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When a sensor output is an analog signal, it needs to be converted to a digital signal by an analog to digital converter (A/D converter) before being fed into the computer input port. There are many A/D converter chips on the market. Most microcontrollers (like the M68HC12 we are using in the lab) include A/D converter circuits on the chip.

A/D is characterized by resolution, linearity, and speed. Resolution is one determiner of conversion accuracy. More digital data lines are needed if higher resolution is desired. If the input voltage is between 0 to 10 volts and the resolution is 8 bits, any variation smaller than 10/256 Volt in the analog signal cannot be detected in the converter output.

Linearity also affects conversion accuracy. Higher linearity means a better A/D converter. This concept is shown in the above figure. The horizontal axis is the analog input. The vertical axis shows the converted digital value. The dashed line shows the accurate linear A/D conversion and the solid line shows the effect of non-linearity.

Some A/D converters have a reference voltage input. This input allows the user to specify the upper bound of the analog input