理解ADCs和DACs基础：采样数据系统的核心

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"这篇资源是关于模拟到数字转换器(ADC)和数字到模拟转换器(DAC)的基础知识，由Analog Devices公司的Walt Kester和James Bryant撰写。内容包括ADC和DAC的工作原理、在信号处理系统中的作用，以及它们引入的噪声源，如量化误差和偏移误差等。" 在数字信号处理(DSP)系统中，ADCs和DACs是至关重要的组件。ADCs将连续的模拟信号转化为离散的数字信号，而DACs则执行相反的操作，将数字信号还原为模拟信号。文章首先介绍了图2-1所示的典型采样数据DSP系统的块图，该系统包括信号调理电路、ADC和DAC。在实际的模拟到数字转换之前，原始的模拟信号通常会通过一些信号调理电路，这些电路可以进行放大、衰减和过滤操作。低通/带通滤波器的作用是为了去除超出我们关心带宽的不必要信号，防止混叠现象的发生。混叠是当高频信号被错误地表示为较低频率信号时发生的一种问题，这是由于没有足够的采样率导致的。图2-1展示了一个实时系统的结构，其中ADC以恒定的采样速率fs连续对输入信号进行采样，并且DSP必须在每个采样间隔(1/fs)内完成所有的计算工作，并在下一个ADC样本到来之前向DAC提供一个新的输出样本。这种实时操作确保了信号处理的连续性。文章的第二部分（未在提供的内容中完全展示）可能涉及到ADC和DAC引入的噪声和误差。例如，量化误差是由于有限的数字分辨率导致模拟信号的精确度损失；偏移误差则是因为ADC或DAC内部的不完美特性，导致输出信号出现固定的直流偏移。此外，还有其他“直流”误差，可能包括增益误差、热噪声和非线性效应等。 ADC和DAC在DSP系统中的核心作用是实现模拟世界和数字世界的转换。理解它们的工作原理、性能指标以及可能出现的问题对于设计高效和精确的信号处理系统至关重要。这篇资源将深入探讨这些主题，为工程师和学生提供宝贵的理论基础和实践指导。

Figure 2-6C shows the sampled signal restricted to the third Nyquist zone. Note that the first

Nyquist zone image has no frequency reversal. In fact, the sampled signal frequencies may

lie in any unique Nyquist zone, and the first Nyquist zone image is still an accurate

representation (with the exception of the frequency reversal that occurs when the signals are

located in even Nyquist zones). At this point we can clearly restate the Nyquist criteria:

A signal must be sampled at a rate equal to or greater than twice its bandwidth in order to

preserve all the signal information.

Notice that there is no mention of the absolute location of the band of sampled signals within

the frequency spectrum relative to the sampling frequency. The only constraint is that the

band of sampled signals be restricted to a single Nyquist zone, i.e., the signals must not

overlap any multiple of fs/2 (this, in fact, is the primary function of the antialiasing filter).

Sampling signals above the first Nyquist zone has become popular in communications

because the process is equivalent to analog demodulation. It is becoming common practice to

sample IF signals directly and then use digital techniques to process the signal, thereby

eliminating the need for the IF demodulator. Clearly, however, as the IF frequencies become

higher, the dynamic performance requirements on the ADC become more critical. The ADC

input bandwidth and distortion performance must be adequate at the IF frequency, rather than

only baseband. This presents a problem for most ADCs designed to process signals in the

first Nyquist zone; therefore, an ADC suitable for undersampling applications must maintain

dynamic performance into the higher order Nyquist zones.

Part 2 explains how ADCs and DACs introduce noise through quantization errors, offset

errors, and other "DC" errors.

Used with the permission of the publisher, Newnes/Elsevier, this five-part series of articles is

based on chapter 2 of "Mixed-signal and DSP Design Techniques," by Walt Kester.

Download Address:

http://www.eetimes.com/design/automotive-design/4017511/Basics-of-ADCs-and-DACs-part-

Basics of ADCs and DACs, part 2

Walt Kester and James Bryant, Analog Devices

7/26/2007 3:00 AM EDT

[Part 1 explains the basics of data sampling and shows how to use undersampling and

antialiasing filters. Part 3 examines distortion and noise in practical ADCs.]

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