ADC.V4 - L303.4.V4 Drexel University Electrical and...

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L303.4.V4 4-1 Drexel University Electrical and Computer Engr. Dept. Electrical Engineering Laboratory III, ECE L303 E. L. Gerber ANALOG-TO-DIGITAL CONVERSION Object The object of this experiment is to learn the elements of an analog-to-digital converter and to utilize an analog-to-digital chip in the Laboratory. Introduction An analog signal is one that can assume any voltage value as a function of time, e.g., sine waves, speech waveforms, etc. In contrast, a digital binary signal can assume only two values called “1” and “0”; for transistor-transistor-logic (TTL) the 1 state is approximately +5 V and the 0 state is 0 V. If we wish to transfer analog signals from one point to another with minimum interference from noise, or if we need to change the form or characteristics of the signal, we need to convert the analog signal to a digital signal. The conversion process consists of taking sample points on the analog waveform (see Fig. 1) and converting each point a digital word. The more points we take, the closer the sampled waveform follows the original waveform. A series of binary words will represent the analog signal over a time interval. Theory In the analog waveform shown in the Figure 1, the signal has a maximum value of +3.1 V. Let us assume that after processing and transmission, we would like the analog signal to be restored to within 0.1 V of its original value. We can determine the required resolution by dividing the 3.1 V into 0.1-V increments. With a signal range of 0 to +3.1 V, we have 32 steps of 0.1 V each, i.e., 0, 0.1, 0.2 3.1V, which gives a resolution of 1/32 = 1/2 n . Also the resolution can be expressed in terms of the full-scale voltage, that is, V FS /(2 n - 1) volts per bit. In this example we have 3.1/(32 - 1) = 0.1 volts/bit. To represent these 32 steps as a digital word requires five bits (2 5 = 32). With five bits we can get 32 combinations of ones and zeros. Zero volts can be represented by the binary-coded decimal count of 00000, and 0.1 V by 00001, and so on. At each particular sample time, a digital word consisting of five bits is generated for each particular analog value. This process must be repeated many times.
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L303.4.V4 4-2 Figure 1. Sampling an Analog Waveform Analog-to-digital (A/D) converters are integrated circuit devices that perform the described conversion process. There are four basic types of A/D converters: 1. Successive approximation 2. Flash 3. Dual slope 4. Tracking The successive approximation A/D converter is used where reasonably fast conversion is required. The flash-type A/D converter is used for fast-changing analog input signals. Dual slope conversion is slow, but offers excellent rejection of power supply noise. Finally, the tracking converter is faster than the dual slope, but slower than the successive approximation or flash types. We will investigate the successive approximation converter in this experiment. • Successive Approximation A/D Converter
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ADC.V4 - L303.4.V4 Drexel University Electrical and...

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