The outputs of the D/A are analog, so the D/A converter has a port structure that differs from the MCU’s digital ports. Obviously, the D/A signal can’t go through the typical digital buffer. Instead, it is controlled (turned on or off) by a two-transistor transmission gate, like the one shown here. This section will discuss the reason for those two transistors in a moment. This diagram illustrates a source of analog signal error. The pin for the D/A output performs multiple functions. Besides outputting the D/A signal, it can be used as a digital input pin that connects to the internal data bus through a digital buffer. The same pin also serves as an analog input for the A/D converter. The error associated with this port is caused by leakage current, a characteristic inherent in any semiconductor structure. Whether the digital I/O is used or not, the MCU pin will have leakage current that might affect the analog circuits external to the MCU. The leakage might be a low level, but in some cases it could degrade analog circuit performance. (To provide perspective, the leakage current for an analog I/O port on the H8S/2148 device is 1.0µA.)