Designing Embedded Hardware

Analog-to-Digital Conversion

A device that converts an analog input voltage to a digital number is known as an Analog-to-Digital Converter, or simply and more commonly as an ADC. You may have also heard the term codec (COder-DECoder) before. A codec is an ADC combined with a Digital-to-Analog Converter (DAC), providing both analog input and analog output in the one chip. We’ll look at DACs in more detail later in this chapter.

ADCs are found in cell phones and digital phones, converting your voice to digital data for transmission. They are also used in your computer to digitize the input from a microphone for speech recognition. Professional recording studios use ADCs to convert audio to digital data in preparation for CD mastering. Similarly, video is sampled using ADCs prior to DVD mastering. Your scanner, web cam, and digital camcorder all have ADCs in them. At the other end of the application spectrum, ADCs are used to sample inputs from sensors. These applications can range from automated weather stations to the system monitoring the processor temperature in your PC.

There are several different types of ADCs. Integrating ADCs use an internal voltage-controlled oscillator to produce a clock signal whose frequency is proportional to the voltage being sampled. The clock signal is used to drive a counter, which provides the digital value for the sample. The higher the sampled voltage, the higher the clock frequency, and therefore the higher the number reached by the counter. ...