Analog vs. Digital: What Does It Even Mean?

2023-10-13 | By Antonio Velasco

License: Public Domain

Something thrown around a lot is that on Arduino boards, there are inherently two different types of pins: analog and digital. They're typically denoted with an "A" or "D" before the number, indicating either Analog or Digital. Each can be used for different applications or, in some cases, the same application. It's important to know what each of them means for any microcontroller application, and as they act as both inputs and outputs, understanding what you’ll be working with is crucial.

Analog I/O

Analog I/O, to put it simply, can take on a variety of values for its output or take on a range of values for its input. Typically, it will output or input a range of voltages. It measures input continuously, meaning that it's great for sensors or any application where you’re trying to get a value that may be changing. 

Here's a graph displaying that the voltage can vary and take any value between the minimum (-20V) and maximum (20V). Please note this is just an example of an analog value and not representative of what voltages can be applied to an Arduino’s analog input. Always verify acceptable voltage input ranges for your board to avoid damage.

For example, say that we have a water tank where I want to measure the water level to display its current height or change it if I desire. I would have an analog input that measures the water level value and returns a voltage of 0-20V, which I would then convert to a value that I can read in the microcontroller. Calibration is key as you will need to determine what value would correspond with what height. Additionally, if I would like to add a certain level of water, I can then utilize an analog output to put in 0-20V “worth” of water, and not just opening or closing a valve.

Digital I/O

Digital I/O can only take on a value of either HIGH or LOW, which is basically ON or OFF. This binary input, while more limited, is typically cheaper to implement in terms of components and is not continuous, meaning that if you don’t need to check it, it won’t take up processing power. Additionally, they tend to be used more commonly in today’s world as they are faster and can be used to transmit a large number of signals at a high speed, which is why they’re used in a lot of today’s computers, phones, and more. This speed and simplicity is good for our scaled-down purposes in checking if a component is detecting an object or if a certain value is returning positive or nothing at all.

Here's a graph displaying that voltage can only hold two values (on and off, or in this case a specific voltage or 0 volts.)

For example, say that we are still working with that same water tank, but instead, I merely want to either turn on a hose that will fill it or know if the tank is full or not. We can use a digital input that has a water sensor at the top of the tank which will let us know if the water level is at the maximum height or not. If the water is not touching the top sensor, then we will know that the tank is not full. Say that I would like to add water now, I can use a digital output that will either turn the hose ON or OFF. Very simple, right?

Comparing the Two

The biggest difference is inevitably in the type of signal that it outputs, either something that can take on a range of values or a binary signal. Think of analog as returning how much water is going through a hose whereas digital is merely if the hose is on or not. There are also speed and cost considerations, which go to digital, and complexity considerations where you need a range, which go to analog.

We can also use them with each other for certain purposes. The Arduino offers both digital and analog pins and offers the capability to use them both at the same time. For example, in our water example, it’d likely be more beneficial to use the analog input that lets us know a specific water level. At the same time though, it may not be particularly helpful to implement an analog output in this case to add water as it may be more expensive and take longer. A digital output here that just turns the hose on and off may be better, and we can use the analog water level value to just determine when we have enough water in the tank.

Analog and Digital I/O are very important in our world, and both offer pros and cons in certain areas. You can’t really say that one is better than the other as their applications are simply different, but having a deep understanding of both will allow you to choose the signal type that will best benefit your project.

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