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Digital to analog conversion
When a peripheral requires an analog signal from the microprocessor, the signal must be generated from a digital signal provided by the microprocessor. To do this, we use a digital to analog (D/A) converter. Like A/D converters, D/A converters are characterized by resolution, linearity, and speed. There are many D/A converter chips on the market. Not many microcontrollers include D/A converters.
Resolution is one determiner of conversion accuracy. If the input digital value is between 0 and 255 and the output voltage range is 0 to 5 volts, voltage variations within plus or minus 5/256 volts cannot be achieved. More digital data lines are needed for higher resolution.
Physical connections of D/A chips include data pins (to be connected to the microprocessor), analog output pins, and calibration support pins. Analog outputs can be either current or voltage signals. The circuit shown above is an example of D/A conversion circuit. The 500 variable resistors are used to calibrate the output. The output (pin 4 of the D/A chip 1408) is a current signal. This output is converted to a voltage signal using an op-amp.