Попробуйте сделать нужную тактовую частоту периферии
EALLOW;
// тактовая частота SPI (всей "медленной периферии")
SysCtrlRegs.LOSPCP.all=0x6;

EDIS;

а также инициализируйте ацп так:
InitAdc();
AdcRegs.ADCTRL1.all = BIT4;
AdcRegs.ADCTRL2.all = BIT15|BIT8;
AdcRegs.ADCTRL3.all |= BIT0;

AdcRegs.ADCMAXCONV.all = 0x0007;
AdcRegs.ADCCHSELSEQ1.bit.CONV00 = 0x0;
AdcRegs.ADCCHSELSEQ1.bit.CONV01 = 0x1;
AdcRegs.ADCCHSELSEQ1.bit.CONV02 = 0x2;
AdcRegs.ADCCHSELSEQ1.bit.CONV03 = 0x3;
AdcRegs.ADCCHSELSEQ2.bit.CONV04 = 0x4;
AdcRegs.ADCCHSELSEQ2.bit.CONV05 = 0x5;
AdcRegs.ADCCHSELSEQ2.bit.CONV06 = 0x6;
AdcRegs.ADCCHSELSEQ2.bit.CONV07 = 0x7;

Где initADC - ихняя функция
C:\tidcs\c28\dsp281x\v100\DSP281x_common\source\DSP281x_Adc.c
void InitAdc(void)
{
extern void DSP28x_usDelay(Uint32 Count);

// To powerup the ADC the ADCENCLK bit should be set first to enable
// clocks, followed by powering up the bandgap and reference circuitry.
// After a 5ms delay the rest of the ADC can be powered up. After ADC
// powerup, another 20us delay is required before performing the first
// ADC conversion. Please note that for the delay function below to
// operate correctly the CPU_CLOCK_SPEED define statement in the
// DSP28_Examples.h file must contain the correct CPU clock period in
// nanoseconds. For example:

AdcRegs.ADCTRL3.bit.ADCBGRFDN = 0x3; // Power up bandgap/reference circuitry
DELAY_US(ADC_usDELAY); // Delay before powering up rest of ADC
AdcRegs.ADCTRL3.bit.ADCPWDN = 1; // Power up rest of ADC
DELAY_US(ADC_usDELAY2); // Delay after powering up ADC
}