F2812内部集成了ADC转换模块。该模块是一个12位、具有流水线结构的模数转换器，内置双采样保持器（S/H），可多路选择16通道输入，快速转换时间运行在25 MHz、ADC时钟或12.5 Msps，16个转换结果寄存器可工作于连续自动排序模式或启动/停止模式。在实际使用中，ADC的转换结果误差较大，如果直接将此转换结果用于控制回路，必然会降低控制精度。（最大转换误差可以达到9%左右）
F2812的ADC转换精度较差的主要原因是存在增益误差和失调误差，要提高转换精度就必须对两种误差进行补偿。
对于ADC模块采取了如下方法对其进行校正：
选用ADC的任意两个通道（如A3，A4）作为参考输入通道，并分别提供给它们已知的直流参考电压作为输入（RefHigh和RefLow），通过读取相应的结果寄存器获取转换值，利用两组输入输出值求得ADC模块的校正增益和校正失调，然后利用这两个值对其他通道的转换数据进行补偿，从而提高了ADC模块转换的准确度。
实现校准的硬件电路在本文中不作描述，在有关资料中可以查到。下面是该算法的C语言实现：
//首先计算两个通道的参考电压转换后的理想结果
// A4 = RefHigh = 2.5V ( 2.5*4095/3.0 = 3413 ideal count)
// A3 = RefLow = 0.5V ( 0.5*4095/3.0 = 683 ideal count)
#define REF_HIGH_IDEAL_COUNT 3413
#define REF_LOW_IDEAL_COUNT 683
#define SAMPLES 63
//定义所需的各个变量
Uint16 Avg_RefHighActualCount;
Uint16 Avg_RefLowActualCount; /
Uint16 CalGain; // Calibration Gain
Uint16 CalOffset; // Calibration Offset
Uint16 SampleCount;
Uint16 RefHighActualCount;
Uint16 RefLowActualCount;
//对各个变量进行初始化
void InitCalib()
{
Avg_RefLowActualCount = 0;
Avg_RefLowActualCount = 0;
Avg_RefHighActualCount = 0;
RefHighActualCount = 0;
RefLowActualCount = 0;
CalGain = 0;
CalOffset = 0;
SampleCount = 0;
}
//获得校准增益和校准失调
// Algorithm: Calibration formula used is:
//
// ch(n) = ADCRESULTn*CalGain - CalOffset
// n = 0 to 15 channels
// CalGain = (RefHighIdealCount - RefLowIdealCount)
// -----------------------------------------
// (Avg_RefHighActualCount - Avg_RefLowActualCount)
//
// CalOffset = Avg_RefLowActualCount*CalGain - RefLowIdealCount
//
// A running weighted average is calculated for the reference inputs:
//
// Avg_RefHighActualCount = (Avg_RefHighActualCount*SAMPLES
// + RefHighActualCount) / (SAMPLES+1)
//
// Avg_RefLowActualCount = (Avg_RefLowActualCount*SAMPLES
// + RefLowActualCount) / (SAMPLES+1)
//
void GetCalibParam()
{
RefHighActualCount = AdcRegs.ADCRESULT4 >>4;
RefLowActualCount = AdcRegs.ADCRESULT3 >>4;
if(SampleCount > SAMPLES)
SampleCount = SAMPLES;
Avg_RefHighActualCount = (Avg_RefHighActualCount * SampleCount
+ RefHighActualCount) / (SampleCount+1);
Avg_RefLowActualCount = (Avg_RefLowActualCount * SampleCount
+ RefLowActualCount) / (SampleCount+1);
CalGain = (REF_HIGH_IDEAL_COUNT - REF_LOW_IDEAL_COUNT)
/ (Avg_RefHighActualCount - Avg_RefLowActualCount);
CalOffset = Avg_RefLowActualCount*CalGain - RefLowIdealCount;
SampleCount++;
}
//在ADC_ISR中，对其他各个通道的结果进行修正：
interrupt void adc_isr(void)
{
GetCalibParam();
......
newResult n= AdcRegs.ADCRESULTn*CalGain - CalOffset;
......
}
通过上面的代码，配合硬件电路改动，可以大幅实现提高ADC采样的精度，实现更灵敏、更精确的控制。
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