AA 320 Summary Notes #6

AA 320 Summary Notes #6 - AA320 Aerospace Instrumentation...

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Page 1 of 5 AA320 – Aerospace Instrumentation Notes on Digital circuits: A-to-D conversion, D-to-A conversion, and digital logic. In Lab 6 you will build a circuit that can convert an analog signal into an approximate digital representation of the signal, then convert that digital representation back into an analog signal. Remember that an analog signal means one that has a continuous range of values (voltages), and a digital signal is one that is either “on” (a specified fixed voltage, such as +5 V) or “off” (usually ground, i.e., 0 volts). Sometimes these two states are referred to as “HI” and “LO”. A-to-D conversion is necessary to acquire signals from the “outside world” to a computer (digital format) for processing (visualization, data analysis, etc.). D-to-A conversion is used to transform a computer-generated, digital signal to a continuous waveform that is useful to the outside world (e.g., an audio signal that can drive speakers, or a video signal that can drive a monitor). A-to-D method: The input analog signal V in is applied to a series of n comparators, each of which is set to “turn on” ( V out goes HI) at a different voltage level V i ( i = 1, 2, . .. n ). Usually the voltage levels are equally spaced: V i = i* Δ V . These voltages levels can be supplied by a simple resistive voltage divider, and are applied to the – inputs of successive comparators, and the signal V in is applied to every comparator's + input. For example, with 5 comparators (labeled A to E), and a voltage increment V = 0.5 V, an example circuit and a table of comparator outputs vs V in is shown below (Comparator Outputs: 0 means LO or ground, 1 means HI or V s ): Comparator Outputs “Level” Binary V in (Volts) A B C D E 0 – 0.499 0 0 0 0 0 0 000 0.5 – 0.999 1 0 0 0 0 1 001 1.0 – 1.499 1 1 0 0 0 2 010 1.5 – 1.999 1 1 1 0 0 3 011 2.0 – 2.499 1 1 1 1 0 4 100 > 2.5 1 1 1 1 1 5 101 Thus the input signal, which can have an infinite number of possible voltages, is represented by only 6 “levels” (0,1,2,3,4, or 5) – not too accurate. The levels, 0 through 5, must be converted to binary format for storage on a computer. The conversion between the comparator outputs and binary format is done with an “ encoder ” – it uses basic logic elements (AND, OR, NAND, NOR).
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Page 2 of 5 Binary encoding: Digital electronics uses only two states for representation of information –
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AA 320 Summary Notes #6 - AA320 Aerospace Instrumentation...

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