Why is AD conversion necessary?

Temperature, sound, light, and other things that we can perceive with our five senses are not present or absent, but constantly change in an analog fashion.
Sensors are used to measure these.
Various sensors convert analog changes such as temperature, volume, and light intensity into analog electrical signals.
To perform some kind of control based on the measured information, a CPU, which is the brain of the electrical circuit, is required, but the CPU itself can only process digital signals, so it cannot process analog signals input from the sensor.

To eliminate this mismatch, an AD conversion circuit is inserted.
A stands for analog and D stands for digital, and as the name suggests, it is a circuit that converts analog voltage into digital data.

• In reality, AFE (analog front-end) circuits such as voltage adjustment amplifiers and FILTERS are often inserted between the sensor and the AD conversion circuit, but since this article is about AD converters, we will omit this.

Let's take the example of automatic air conditioning.
The diagram below shows the temperature changes over the course of a day. We will assume that the air conditioner will be on when the temperature is above 20°C, off when it is between 10 and 20°C, and on when it is below 10°C.

The above is converted into voltage information using a sensor such as THERMISTORS.

Since the CPU cannot process this, it is converted into digital data represented by "0" or "1".

Now that the data has been successfully converted into digital data, the CPU can determine whether the temperature is hot or cold and issue commands to turn the air conditioning and heating on or off.

However, the original specifications assumed three states: cooling, stopped, and heating. However, when converted to digital data, only two states, "0" or "1," could be expressed, and the specification for the stopped state disappeared.

As this would not satisfy the expected specifications, the AD converter combines the following technologies to convert the original analog data into digital data while reproducing it as closely as possible.

• Extracting at a fixed interval [Sampling]
• Converting voltage into digital data within a certain range [Quantization]

For more details on this, please see the article Analog to Digital Conversion of Time? Learn about Sampling!​