【正文】 lowing equation for the time elapsed since the capacitor began to discharge:VC = VCC?et/RC (4) The size and contents of the table may vary from application to application depending on the sample interval and conversion resolution. As the resolution increases, the number of entries in the table grows.In the voltmeter application, with resolution equal to Volt, the lookup table contains 158 entries, which is twice the number of samples per half cycle calculated above.Voltages corresponding to samples taken during the charge half cycle are calculated by replacing ’t’ with ’N Dt’ in Equation 1, where N represents the sample number (078). By setting Dt equal to the sample interval of 5 microseconds, R to 267 kilohms, C to 2 nanofarads, and VCC to , Equation 1 bees:V = 5(1eN (.0093633))Voltages corresponding to samples taken during the discharge half cycle are calculated by replacing ’t’ with ’N Dt’ in Equation 4, where N represents the sample number (078). Using the same values as for the charge half cycle, Equation 4 bees:V = 5?eN(.0093633))An abbreviated list of the voltages calculated for the capacitor charge/discharge cycle is shown below. The ordering of the voltages, increasing in the first half, decreasing in the second, tracks the voltage on the capacitor and defines the ordering of the table entries.N = 0 V= N = 1 V= . .. .. .N = 74 V= N = 75 V= N = 76 V= N = 77 V= N = 78 V= N = 0 V= N = 1 V= . .. .. .N = 74 V= N = 75 V= N = 76 V= N = 77 V= N = 78 V= As shown by the list, the number of samples in each half cycle is greater than required to reach the midrange value of . This allows for “fast” cycles which overshoot the nominal midrange value before the last sample is taken in each half cycle. Note that the difference between the calculated voltages at samples N=0 and N=1 is within the desired resolution of , but the difference in voltage between adjacent samples decreases as N increases. This reflects the nonlinear relationship between voltage and time in the circuit.The calculated voltages shown in the list are not entered into the lookup table, but are used to determine the values of the table entries. In the voltmeter application, the calculated voltages are rounded to tenths of a volt and the result stored in the table in packedBCD form, two digits per byte. Example: the table entry corresponding to is 25 hex, which displays as .The voltmeter prototype demonstrated accuracy of +/ one count ( Volt), but accuracy of somewhat less than a tenth of a Volt is about the best that can be expected from the RC precision ponents, variations in ponent values may contribute an error of 177。 一臺(tái)12 MHz 處理器時(shí)鐘和一微秒指令周期的輸出結(jié)果，8 位的變換可以在被300微秒內(nèi)進(jìn)行。5. 0伏和177。電路不提供補(bǔ)償調(diào)整。 。DAC輸出設(shè)定時(shí)間為300十億分之一秒DAC包含二進(jìn)制加權(quán)，用的二進(jìn)制代碼檢測(cè)輸入電流的電流導(dǎo)引開關(guān)。在一個(gè)8位的轉(zhuǎn)換器中，要八次反復(fù)才能找到正確的二進(jìn)制編碼。沒(méi)有被測(cè)試的誤差源包括：比較器的局限性，充放電周期的不對(duì)稱性，電容器電壓達(dá)不到起點(diǎn)或是VCC,VCC的變化。在伏特計(jì)應(yīng)用中，結(jié)果儲(chǔ)存在packedBCD式的表中，兩個(gè)數(shù)字一比特。如表所示。是先前確定的采樣間隔。所需電壓為:VC=VCC(1/2)()=由方程三:所需測(cè)量回路采樣最小值通過(guò)計(jì)算電容器電壓達(dá)到VCC/2得到，根據(jù)不同采樣間隔劃分。