In this section, the next two slides will be discussing a practical application for the PWM D/A: a digital servo mechanism that positions something very accurately under software control. Simplistically, there are two implementations that could be used. One solution, the one shown here, puts the microcontroller outside the feedback loop. The other solution, the one shown on the next page, puts the MCU inside the feedback loop. This design outputs a simple binary value into a D/A and has the closed-loop positioning circuitry in the analog portion of the design. To obtain good performance, tight calibration requirements are imposed on the analog control circuits in the feedback loop. In a typical application, this control system must move an antenna to track a mobile signal source. Assume that the antenna gets to the correct position. With this design, it is held there by the analog feedback circuits. If a strong wind comes up and pushes the antenna, the analog feedback keeps the antenna in position. If the wind becomes strong enough to possibly break the antenna (that is, if it becomes so high that excessive correction force is required), this system would require a multitude of limit sensors just to detect the problem.