There's no AD/DA function on the Raspberry Pi GPIO interface, this may trouble you in the Pi development. However, it won't be a problem anymore. The RPI GPIO-PCF8591 Shield allows you to add high-precision AD/DA functions to the Raspberry Pi.
The PCF8591 features four-8-bit analog to digital converters and a single 8-bit digital to analog converter. This will operate through the I2C interface on the Raspberry Pi.