Analog Fundamentals: Getting to know the signal chain

-October 18, 2013

A signal chain consists of several components, such as amplifiers, data converters, interface, clocks and timing. The purpose of the signal chain is to gather and process data or apply system controls based on analysis of real-time information.


In this post, we’ll focus in on one part of the signal chain: the data converter, see Figure 1 below, but it’s important to understand the analog and digital world first.  


Figure 1. Basic ADC with External Reference


Signals need to be processed so that the information that they contain can be displayed, analyzed or converted to another type of signal that may be of use. In the real-world, analog products detect signals such as sound, light, temperature or pressure and manipulate them. Converters such as an analog-to-digital converter (A/D Converter) then take the real-world signal and turn it into the digital format of 1's and 0's. From here, the digital signal processors take over by capturing the digitized information, processing it and then feeding it back for use in the real world. It does this in one of two ways, either digitally or in an analog format by going through a digital-to-analog converter (D/A Converter). All of this occurs at very high speeds. Analog signals are continuous, while digital signals capture only a portion of the signal.


When learning about data converters, either A/D converters or D/A converters, it’s important to understand a few of the basics.


For instance, why digitize an analog signal in the first place? There are several reasons, which include improved signal analysis potential, more robust storage and more accurate transmission. On the flip side, it’s more complex and requires more processing time. Unfortunately, one cannot live without the other, but the good news is that high-performance converters are designed to help remove complexity and boost performance.


The purpose of an A/D converter involves quantization of the input, which means the converter introduces a small amount of error. The A/D converter’s total performance is really a number of parameters like thermal noise, jitter and quantization noise, i.e. — what’s known as signal-to-noise ratio (SNR) over a specified bandwidth (BW). SNR reported in a converter datasheet, can give the designer a realistic expectation when trying to understand the converter’s lowest resolvable “step” in the signal being sampled.

Instead of doing a single conversion once, an A/D converter often performs the conversions, called samples, periodically. What you end up getting is a sequence of signal samples or digital values that have converted a continuous analog signal to a discrete signal or discontinuous signal. And as you’ll see this very process has enormous impact on the quality of the signal captured.


Figure 2. Sampled Data System: Sampling and Quantization. Here, sampling is the impact along the time domain. While quantization is the impact along the amplitude domain. the very act of sampling does cause both to take place at the same time, causing the signal to be parsed in two axes.

Understanding data converters is no small task. For a deeper dive into converter fundamentals, my ADI colleague’s presentation, Hank Zumbahlen, goes into much further detail at ADI’s Virtual Design Conference. He specifically covers, “Data Converters for Solving Hard Problems.”

If you have any questions please leave a comment.

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