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--- dac [2016/07/14 02:48]
gongyu
+++ dac [2022/11/07 01:55] (当前版本)
gongyu
@@ 行 1: / 行 1: @@
+## DAC - 数模转换器
 首先得说，模数转换器（ADC）和数模转换器（DAC）是连接模拟世界和数字世界的桥梁，对于信号的数字化革命起到了巨大的作用。
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-====概述====
+### 1. 概述
 {{ :220px-sampled.signal.svg.png |}}理想的被取样信号.
@@ 行 23: / 行 25: @@
 As per the Nyquist–Shannon sampling theorem, a DAC can reconstruct the original signal from the sampled data provided that its bandwidth meets certain requirements (e.g., a baseband signal with bandwidth less than the Nyquist frequency). Digital sampling introduces quantization error that manifests as low-level noise added to the reconstructed signal.
-====实际的工作====
+### 2. 实际的工作
 {{ :220px-zeroorderhold.signal.svg.png |}}
 实际的应用中，需要在DAC的输出端加入一个滤波器或有限带宽的器件将DAC输出的台阶响应平滑为连续的曲线。将时间上分离的点通过类似内插机制的平滑重构得到连续变化的信号电平。
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 The fact that DACs output a sequence of piecewise constant values (known as zero-order hold in sample data textbooks) or rectangular pulses causes multiple harmonics above the Nyquist frequency. Usually, these are removed with a low pass filter acting as a reconstruction filter in applications that require it.
-====应用====
+### 3. 应用
 {{ :220px-8_bit_dac.svg.png |}}
 这是一个8为DAC的简化功能框图
-====DAC类型====
+### 4. DAC类型
 最常见的类型:
-  * 脉宽调制型，也是最简单的DAC类型。 在一个低通模拟滤波器上以开关的方式施加一个稳定的电流货电压信号，开关切换的持续时间取决于数字输入量。这种方式常被用在电机马达的控制以及其它很多应用中。
+  * 脉宽调制型，也是最简单的DAC类型。 在一个低通模拟滤波器上以开关的方式施加一个稳定的电流或电压信号，开关切换的持续时间取决于数字输入量。这种方式常被用在电机马达的控制以及其它很多应用中。
   * 过取样DAC，又叫内插DAC，例如delta-sigma DAC采用了脉冲密度转换技术. 这种技术允许使用比较低分辨率的DAC，比较常用的是1-bit DAC
-  * The binary-weighted DAC, which contains individual electrical components for each bit of the DAC connected to a summing point. These precise voltages or currents sum to the correct output value. This is one of the fastest conversion methods but suffers from poor accuracy because of the high precision required for each individual voltage or current. Such high-precision components are expensive, so this type of converter is usually limited to 8-bit resolution or less.[citation needed]
+  * The binary-weighted DAC, which contains individual electrical components for each bit of the DAC connected to a summing point. These precise voltages or currents sum to the correct output value. This is one of the fastest conversion methods but suffers from poor accuracy because of the high precision required for each individual voltage or current. Such high-precision components are expensive, so this type of converter is usually limited to 8-bit resolution or less.
+{{ :weighted-resistor-dac.jpg |}} <WRAP centeralign>权重-电阻的DAC</WRAP>
     * 开关电阻DAC包含了一个并行的电阻网络。 contains a parallel resistor network. Individual resistors are enabled or bypassed in the network based on the digital input.
     * 开关电容源DAC, from which different current sources are selected based on the digital input.
     * 开关电容DAC包含了一个并行的电容网络. Individual capacitors are connected or disconnected with switches based on the input.
-    * The R-2R ladder DAC which is a binary-weighted DAC that uses a repeating cascaded structure of resistor values R and 2R. This improves the precision due to the relative ease of producing equal valued-matched resistors (or current sources).
+    * 优点:
-    * The Successive-Approximation or Cyclic DAC, which successively constructs the output during each cycle. Individual bits of the digital input are processed each cycle until the entire input is accounted for.
+      * 构建起来很简单.
-    * The thermometer-coded DAC, which contains an equal resistor or current-source segment for each possible value of DAC output. An 8-bit thermometer DAC would have 255 segments, and a 16-bit thermometer DAC would have 65,535 segments. This is perhaps the fastest and highest precision DAC architecture but at the expense of high cost. Conversion speeds of >1 billion samples per second have been reached with this type of DAC.
+      * 转换速度很快.
