2812搜查令+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 #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; ...... }
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