发现与高精度AD转换单片机

　　发现与高精度A/D转换单片机

　　越来越多的嵌入式系统基于采集各种传感器的测量数据，典型的应用包括医疗、智能电网、能量控制以及工业控制系统。因此，对精密A/D转换的需求日益增长。大多数微控制器来与A/D转换器，但他们通常是10或12位转换器，通常有4或5比特错误（计数）–使他们有效的8 -或9位转换器。

　　换句话说，8的1位的错误大约是0.4%。不太坏，真的吗？。对于一个真正的8位分辨率，我通常计算为1%，或者稍微少一点——分解一些参考电压随时间和温度的漂移。然而，如果你需要更精确，可能更多，你必须仔细寻找一个能满足这种需求的MCU。更重要的是，让我们说你需要真正的12位精度（0.025%）；现在你必须非常努力地看。你能在MCU中用A/D来达到那种精度吗？或者你必须使用离散A/D芯片？

　　To answer that question you must first study manufacturer’s datasheets very closely， as they tend to express accuracy in different ways. The two types of MCU A/D’s are Successive Approximation （SAR） and Sigma-Delta （Σ-Δ）。 Generally speaking the latter is slower and more accurate – usually with 16- or 24-bit resolution. The 16-bit versions of a Sigma-Delta A/D are usually viable for about 13-bits and the 24-bit for about 16.

　　SAR A/D suppliers usually give accuracy in terms of integral non-linearity （INL）， offset， and gain errors. The latter two can usually be removed with some sort of calibration （Mx+b） technique. INL is the real accuracy term and it is usually given in bits and is therefore very easy to use. As an example， for a 12-bit A/D an INL of ±0.8 bits typ.， ±1.5 bits maximum is very good and fairly common – but beware that many 12-bit A/Ds have INL of ±4 bits so they are really 8-bit converters.