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PGA of ADC0 is generating periodic spikes when no input is applied

Question asked by renganathan bs on Dec 18, 2014
Latest reply on Jan 6, 2015 by xiangjun.rong

I am trying to amplify an input signal of 500 uV Vp-p and when i saw the output it was generating a spike going from 65000 and 0. The output looks how an unstable amplifier behaves. This happens even if I don't apply the signals. This is the code I have used to make the PGA work. Also I don't have good idea on how to select the Vref for the PGA. As of now my ADC0 is connected to external Vref.


Code :

This is done to enable the PGA

void adc_pga_config()


  ADC0_PGA = 0x960000;



// intialise the adc0

void ADC0_Init16b(void)


        SIM_SCGC6 |= SIM_SCGC6_ADC0_MASK; //Gives clock to ADC0


        ADC0_CFG1 = (ADC_CFG1_MODE(3) | ADC_CFG1_ADIV(2)); //16bit mode

  ADC0_SC1A = ADC_SC1_ADCH(31); //disable module



        /* Configure ADC pins as ADC Inputs manually */


        /*In Kinetis the ADC pins are ADC inputs by default */



// Function used to get the adc value

unsigned short ADC0_Read16b(unsigned char channelNumber)


    ADC0_SC1A = 0x22;                /* Write to ADCSC1 to start conversion */

    while (!(ADC0_SC1A & 0xA2)); /* Wait until the conversion is complete */

    return ADC0_RA;



Connection :

Input is connected to PGA0_DP0 and PGA0_DM0 is connected to ground. ADC0 channel 2 is used to capture the signal .


Help me out with this one. Thanks in advance.