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CHAPTER 3 DIGITAL CODING OF SIGNALS Computers are often used to automate the recording of measurements. The transducers and signal conditioning circuits produce a voltage signal that is proportional to a quantity we wish to measure. This signal may be constant or it may be varying with time. We cannot directly input this voltage into a computer, so we use an analog to digital converter (ADC) to turn the voltage we wish to measure into an integer code which can be handled by the computer. Once inside the computer the integer code can be used to produce an estimate of the measured voltage. This in turn can be converted into the quantity that we wish to measure, e.g., temperature, acceleration, flow rate, by using the results of a static calibration. In this chapter we will concentrate on how this voltage to integer coding is done. We will not go into any details on the hardware structure of an ADC, there are many types available from suppliers; your choice of ADC will depend on cost, speed requirements, compatibility with the computer you are using and the software available to manipulate the ADC from your computer. Here, all we are interested in is the result of the analog to digital conversion and how it should be interpreted. The coding-decoding that takes place in the ADC and computer gives rise to errors, which may be small or large compared to the true voltages depending on how well the analog to digital converter is being utilized. Below is a description of some of the components of analog to digital conversion; this should give you some ideas on how to use these devices effectively. An analog to digital converter converts a voltage into an n-bit integer code which can then be stored on a computer. Many ADC's are set up to sample a signal at equally spaced time intervals and store an integer code each time the signal is sampled. There are many issues here. For example: Can we convert the integer code back to the voltage we sampled? Do we have enough information to reconstruct the original signal? What happens if the signal voltage changes while the ADC is calculating the integer code? Clipping An analog to digital converter has an input range, e.g. 0→10 volts (unipolar), or -5 to +5 volts (bipolar). Different ADC's have different input ranges. You must make sure your signal lies within this range. For example, for an input range of ± 5V, signals greater than 5 volts have the same codes as 5 volt signals and signals less than -5 volts will have the same codes as
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3-2 -5 volt signals. That is, the ADC perceives signal A in Figure 1 to be signal B shown in Figure 1. This phenomenon is known as clipping . This is a highly nonlinear effect. Figure 1: An illustration of signal clipping. Voltages to Integer Codes Having ensured that clipping will not take place we now need to sample the signal and generate integer code. Since a computer uses binary coding (1's and 0's), only a finite number of bits (binary digits) are available to represent the input voltage. At this point let's assume that the integer conversion happens instantaneously. So we are measuring the signal at intervals T seconds apart and converting the voltage to a code.
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