在伏特計(jì)應(yīng)用中，R的值選擇為267歐姆，此值遠(yuǎn)遠(yuǎn)大于上拉內(nèi)阻。用帶1％公差電阻和5％公差的電容：（Rnorm1%）(Cnorm5%)*104在伏特計(jì)中，R和C的值選擇分別為267歐姆和2毫微法。在圖中，由于采樣在電壓間隔中心進(jìn)行，所以曲線的斜面是理想的。時(shí)間恒量RC影響著電容器充放電的波形。電容器充放電周期的對(duì)稱減小了電容電阻乘積值變化帶來(lái)的影響，提高了變換準(zhǔn)確性。是這種方法失敗的原因。除了已給出的說(shuō)明的地方，放電部分周期運(yùn)用了下面的方程和討論：下列指數(shù)方程中，電容器的電壓是時(shí)間的函數(shù)：其中VC是t時(shí)刻的電容器電壓，VCC是給定電壓，RC是電容器和電阻器值的乘積。這個(gè)電容器與內(nèi)部比較器的非反向輸入相連（12腳）。2005（3）：9293【8】何希才《常用集成電路實(shí)用實(shí)例》 電子工業(yè)出版社，2007【9】陳有卿《通用集成電路應(yīng)用于實(shí)例分析》中國(guó)電力出版社，2007【10】馬中梅《單片機(jī)C語(yǔ)言程序設(shè)計(jì)》北京航空航天大學(xué)出版社，2007外文文獻(xiàn)中文譯文AT89CX051微控制器的模擬數(shù)字變換器應(yīng)用Atmel AT89C1051和AT89C2051微控制器是具有低引腳數(shù)和寬工作電壓范圍的單片閃光器（Flash）和不可缺少的比較器。發(fā)光二極管點(diǎn)越亮，酒精濃度越高燃燒產(chǎn)生的電壓值越大，超過(guò)設(shè)定值，電路報(bào)警。輸入電壓為05V。與半導(dǎo)體型相比，燃料電池型的呼吸酒精測(cè)試儀具有穩(wěn)定性好、精度高、抗干擾性能好等優(yōu)點(diǎn)。但由于價(jià)格方面的原因目前市場(chǎng)上用的是燃料電池型和半導(dǎo)體型。此外酒精測(cè)儀還可以測(cè)定某一特定環(huán)境下的酒精濃度如酒精生產(chǎn)車間可以避免發(fā)生火災(zāi)。各種應(yīng)酬也越來(lái)越多酒這東西貼近了我們的生活。通過(guò)酒精濃度的輸入，進(jìn)行響應(yīng)的電路報(bào)警。所以研究一個(gè)酒精測(cè)試是非常有必要和意義的事。發(fā)展MEMS技術(shù)。使進(jìn)入燃燒室內(nèi)的酒精能進(jìn)行充分燃燒轉(zhuǎn)換為電能?？梢圆捎命c(diǎn)狀顯示也可采用條狀顯示。 結(jié)構(gòu)方案圖傳感器——信號(hào)調(diào)制——A\D—單片機(jī)重點(diǎn)與難點(diǎn)(此部分要求條理清晰，分一，二，三……等小點(diǎn)描述清楚，以及解決途徑) 本次設(shè)計(jì)的酒精測(cè)試儀系統(tǒng)的關(guān)鍵問(wèn)題是：?！?】【M】.北京：北京航空航天大學(xué)出版社，2005。變換時(shí)間為7毫秒或是更少如圖一所示，如果采用RC模擬數(shù)字轉(zhuǎn)換方法只需要一個(gè)AT89CX051微控制器，兩個(gè)電阻器和一個(gè)電容器。電壓分辨率不利用RC轉(zhuǎn)換軟件的判別，它在提供調(diào)試工具的同時(shí)也給出了一個(gè)方法。不能用浮點(diǎn)計(jì)算和超函數(shù)來(lái)求解指數(shù)方程是RC變換方法的首要問(wèn)題。RC轉(zhuǎn)換方法的第二個(gè)問(wèn)題是方程各項(xiàng)值變化引起的固有誤差。采樣間隔受執(zhí)行必要編碼所需時(shí)間限制。在圖中，是變換器達(dá)到所需分辨率的所需電壓。為了求得第一次采樣所需斜面，要獲得時(shí)間恒量的最小值，解方程一得RC然后設(shè)為所需分辨率得最小值（）,時(shí)間為先前確定的采樣間隔（5毫秒）。它產(chǎn)生不對(duì)稱的充放電波。波形的充電部分，漸近線在VCC。查表包含軟件一個(gè)專門值。方程4變成：V=5*eN(.0093633)電容器充放電周期電壓計(jì)算略表如下。電壓和時(shí)間表現(xiàn)非線性關(guān)系。最差的變換誤差可以通過(guò)用較小公差元件來(lái)進(jìn)一步減小。比較器比較DAC未知電壓和輸出，并返回SAR的結(jié)果。7和6比特型相對(duì)來(lái)說(shuō)適合于MC1407和MC14086。 當(dāng)被編譯電壓超過(guò)未知的電壓時(shí)，比較器的輸出變大，這被軟件檢測(cè)。 輸出電壓由DAC輸出電流( Io)以i/ V變換器的值得乘積來(lái)確定。 1240歐電阻器連接 DAC的腳15 ，2500歐電阻器和運(yùn)算放大器腳3 連接可能相抵消，性能稍微下降。DAC輸出設(shè)定時(shí)間和比較器執(zhí)行SA算法所需的響應(yīng)時(shí)間是可以忽略的。 并行輸入DAC可被連續(xù)的DAC輸入替換(更昂貴)。 variations in VCC. The contributions to conversion error made by these sources can be expected to increase error to somewhat more than the value due to ponent tolerances alone.Successive Approximation AnalogtoDigital ConverterThis conversion method offers good resolution and accuracy and a short conversion time at the expense of increased ponent count.Successive approximation (SA) ADCs incorporate a digitaltoanalog converter (DAC), a parator and a successive approximation register (SAR). The SAR controls the conversion by performing a search for the binary code which, when fed to the DAC, will produce an output matching the voltage to be converted. The parator pares the DAC output to the unknown voltage and returns the result to the SAR.The SAR begins the search with the most significant DAC bit, which controls the widest o