As an analogy, see the conventional water tap as an analog control, an electric light ON-OFF switch as a digital control.
Bob Pease - Analog Guru
Among the semiconductor products Bob designed are temperature-voltage frequency converters used in groundbreaking medical research expeditions to Mt. Everest in the 1980s. He also designed a seismic pre-amplifier chip used to measure lunar ground tremors in the US Apollo moon landing missions. Among his more memorable designs are the LM331 voltage to frequency converter and the LM337 adjustable voltage regulator. - Remembering Bob Pease
Even in digital, going from a low level say 0V to a high level say 5V is not instantaneous, the side of the square wave or the rising edge is not as straight as it looks. On zooming only the edge, you see that it is also an analog phenomena. The 'square wave' itself appears to be a summation of many sine waves, 'harmonics'. This is because nature is analog, the sine wave is analog, the movements of the planets or even the spin of electrons in an atom too.
After the handheld Abacus calculator (low-power, durable, bio-degradable), Mechanical and electro-mechanical computers were made and studied, The very first electronic computers were Analog Computers using an opamp as a building block.
Then came huge Digital Computers, Later LSI and other Microelectronic Innovations have precipitated handheld computers.
Have a look at Op Amp Applications Handbook
The Rising edge of the square wave depends on the Transient Response of the devices, the slew rate and bandwidth too. Fast Op-Amps are required for many applications, in switching and also AC amplification. Beyond that, study goes into Power Electronics and RF-Microwave which are specialized extensions of Analog. Similarly study of Digital Circuits extends into Computers.
Analog Design and Circuits
Analog Circuits Instrumentation
- Industrial Process Control Circuits
- Instrumentation Circuits
- Analog Circuits added after 2004
- Temperature Measurement Control
Circuits in Above
- Auto ranging 4-1/2 digit digital voltmeter icl7135 of intersil.
- Precision attenuator with digital control - use with above.
- Precision amplifier with digital control - use with above.
- Precision attenuator and amplifier - write up for above two.
- Fluid or water level sensor using reed relays
- Fluid or water level sensor using reed relays - circuit example.
- Voltage to pulse converter, for impulse counter
- Current Source mA for calibration
- AC voltmeter digital
- 8 channel analog mux with sample hold