, /*  7 - AFE_ADC_CFG: MUX_SEL = 0 ANEXCITESW_EN = 0                                                                                  */
        0x82000002  /*  8 - AFE_SEQ_CFG: SEQ_EN = 0                                                                                                     */
    };

    Best,

    Jimmy

  • 0
    •  Analog Employees 
    on Oct 19, 2020 1:11 PM 1 month ago in reply to JimmySIP

    Hi Jimmy,

    The schematic is correct but are you sure that the VREF and VBIAS are not physically shorted on the board? You see the BGA balls of these pins are beside each other.

  • 0
    •  Analog Employees 
    on Oct 19, 2020 1:12 PM 1 month ago in reply to JimmySIP

    Hi Jimmy,

    Just make sure that you followed the initialization sequence of the ADUCM350 LDOs.See user guide page 389 for details. Our example codes are also implementing the sequence so you can just re-use those.

    www.analog.com/.../aducm350_ug-587.pdf