topic Re: TWR-KM34Z75M AFE Readings in Kinetis Microcontrollers

TWR-KM34Z75M AFE Readings

Edrianocarlos — Thu, 17 Sep 2015 19:18:02 GMT

Hello All.

I am testing the AFE 24 bits and it is working but.

I short circuit the ADP0 and ADM0 and get readings of -48000 the readings are steady they are not varying a lot.

Is that normal this offset?

the clock i am using is 4MHz internal.

void Configura_Afe_24bits(void)
{
  SIM_SCGC6 |=  (SIM_SCGC6_AFE_MASK);  //habilita modulo AFE 24 bits
  SIM_SCGC4 |=  SIM_SCGC4_VREF_MASK; // HABILITA PERIFERICO GERADOR DE REFERENCIA
  
  VREF_VREFH_SC = VREF_VREFH_SC_VREFEN_MASK + VREF_VREFH_SC_REGEN_MASK
                  + VREF_VREFH_SC_ICOMPEN_MASK + VREF_VREFH_SC_MODE_LV(1);
  
  AFE_CH0_CFR = (   AFE_CFR_DEC_OSR(5)
                  + AFE_CFR_PGA_EN_MASK
                  + AFE_CFR_PGA_GAIN_SEL(1)
                  + AFE_CFR_BYP_MODE(0)
                  + AFE_CFR_SD_MOD_EN_MASK
                  + AFE_CFR_DEC_EN_MASK
                  + !AFE_CFR_CC_MASK
                  + AFE_CFR_DEC_CLK_EDGE_SEL(0)
                  + AFE_CFR_DEC_CLK_INP_SEL(0)
                  + !AFE_CFR_HW_TRG_MASK
                 );
  
  AFE_CH1_CFR = AFE_CH0_CFR;
  AFE_CH2_CFR = AFE_CH0_CFR;
  AFE_CH3_CFR = AFE_CH0_CFR;
  AFE_CR = (  AFE_CR_MSTR_EN_MASK
              + !AFE_CR_LPM_EN_MASK
              + AFE_CR_RST_B_MASK   
              + !AFE_CR_DLY_OK_MASK
              + AFE_CR_RESULT_FORMAT_MASK
              + AFE_CR_STRTUP_CNT(0)
            );  
}
void Ler_Afe_Adc(void)
{
  AFE_CR |= AFE_CR_SOFT_TRG0_MASK;
  while((AFE_SR & AFE_SR_COC0_MASK)==0)
  {
  }
  resultado = AFE_CH0_RR;
}

Re: TWR-KM34Z75M AFE Readings

xiangjun_rong — Fri, 18 Sep 2015 07:46:14 GMT

Hi, Edriano,

i do not think the result -48000 is normal, how about connect both ADP0 and ADM0 to GND so that the common mode voltage is zero, then have a test.

I will test your code on my tower board and give your my test result next week.

Xiangjun Rong

Re: TWR-KM34Z75M AFE Readings

Edrianocarlos — Fri, 18 Sep 2015 13:46:12 GMT

Hi Xiangjun Rong.

I did that and the value decrease to -30000.

Re: TWR-KM34Z75M AFE Readings

Edrianocarlos — Mon, 21 Sep 2015 15:49:29 GMT

Hi all.

I am keeping studying this Analog front end and know i got better measures changing the Vref settings

VREF_VREFH_TRM |= VREF_VREFH_TRM_CHOPEN_MASK;

VREF_VREFH_SC = VREF_VREFH_SC_VREFEN_MASK + VREF_VREFH_SC_REGEN_MASK

+ VREF_VREFH_SC_ICOMPEN_MASK + VREF_VREFH_SC_MODE_LV(1);

My problem now is that i can't find the Input impedance and i think i will have to use an external buffer to get measures.

As an example if i use a 50R to short circuit the +-input i got 0.0012V offset if i use a 4700R i get 0.088V if i turn the PGA on the results are even worse.

Does anyone know what is the input impedance for this AFE? i was not able to find the information.

thanks.