93 from Ch 21 A _ D - Making sense of EE 2nd ed Analog and...

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Today’s world is becoming more and more digital. But what exactly does it mean? Simply, many signals are stored, transferred, and processed not as analog voltages (which continuously vary with time) but as digital codes or voltages, which can take only two values – LOW and HIGH (for example, 0 and 5 V). We are using such digital codes more often than we realize – when we talk over a cell phone, swipe a credit card, watch a DVD, and drive a car (most likely, it has quite a few embedded microprocessors that control the engine, monitor the exhaust, and ensure comfortable temperature for passengers and help the driver by operating cruise control and anti-lock breaks). Signals in the real world are analog such as continuously varying loudness and frequency of sound of your favorite songs. These signals have to be encoded in the digital form so that you can store them in your MP3 player and share as computer files. Before playing back, the digital signals have to be converted back to analog voltages that will be fed to the amplifiers and speakers. In this chapter, we explore a broad range of questions that relate to the conversion of analog signals into digital – and digital to analog. We start with the accuracy of sampling the analog signals and consider both the resolution (number of bits) and the sampling rate. From these parameters, we arrive to the size of memory needed to store the data. Also, we consider voltage levels of digital signals and timing diagrams used for their representation, conversion from binary to decimal and from decimal to binary. Interestingly, both AD and DA converters can be built with Op Amps – and we will learn how some of them work. Many of these questions may arise in your future projects. Making sense of EE // 2nd ed Analog and Digital © 2008 A. Ganago Page 1 of 18
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1 © 2008 A. Ganago Analog & Digital, step-by-step: 1 Analog signals (continuous) are sampled at discrete moments of time –Big idea: Sample-and-Hold circuits –How often to sample? –How to avoid under-sampling (aliasing)? –How accurate should be each sample? © 2008 A. Ganago Analog & Digital, step-by-step: 2 Samples of analog signals are encoded in the digital, or binary form –Big idea: Binary means that only two voltage levels are allowed = LOW and HIGH (for example, 0 and 5 V) © 2008 A. Ganago Analog & Digital, step-by-step: 3 Analog signal, which changes continuously and can be measured with any accuracy, carries a lot of information To preserve this rich information in the binary form, each sample is recorded with several bits, or bi nary digit s • Big idea: AD converter circuits © 2008 A. Ganago Analog & Digital, step-by-step: 4 Digitized signals in binary form can be transferred without loss of quality, stored in memory, and processed –Big idea: digital circuits are based on transistors in saturation mode (for example, output voltages always are either ~0 V or ~5 V) –Big idea: synchronize all digital signals in the circuit with the same clock pulse train –Big idea: parallel and serial communication © 2008 A. Ganago
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