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1、ADC分类及参数ADC分类 直接转换模拟数字转换器（Direct-conversion ADC），或称Flash模拟数字转换器（Flash ADC） 循续渐近式模拟数字转换器（Successive approximation ADC） 跃升-比较模拟数字转换器（Ramp-compare ADC） 威尔金森模拟数字转换器（Wilkinson ADC 集成模拟数字转换器（Integrating ADC） Delta编码模拟数字转换器（Delta-encoded ADC） 管道模拟数字转换器（Pipeline ADC） Sigma-Delta模拟数字转换器（Sigma-delta ADC） 时间交织
2、模拟数字转换器（Time-interleaved ADC） 带有即时FM段的模拟数字转换器 时间延伸模拟数字转换器（Time stretch analog-to-digital converter, TS-ADC1、闪速型2、逐次逼近型3、Sigma-Delta型1. 闪速ADC闪速ADC是转换速率最快的一类ADC。闪速ADC在每个电压阶跃中使用一个比较器和一组电阻。2. 逐次逼近ADC逐次逼近转换器采用一个比较器和计数逻辑器件完成转换。转换的第一步是检验输入是否高于参考电压的一半，如果高于，将输出的最高有效位(MSB)置为1。然后输入值减去输出参考电压的一半，再检验得到的结果是否大于参考电压
3、的1/4，依此类推直至所有的输出位均置“1”或清零。逐次逼近ADC所需的时钟周期与执行转换所需的输出位数相同。3. Sigma-delta ADCSigma-delta ADC采用1位DAC、滤波和附加采样来实现非常精确的转换，转换精度取决于参考输入和输入时钟频率。Sigma -delta转换器的主要优势在于其较高的分辨率。闪速和逐次逼近ADC采用并联电阻或串联电阻，这些方法的问题在于电阻的精确度将直接影响转换结果的精确度。尽管新式ADC采用非常精确的激光微调电阻网络，但在电阻并联中仍然不甚精确。sigma-delta转换器中不存在电阻并联，但通过若干次采样可得到收敛的结果。Sigma-del
4、ta转换器的主要劣势在于其转换速率。由于该转换器的工作机理是对输入进行附加采样，因此转换需要耗费更多的时钟周期。在给定的时钟速率条件下，Sigma-delta转换器的速率低于其它类型的转换器；或从另一角度而言，对于给定的转换速率，Sigma-delta转换器需要更高的时钟频率。Sigma-delta转换器的另一劣势在于将占空(duty cycle)信息转换为数字输出字的数字滤波器的结构很复杂，但Sigma-delta转换器因其具有在IC裸片上添加数字滤波器或DSP功能而日益得到广泛应用。Atmel AVR127: Understanding ADC ParametersThis applica
5、tion note is about the basic concepts of analog-to-digital converter (ADC) and the parameters that determine the performance of an ADC.These ADC parameters are of good importance since they determine the accuracy of the ADCs output.The parameters can be broadly classified into static performance par
6、ameters and dynamic performance parameters.Static performance parametersare those parameters that are not related to ADCs input signal.These parameters are measured and analyzed for all types of ADCs(ADCs integrated within the microcontroller or standalone ADCs whose operating frequency are usually
7、higher).Instead,dynamic performance parametersare related to ADCs input signal and their effects are significant with higher frequencies.Major static parametersinclude gain error, offset error, full scale error and linearity errorswhereas some important dynamic parametersinclude signal-to-noise rati
8、o (SNR), total harmonic distortion (THD),signal to noise and distortion (SINAD) and effective number of bits (ENOB).Basic ConceptsAn ADC is an electronic system or a module that has analog input, reference voltage input and digital outputs.The ADC convert the analog input signal to a digital output
9、value that represents the size of the analog input comparing to the reference voltage.It basically samples the input analog voltage and produces an output digital code for each sample taken.Figure 1-1. Basic diagram of ADCTo get a better picture about the ADC concepts, let us first look into some ba
10、sic ADC terms used.1.1 Input Voltage RangeThe input voltage range of an ADC is determined by the reference voltage (VREF) applied to the ADC.A reference voltage can be either internal voltage or external voltage by applying a voltage on an external pin of the microcontroller.Generally reference volt
11、age can be selected by configuring the corresponding registers bit field of the microcontroller.ADC will saturate with a analog voltage higher than the reference voltage,so the designer must make sure that the analog input voltage does not exceed the reference voltage.The input voltage range is also
12、 called asconversion range.If ADC runs in signed mode (the mode produces signed output codes), it allows negative analog input voltages.In such cases the analog input range is from VREF to +VREF.An ADC which accepts both positive and negative input voltages is called asbipolar ADCwhereas an ADC that
13、 accepts only positive input voltage is called asunipolar ADC.1.2 ResolutionThe entire input voltage range (from 0V to VREF) is divided into a number of sub-ranges.Each sub range is assigned a single output digital code.A sub range is also called LSB (least significant bits) and the number of sub ra
14、nges is usually in powers of two.The total number of sub ranges is called the resolution of the ADC.For an example, an ADC with eight LSBs has the resolution of three bits (23 = 8).If an ADCs resolution is three bits then it also means that the code width of the output is three bits.1.3 Quantization
15、The LSB is determined if input analog voltage lies in the lowest sub-range of the input voltage range.For example, consider an ADC with VREF as 2V and resolution as three bits.Now the 2V is divided into eight sub-ranges, so the LSB voltage is within 250mV.Now an input voltage of0V as well as 250mVis
16、 assigned to the same output digital code000.This process is called as quantization.1.4 Conversion ModeA conversion mode determines how the ADC processes the input and performs the conversion operation.A standard ADC has basically two types of conversion modes.1. Single ended conversion mode.2. Differenti
