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Hello, Mr. Yee. These days, it is much simpler to buy an analog to digital converter (ADC) integrated circuit, and apply it per manufacturer application notes, than to build one. An ADC represents on input analog voltage by a digital output bit pattern. An ADC does this by "sampling" the input during a short time interval, and "quantizing" — choosing the bit pattern the voltage fits into. Here's a simple example: an ADC with 0-8 vDC input, and a 3-bit output. This means we can have 2^3 quantization levels -- eight of them. So, here's a table indicating what input ranges give what output bit patterns: 0 - 1v 000 1 - 2 001 2 - 3 010 3 - 4 011 4 - 5 100 5 - 6 101 6 - 7 110 7 - 8 111 This ADC has 3-bit resolution, which can only represent to the nearest volt. 4 bits would allow representation to the nearest 0.5 volts. because you would then have 2^4 quantization levels. You choose ADCs based on what resolution your application requires. This is not the only consideration. How many samples per second needs is another big one. How the input voltage is evaluated comprises several methods like "flash," single slope integration, successive approximation, and dual-slope integration. How the output bit pattern is presented also varies. It could be on parallel wires, or on one wire in serial form. What I'm saying is there are so many flavors of ADCs, you want to read a good tutorial on them so you can choose intelligently. Either find an electronics text whose index devotes pages to the kinds of ADCs in the previous paragraph, and/or try this link: http://www.win.net/~radiosky/atod.html Good luck! Larry Skarin

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