Simply we can say,a signal which varies continously with time.From the real world itsel we can found out so many examples for analog signals.sound,light etc..But in the case of instrumentaton we are using 4-20 ma or 0-20 ma as analog signals.Here the voltage between controller and instrument is kept constant and taking the current as signal.
Digital signal is discrete to time. Actually digitally signal is sampled from analog signal.In the conventional method they use two discreet levels for transmitting the signal from instrument to controller. Logic high And Logic low. Relays are used for implement the digital signal transmission. Recently, different types of digital signals are introduced for communication between instrument and controller. Some signals are Profibus, HART, FieldBus. I will explain about each signals in my coming posts.