EK-TM4C1294XL: ADC differential mode

Hi,

I had set up my ADC in single-ended operation and it gave me accurate readings.

However when I changed it to differential mode I am getting values which do not make much sense. I followed the differential example, I am not sure what is happening; when I tie PE2 to ground and PE3 to 3.3V I get an ADC reading of 4095 which is correct. However if I tie both PE2 and PE3 to ground I get a reading of about 1900 and it jumps around a lot in this range +/- a few hundred readings. In single ended operation when I would tie PE3 to ground I would get the expected reading of 0 with a peak of 4 every once in a while.

I have a current sensor (HXS 20-NP by LEM) which gives a voltage output, the sensor has a reference voltage, I am trying to measure the difference between the reference voltage (around 2.5V) to the voltage output. if there is no current the sensor output and reference value give me me the same value of  2.5V. I was expecting when PE2 is tied to the voltage reference pin and PE3 is tied to the Vout pin, with no current passing through, I would get a 0 value for my ADC, however, I am getting the same type of values when both pins are tied to ground.

Here is my code, any help would be appreciated.

void ConfigureADC(void)
{
    SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOE);
    SysCtlPeripheralEnable(SYSCTL_PERIPH_ADC0);
    GPIOPinTypeADC(GPIO_PORTE_BASE, (GPIO_PIN_3 | GPIO_PIN_2));
    while(!SysCtlPeripheralReady(SYSCTL_PERIPH_ADC0))
    {
    }
    ADCSequenceConfigure(ADC0_BASE, 3, ADC_TRIGGER_PROCESSOR, 0); 
    ADCSequenceStepConfigure(ADC0_BASE, 3, 0, (ADC_CTL_D | ADC_CTL_CH0 | ADC_CTL_IE | ADC_CTL_END));
    ADCSequenceEnable(ADC0_BASE, 3);
    ADCIntClear(ADC0_BASE, 3);
}
void ADCGetValue (void)
{
    uint32_t ADCValues[1];
    uint32_t VoltageValue = 0;
    uint32_t AmperageValue = 0;
    TimerIntClear(TIMER1_BASE, TIMER_TIMA_TIMEOUT);
    ADCProcessorTrigger(ADC0_BASE, 3);
    while(!ADCIntStatus(ADC0_BASE, 3, false))
    {
    }
    ADCIntClear(ADC0_BASE, 3);
    ADCSequenceDataGet(ADC0_BASE, 3, ADCValues);
    VoltageValue = (uint32_t)((float)(ADCValues[0]*3300)/4096);
    AmperageValue = (10000*((uint32_t)(ADCValues[0] - 3103)))/775;
    UARTprintf("ADC Value = %d\n", ADCValues[0]);
    UARTprintf("Voltage Value (mV): %3d\n", VoltageValue);
    UARTprintf("Current Value (mA): %3d\n", AmperageValue);
//    SysCtlDelay(10000000);
}

void ADCTimerConfig (void)
{
    SysCtlPeripheralEnable(SYSCTL_PERIPH_TIMER1);
    TimerConfigure(TIMER1_BASE, TIMER_CFG_PERIODIC);
    TimerClockSourceSet(TIMER1_BASE, TIMER_CLOCK_SYSTEM);
    TimerLoadSet(TIMER1_BASE, TIMER_A, g_ui32SysClock);
    IntRegister(INT_TIMER1A, ADCGetValue);
    IntPrioritySet(INT_TIMER1A, 2); // change priorities
    TimerIntClear(TIMER1_BASE, TIMER_TIMA_TIMEOUT);
    TimerIntEnable(TIMER1_BASE, TIMER_TIMA_TIMEOUT);
    IntEnable(INT_TIMER1A);
    TimerEnable(TIMER1_BASE, TIMER_A);



}