ADC / DAC的教程

　　今天的世界充滿了數字和模擬信號。雖然這些信號的表現不同，但它們通常都用來幫助實現更大的目標。想象工程師負責控制空調機組。如果計劃使用任何一種微控制器或微處理器，就必須能夠讀取具有無限量值的模擬溫度，并將其轉換為離散步驟中的二進制表示形式。這種模擬值的二進制表示將由微控制器或微處理器處理。這些數據將被用來幫助執行一個過程的暖通空調單位，以幫助維持穩定的環境。在處理需要由數字系統處理的模擬值時，模數轉換器（ADC）是勢在必行的。同樣的理論可以應用于需要轉換成模擬信號的數字信號。在線播放一首歌曲涉及到使用數字信號轉換為模擬信號的幾個不同步驟。主機設備從服務器接收到的信號將是原始模擬信號的二進制表示。這個二進制數據的可聽響應對聽者來說是不可理解的。原始信號是模擬的，所以最后的表示也需要模擬。通過使用數模轉換器（DAC）解決了這個問題。這種類型的設備需要一個二進制代碼，它可以由模數轉換器進行編碼，并將其轉換成模擬電壓。

　　Converting signals from analog to digital or digital to analog is an unavoidable task for today‘s engineer. There are many different kinds of analog to digital converters and digital to analog converters. While these differ in their architecture， they all work to achieve a similar end. Since digital signal processing cannot be done with analog values， it would be analogous to a French speaking person trying to speak with a German speaking person. It would not work without a translator. ADC and DAC devices can help to act like that translator. When an ADC sees an analog voltage， its job is to turn the analog voltage into a binary code at a given period of time. This means the ADC will sample the analog voltage at an instant， and then it determines what the value would be in binary on the output side of the ADC. The amount of samples that the device takes every second will be called out on its documentation. An example of this would be the MAX1118EKA+T from Maxim Integrated. This device has a 100 kHz sampling rate so it can sample the analog voltage on its input side 100，000 times per second. By being able to take that many samples in a second， it is possible to accurately log what the analog voltage looked like by using a binary interpretation. Sometimes the sample rate of an ADC is not high enough to accurately recreate its input which causes aliasing. This is where signals start to be indistinguishable from one another， or aliases of one another. Imagine a video camera that can take 24 frames per second when recording. For most applications this will be fine; however， if trying to view something that is moving very fast， this can distort the image. Recall the effect of watching a television on a recording in the late nineties. The image on the television would be flickering. This is because the refresh rate on the television itself is much faster than the recording can capture at its given frames per second. The image would be distorted because the video is actually a succession of pictures. There is more happening between each picture than is actually being represented on the video. The same kind of effect can happen with an ADC. To avoid this， it is pertinent to make sure the sample rate is at least two times higher than the highest frequency that needs to be transmitted. This is referred to as the Nyquist rate.