topic Re: How to Configure ADC? in MPC5xxx

How to Configure ADC?

Charith — Thu, 09 Feb 2023 05:10:09 GMT

Hi,

I am using MPC5748G Micro for ADC Conversion. I am using a circuit with 12Volts input. I need to monitor the input to that circuit using ADC, for that I am using Voltage Divider Circuit with 3.3Volts reference. I am continuously taking the readings through ADC channel but the readings are not constant, it is varying. I am using the below configurations for ADC. Please suggest if any corrections required.

void ADC1_PadConfig_ChanSelect(void) {
SIUL2.MSCR[PD0].B.APC = 1; /* PD0 = function ADC1_P[0] = ADC 1 channel 0 */
ADC_1.NCMR0.B.CH0 = 1; /* Enable channel 0 for normal conversion on ADC1 */

}

void ADC1_Calibration(void) {
uint32_t __attribute__ ((unused)) ADC1_Calibration_Failed = 1; /* Calibration has not passed yet */

ADC_1.MCR.B.PWDN = 1; /* Power down for starting calibration process */
ADC_1.MCR.B.ADCLKSEL = 0; /* ADC clock = bus clock/2 (80/2 MHz FS80) */
/* Note: Calibration requires it to be set at max 40MHz for best results */
ADC_1.CALBISTREG.B.TEST_EN = 1; /* Enable calibration test */
ADC_1.MCR.B.PWDN = 0; /* Power back up for calibration test to start */
while(ADC_1.CALBISTREG.B.C_T_BUSY); /* Wait for calibration to finish */
if(ADC_1.MSR.B.CALIBRTD) { /* If calibration ran successfully */
ADC1_Calibration_Failed = 0; /* Calibration was successful */
}
else {
ADC1_Calibration_Failed = 1; /* Calibration was not successful */
}
}

void ADC1_Init(void) { /* Initialize ADC1 module & start normal scan mode */
ADC_1.MCR.B.PWDN = 1; /* Power down for starting module initialization */
ADC_1.MCR.B.OWREN = 0; /* Enable overwriting older conversion results */
ADC_1.MCR.B.MODE = 1; /* Scan mode (1) used instead of one shot mode */
ADC_1.MCR.B.ADCLKSEL = 1; /* ADC clock = FS80 bus clock (80 MHz here) */
ADC_1.MCR.B.PWDN = 0; /* ADC_1 ready to receive conversation triggers */
ADC_1.MCR.B.NSTART = 1; /* Initiate trigger for normal scan */
}

void ADC1_Read_Chan_0 (void) { /* Read conversion results */
Result= ADC_1.CDR[0].B.CDATA; /* Read channel 0 conversion result data */
ResultInMv = (uint16_t) (ADC_VREF*Result/0xFFF); /* Conversion in mV */
}

Thanks

Re: How to Configure ADC?

davidtosenovjan — Thu, 09 Feb 2023 11:52:32 GMT

Could you specify how much it is varying? Conversion accuracy level depends also on noise level and ADC input impedance, usually you can see conversion result floating a bit unless it is high precison analog design what not the case of EVB.

Re: How to Configure ADC?

Charith — Fri, 10 Feb 2023 11:03:03 GMT

I am giving 12V input to the voltage divider circuit having 220k as R1 and 33k as R2, the output from voltage divider is 1.565V.

If I read the ADC readings from the Voltage Divider which should give 1.565V(or 1565mV) I am reading in the range of 1410mV to 1540mV.

Re: How to Configure ADC?

davidtosenovjan — Fri, 10 Feb 2023 12:29:11 GMT

These are very weak resistors and it'll be apparently reason why you are facing conversion inaccuracy.

Pay attention to following appnote, section "Input Leakage Offset Error":
https://www.nxp.com/docs/en/application-note/AN2438.pdf

I would recommend to use resistor divider consisting of resistors in orders units of kiloohms, not hundreds.

Re: How to Configure ADC?

Charith — Fri, 10 Feb 2023 12:42:33 GMT

Actually I am using 220kiloohms and 33kiloohms

Re: How to Configure ADC?

davidtosenovjan — Fri, 10 Feb 2023 16:04:28 GMT

Yes, I understand. These are weak resistors (i.e. high resistance). Use stronger resistors (i.e. with lower resistance).

Re: How to Configure ADC?

Charith — Wed, 15 Feb 2023 05:22:11 GMT

If I use lower resistor values 2.2Kohms and 330ohms the ADC values is stable.

Can I know how to configure 10bit ADC for the same.

Re: How to Configure ADC?

davidtosenovjan — Wed, 15 Feb 2023 09:00:36 GMT

This is common topic for all kind of ADCs.

Re: How to Configure ADC?

Charith — Thu, 16 Feb 2023 05:39:11 GMT

Can You Please Help me with Configuration changes to be done in code for 10bit ADC.