ADC / DAC的教程

　　ADC / DAC的教程

　　今天的世界充满了数字和模拟信号。虽然这些信号的表现不同，但它们通常都用来帮助实现更大的目标。想象工程师负责控制空调机组。如果计划使用任何一种微控制器或微处理器，就必须能够读取具有无限量值的模拟温度，并将其转换为离散步骤中的二进制表示形式。这种模拟值的二进制表示将由微控制器或微处理器处理。这些数据将被用来帮助执行一个过程的暖通空调单位，以帮助维持稳定的环境。在处理需要由数字系统处理的模拟值时，模数转换器（ADC）是势在必行的。同样的理论可以应用于需要转换成模拟信号的数字信号。在线播放一首歌曲涉及到使用数字信号转换为模拟信号的几个不同步骤。主机设备从服务器接收到的信号将是原始模拟信号的二进制表示。这个二进制数据的可听响应对听者来说是不可理解的。原始信号是模拟的，所以最后的表示也需要模拟。通过使用数模转换器（DAC）解决了这个问题。这种类型的设备需要一个二进制代码，它可以由模数转换器进行编码，并将其转换成模拟电压。

　　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.