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Analog to digital conversion
Supported by A/D chip or internal A/D converter.
- Resolution = (VRH – VRL) / (# of quantization steps)
- # of quantization steps = 2^b where b is # bits used for encoding
- Need data rate to satisfy the Nyquist criterion.
Recall that Nyquist criterion says sampling frequency should be at least twice the frequency of the fastest harmonic of the signal to be sampled.
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