TMS320F2812 DSP编程之AD采样精度的校准算法
TMS320F2812 DSP编程之AD采样精度的校准算法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
[*]#defineSAMPLES 63
[*]//定义所需的各个变量
[*]Uint16Avg_RefHighActualCount;
[*]Uint16Avg_RefLowActualCount; /
[*]Uint16CalGain; // Calibration Gain
[*]Uint16CalOffset; // Calibration Offset
[*]Uint16SampleCount;
[*]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 voidadc_isr(void)
[*]{
[*]GetCalibParam();
[*]......
[*]newResult n= AdcRegs.ADCRESULTn*CalGain - CalOffset;
[*]......
[*]}
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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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