目录

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。

其中的数模转换器,英文有多种说法,主要的有DAC,D/A,D-A, D2A, D-to-A,它是将数字信号(一般为二进制)转换为模拟量(电流、电压或电荷)的一种功能。模数转换器(ADC)则正好相仿。与模拟信号不同的是,数字信号即便经过很复杂的系统也可以无损耗低进行传输、处理以及存储。

DAC有几种不同的架构,根据应用可以通过以下的6个主要参量选择相应的结构:

DAC最先被广泛用在音乐播放器中,将数字信号流转换为模拟音频信号,后期也广泛应用在数字电视、手机中。在数字音频中使用的是低速、高分辨率类型的DAC,而在数字视频中则使用高速、中低分辨率的DAC。在军用雷达系统中一般使用分离的DAC,这种DAC的速度非常高、分辨率较低、功耗较高。高速的测试测量仪器,尤其是取样示波器中使用的也基本都是这种分离的高速、低分辨率数模转换器(DAC)。

概述

理想的被取样信号.

数模转换器是将一个抽象的有限精确的数字(通常为定点二进制数)转换为一个物理量(比如电压或压力)更多地是将有限精确的时间序列数字转换为一个连续变化的物理信号。 一个理想的DAC An ideal DAC converts the abstract numbers into a conceptual sequence of impulses that are then processed by a reconstruction filter using some form of interpolation to fill in data between the impulses. A typical practical DAC converts the numbers into a piecewise constant function made up of a sequence of rectangular functions that is modeled with the zero-order hold. Other DAC methods (e.g., methods based on delta-sigma modulation) produce a pulse-density modulated signal that can then be filtered in a similar way to produce a smoothly varying signal.

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

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。

These numbers are written to the DAC, typically with a clock signal that causes each number to be latched in sequence, at which time the DAC output voltage changes rapidly from the previous value to the value represented by the currently latched number. The effect of this is that the output voltage is held in time at the current value until the next input number is latched, resulting in a piecewise constant or staircase-shaped output. This is equivalent to a zero-order hold operation and has an effect on the frequency response of the reconstructed signal.

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

这是一个8为DAC的简化功能框图

DAC类型

最常见的类型:

DAC性能

DACs are very important to system performance. The most important characteristics of these devices are:

Other measurements, such as phase distortion and jitter, can also be very important for some applications, some of which (e.g. wireless data transmission, composite video) may even rely on accurate production of phase-adjusted signals.

Linear PCM audio sampling usually works on the basis of each bit of resolution being equivalent to 6 decibels of amplitude (a 2x increase in volume or precision).

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