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The output of embedded systems may drive many different kinds of external devices. An external device can be as simple as an on/off switch or as complex as an actuator. The required output digital control signal can take many different forms depending on the needs of the external device. Some external devices require a simple on/off digital signal. Some external devices require analog control inputs so that a digital to analog conversion is needed for the computer output. Some external devices require pulse width modulated signals (to be explained later).
In many cases, the digital signal needs to be amplified in order to make full use of the range of the A/D converter. In general, a special circuit is needed for each external device. An in-depth description and examples related to this topic can be found in Introduction to Instrumentation and Measurements, CRC Press, authored by Robert B. Northrop.