MSP430_Microcontroller_Basics_Chapter 9.2, 4-6

MSP430_Microcontroller_Basics_Chapter 9.2, 4-6 -...

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Mixed-Signal Systems: Analog Input and Output 393 Schmitt trigger and interrupts on standard digital inputs can be used for the measurement instead of a comparator. 9.2 Analog-to-Digital Conversion: General Issues Although Comparator_A produces digital values from some types of sensor, many applications call for a “real” analog-to-digital converter (ADC). I explain some general features of ADCs here and go on to describe features of the speciFc types of ADC provided in the MSP430 in subsequent sections. The purpose of an ADC is to convert an analog input, which I take to be a voltage V , into a binary value that the digital processor can handle. The input V(t) is a continuous function, meaning that V can take any value within a permitted range and can change in any way as a function of time t . In contrast, the output V [ n ] is a sequence of binary values. Each has a Fxed number of bits and can therefore represent only a Fnite number of values. Typically the input is sampled regularly at intervals of T s , so the continuous nature of time has also been lost. Thus the process of conversion damages both V and t and we examine the effects next. Mixed-signal systems present serious challenges and it is no surprise that whole books are devoted to them. I Fnd A Baker’s Dozen: Real Analog Solutions for Digital Designers [39] to be the most accessible and the author regularly covers mixed-signal issues in her column in Electronic Design News . There are also good books on data conversion from Analog Devices [43, 44] and books on op-amps [45, 50] devote much space to the analog–digital interface. There is excellent material on the Web sites of all the major semiconductor manufacturers, including TI. 9.2.1 Resolution, Precision, and Accuracy My dictionary lists accurate and precise as synonyms, which is unfortunate because their meanings are quite distinct in engineering. They must be among the technical terms that cause most misunderstanding but the difference is vital for data converters: Accuracy: How close a measurement is to its “true” value, which would be produced by an ideal system. This is easy to deFne but hard to measure. Resolution or precision: The number of distinct output values that a measurement can provide. Alternatively, it can be speciFed as the change in input that corresponds to the minimum change in output, 1 bit.
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394 Chapter 9 The resolution or precision can be quoted in different ways for an ADC, depending on whether you are looking at the ADC alone or its behavior in the system. Consider a 10-bit ADC, for instance. This means that its output is a binary value of 10 bits, which can represent 2 10 = 1024 distinct values. We also need to know the range of input voltage to determine the resolution on the input. Suppose that the range is from 0 to a full-scale value of V FS = 3V. Then a change of 1 bit in the output corresponds to a change of ( 3V )/ 1024 3mV on the input. This is called the LSB for least signi±cant bit and is another way of de±ning the resolution. We could also say that the ADC converts its input
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MSP430_Microcontroller_Basics_Chapter 9.2, 4-6 -...

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