Analog vs. Digital Sensors

Analog Sensors

Analog sensors provide a signal that is continuous in both its magnitude and its temporal (time) or spatial (space) content.

Most physical variables such as, current, pressure, temperature, displacement, acceleration, speed, light intensity and strain tend to be continuous in nature and are readily measured by an analog sensor and represented by an analog signal.

For example, the temperature within a room can take on any value within its range, will vary in a continuous manner in between any two points in the room, and may vary continuously with time at any position within the room. An analog sensor such as a thermocouple, will continuously respond to such temperature changes. Figure 2 shows such a continuous signal where the signal magnitude is analogous to the measured variable (temperature) and the signal is continuous in both magnitude and time.

Digital Sensors

Digital sensors provide a signal that is a direct representation of the measurand. Digital sensors are essentially binary (“on” or “off”) devices. Basically, a digital signal exists at only discrete values of time (or space). And within that discrete period, the signal can represent only a discrete number of magnitude values. A typical variation is the discrete sampled signal representation, which represents a sensor output in a form that is discrete both in time or space and in magnitude.

Digital sensors use some variation of a binary numbering system to represent and transmit the signal information in digital form. With a suitable input, the value of the bit transmitted is reset corresponding to the behaviour of the measured variable. A digital sensor that transmits information one bit at a time uses serial transmission. By combining bits or transmitting bits in groups, it is also possible to define logical commands or integer numbers beyond 0 or 1. A digital sensor that transmits bits in groups uses parallel transmission. With any digital device, an n-bit signal can express 2ⁿ different numbers. This provides the limit for the different values that a digital device can discern. Therefore, the resolution in a magnitude discerned by a digital sensor is inherently limited to 1 part in 2ⁿ.

An illustration of the concept of a digital sensor is shown below:

The above device is used to sense the revolutions per minute of a rotating shaft. In this case, the sensor is a magnetic pick-up/voltage converter that outputs a pulse with each pass of a magnetic stud mounted to a hub on the rotating shaft. The output from the pick-up normally is “off” but is momentarily turned “on” by the passing stud. This pulse is a voltage spike sent to a digital register whose value is increased by a single count with each spike. The register can send the information to an output device, such as the digital display. The output from the sensor can be viewed in terms of voltage spikes with time. The count rate is related to the rotational speed of the shaft. The signal is discrete in time. A single stud with pick-up will increase the count by one for each full rotation of the shaft. In this example, the continuous rotation of the shaft is analog but the revolution count is digital. The amplitude of the voltage spike is set to activate the counter and is not related to the shaft rotational speed.

You can also read: Types of Sensors used in Measurement and Process Control