-    * Hybrid DACs, which use a combination of the above techniques in a single converter. Most DAC integrated circuits are of this type due to the difficulty of getting low cost, high speed and high precision in one device.
+    * 缺点:
-    * The segmented DAC, which combines the thermometer-coded principle for the most significant bits and the binary-weighted principle for the least significant bits. In this way, a compromise is obtained between precision (by the use of the thermometer-coded principle) and number of resistors or current sources (by the use of the binary-weighted principle). The full binary-weighted design means 0% segmentation, the full thermometer-coded design means 100% segmentation.
+      * 这种类型的转换器需要阻值比较宽范围的不同电阻，即便比较低的阻值也需要很高的精度.
-    * Most DACs, shown earlier in this list, rely on a constant reference voltage to create their output value. Alternatively, a multiplying DAC[1] takes a variable input voltage for their conversion. This puts additional design constraints on the bandwidth of the conversion circuit.
+      * 在晶体管中需要非常低的开关阻抗.
+      * 会比较贵，因此分辨率一般限定在8位。
+  * [[R-2R]]阶梯DAC是一种二进制权重的DAC，它使用电阻值R和2R重复级联的结构，等值的电阻（用作电流源）比较容易实现，因此这种方法可以做得比较精准。
+{{ :R-2R-ladder-non-inverting-DAC.jpg |}}<WRAP centeralign>R-2R阶梯正向DAC</WRAP>
+{{ :DAC-output-non-inverting-type-equation1.jpg |}}<WRAP centeralign>采用正向放大器的DAC的等效输出</WRAP>
+{{ :R-2R-ladder-inverting-DAC.jpg |}}<WRAP centeralign>R-2R阶梯反相的DAC</WRAP>
+{{ :DAC-output-inverting-type-equation2.jpg |}}<WRAP centeralign>采用反向放大器的DAC的等效输出</WRAP>
+    * 优点
+      * 只需要两种阻值的电阻
+      * 相对于二进制权重的DAC，它不需要电阻值非常精确
+      * 生产起来容易、成本低
+    * 缺点：
+      * 转换率比较低
+  * The Successive-Approximation or Cyclic DAC, which successively constructs the output during each cycle. Individual bits of the digital input are processed each cycle until the entire input is accounted for.
+  * The thermometer-coded DAC, which contains an equal resistor or current-source segment for each possible value of DAC output. An 8-bit thermometer DAC would have 255 segments, and a 16-bit thermometer DAC would have 65,535 segments. This is perhaps the fastest and highest precision DAC architecture but at the expense of high cost. Conversion speeds of >1 billion samples per second have been reached with this type of DAC.
+  * Hybrid DACs, which use a combination of the above techniques in a single converter. Most DAC integrated circuits are of this type due to the difficulty of getting low cost, high speed and high precision in one device.
+  * The segmented DAC, which combines the thermometer-coded principle for the most significant bits and the binary-weighted principle for the least significant bits. In this way, a compromise is obtained between precision (by the use of the thermometer-coded principle) and number of resistors or current sources (by the use of the binary-weighted principle). The full binary-weighted design means 0% segmentation, the full thermometer-coded design means 100% segmentation.
+  * Most DACs, shown earlier in this list, rely on a constant reference voltage to create their output value. Alternatively, a multiplying DAC[1] takes a variable input voltage for their conversion. This puts additional design constraints on the bandwidth of the conversion circuit.
-====DAC性能====
+### 5. DAC性能
 DACs are very important to system performance. The most important characteristics of these devices are:
   * **分辨率**: The number of possible output levels the DAC is designed to reproduce. This is usually stated as the number of bits it uses, which is the base two logarithm of the number of levels. For instance a 1 bit DAC is designed to reproduce 2 (21) levels while an 8 bit DAC is designed for 256 (28) levels. Resolution is related to the effective number of bits which is a measurement of the actual resolution attained by the DAC. Resolution determines color depth in video applications and audio bit depth in audio applications.
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 Non-linear PCM encodings (A-law / μ-law, ADPCM, NICAM) attempt to improve their effective dynamic ranges by a variety of methods - logarithmic step sizes between the output signal strengths represented by each data bit (trading greater quantisation distortion of loud signals for better performance of quiet signals)
-====DAC品质因素====
+### 6. DAC品质因素
   * 静态性能:
     * 差分非线性(DNL) shows how much two adjacent code analog values deviate from the ideal 1 LSB step.[2]