DSP28335ADC采样正弦信号波形失真的问题

采用28335ADC采样一个由信号发生器发出的正弦信号，信号频率为220Hz（模仿交流电信号频率，因为最终要采样交流电），方法是将数据由串口传PC，然后matlab绘图，但是结果波形有失真。主要程序和绘图结果如下。请问这主要是什么原因。

Assumes ePWM1 clock is already enabled in InitSysCtrl();

   EPwm1Regs.ETSEL.bit.SOCAEN = 1;        // Enable SOC on A group

   EPwm1Regs.ETSEL.bit.SOCASEL = 4;       // Select SOC from from CPMA on upcount

   EPwm1Regs.ETPS.bit.SOCAPRD = 01;        // Generate pulse on 1st event

   EPwm1Regs.CMPA.half.CMPA = 0x0080;   // Set compare A value

   EPwm1Regs.TBPRD = 0x1D4B;              // Set period for ePWM1

   EPwm1Regs.TBCTL.bit.CTRMODE = 00;   // count up and start

interrupt void  adc_isr(void)

{

  VoltageH[ConversionCount] = AdcRegs.ADCRESULT0 >>12;

  VoltageL[ConversionCount] = AdcRegs.ADCRESULT0 >>4;

  // If 40 conversions have been logged, start over

  if(ConversionCount == 127)

  {

    for (ConversionCount=0;ConversionCount<128;ConversionCount++)

    {

     scib_xmit(VoltageH[ConversionCount]);

     scib_xmit(VoltageL[ConversionCount]);

    }

    for(;;);

     ConversionCount = 0;

  }

  else ConversionCount++;

  // Reinitialize for next ADC sequence

  AdcRegs.ADCTRL2.bit.RST_SEQ1 = 1;         // Reset SEQ1

  AdcRegs.ADCST.bit.INT_SEQ1_CLR = 1;       // Clear INT SEQ1 bit

  PieCtrlRegs.PIEACK.all = PIEACK_GROUP1;   // Acknowledge interrupt to PIE

  return;

}