Any Signal input or output of a device or system belongs is either Analog or Digital in nature. and you could find any of them within a circuit. However, primary input of any device is usually physical in nature like sound, light, pressure, heat, movement etc. are analog because of diverse variations in their characteristics which makes them vulnerable to noises creating the need for more efforts to separate the noise from the wanted signals.
Signal processing involves converting a raw analog to a more preferable digital signals but in circumstances where the desired output is analog, what we obtain at the output of a speaker for example is clean sound wave due to digitized inputs within the system. We have analog to digital converters and many types of digital to analog converters. Physiological input in form of electrical signals like ECG, Ultrasound, EEG, MRI etc. from a patient is analog in nature but is stored in a computer or any form of digital analyzer in digital form. A Digital to Analog converter will convert this form back to Analog electrical output readable from the output display or print out.
This is the reason you may find Analog-to-Digital Converter and Digital-to-Analog Converter with associated circuits block within a complex system. You may find it interesting to check the two links below for more insight.
Analog vs Digital Signals There are two types of signals that carry information – analog and digital signals. The difference between analog and digital signals is that analog is a continuous electrical signal, whereas digital is a non-continuous electrical signal. Analog Signals vary in time, and the variations follow that of the non-electric signal.