發現與高精度A/D轉換單片機

　　越來越多的嵌入式系統基于采集各種傳感器的測量數據，典型的應用包括醫療、智能電網、能量控制以及工業控制系統。因此，對精密A/D轉換的需求日益增長。大多數微控制器來與A/D轉換器，但他們通常是10或12位轉換器，通常有4或5比特錯誤（計數）–使他們有效的8 -或9位轉換器。

　　換句話說，8的1位的錯誤大約是0.4%。不太壞，真的嗎？。對于一個真正的8位分辨率，我通常計算為1%，或者稍微少一點——分解一些參考電壓隨時間和溫度的漂移。然而，如果你需要更精確，可能更多，你必須仔細尋找一個能滿足這種需求的MCU。更重要的是，讓我們說你需要真正的12位精度（0.025%）；現在你必須非常努力地看。你能在MCU中用A/D來達到那種精度嗎？或者你必須使用離散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.