To understand why analog circuits are not considered digital, it is necessary to investigate the meaning of the words analog and digital.
As most people today are more familiar with the term digital, we will start with investigation of that term initially.
The phrase digital circuit, as applies to electronic circuits, means a circuit that produces a binary signal of easily discernible value for evaluation. More simply put, a digital circuit has a value of either zero or one. That is because a digital circuit is either switched on, representing one, or off, which represents zero. In electronic equipment series of digital circuits are combined and switched off and on at incredibly fast rates. This is why digital circuits and equipment operate very quickly.
Think about the digital circuits around you every day. What would you think is the simplest digital circuit in your home? You might pick your cell phone or laptop, while some would say the thermostat on the wall. Regardless of what you choose, very few would choose the light switch. Think about how that switch operates. You turn it on or off when you enter or leave a room. A light switch is the simplest form of every day digital circuit that we deal with in our lives.
Now you know that the term digital does not necessarily mean an item should be more expensive. For example, a digital garage door opener is simply a switch on the wall of the wall in your garage.
Analog circuits operate much differently, which makes analog circuit design an entirely different ballgame as they are also an older type of technology than digital circuitry. Analog circuits also constitute electronic system but with a big difference from digital systems. The signals received from an analog circuit are changing constantly and continuously. Thus an analog circuit measurement is totally different when compared to the on and off nature of digital signals.
The signal from an analog circuit represents the relationship, proportionally, between two electrical measurements. This can be the comparison between a base signal and the voltage applied or a comparison to a known value of current. Analog circuits receive measurements that come from physical change in form or nature. This physical change is converted to an electrical signal by a device that is called a transducer. A transducer is a device that converts one type of energy into another.
Analog versus Digital Circuits
Analog circuitry measurements are, by nature, less stable and accurate than digital. One reason this is so rests with the fact that there is little room for ambiguity when a signal is based on the presence or absence of current. Either the current is there and the circuit is on, or it is not and the circuit is off.
However, in order to ensure all technology available today functions properly, it is necessary to have both analog and digital circuits.