[求助] MSP430 ADC采样悬空时采样有1.8V电压

到底是什么原因 请高人给我分析一下    不悬空可以采样别的电压  就是悬空有1.8V电压

void adinit()
{
  P6SEL|=BIT1;
  ADC12CTL0|=SHT0_15+ADC12ON+MSC;  //设置采样周期不开内部参考电压
  ADC12CTL1|=SHP+CONSEQ_3;                       // 表示采样直接由ADC12SC控制,时钟源默认内部时钟
  ADC12MCTL1|=INCH_1;                 //用输入通道1即A1

  ADC12IE|=BIT1;
  //ADC12IFG=0xff;
//ADC12采样中断
  ADC12CTL0|=ENC;
  ADC12CTL0 |= ADC12SC; //开启转换
  _EINT();   //开启总中断
}   

#pragma vector=ADC12_VECTOR
__interrupt void ADC12ISR (void)
{
  //static uchar index = 0;
  //average=ADC12MEM1;
 // while((ADC12IFG & BIT1)!=0);

    results[index++] = ADC12MEM1;                      // Move results, IFG is cleared
    if(index == 31)
    {
         uchar n;        
         average = 0;
        
         for(n = 0; n < 32; n++)
         {
           average += results[n];
         }
           average >>= 5;                           //除以32求平均值
           average=(average*3300)/4096;
       index = 0;

    }
     ADC12CTL0 &= ~ADC12SC;
}