# Analog vs Digital Waveforms

#### bwilliams60

Joined Nov 18, 2012
1,156
Just wanted to throw a quick question out there and see what the response is. What is your definition of an analog and digital waveform?
The reason I ask is that if you look around the Internet, people have a lot of perceptions about what an analog signal is. I have always believed it to be a waveform that can vary in voltage over time. It can be AC or DC and can be a sine wave or just a wave which varies in amplitude over time. I would relate this to a throttle position sensor or thermistor which varies voltage over a given time period. A lot of sites I see, say it is only an AC signal which varies positive and negative voltage over time. Again, I think it can be just positive, just negative or both depending on purpose and source voltage being used.

Joined Mar 10, 2018
3,509
Generally speaking an analog signal is a continuous signal in time, taking on
many values with minimum discontinuities. Whereas most would consider a
digital signal to be a signal with a finite set of values post rise time and many
discontinuities.

Regards, Dana.

#### wayneh

Joined Sep 9, 2010
16,102
I agree. A digital signal will be identifiable as such if the time resolution is fine enough. An analog signal will not show a fine structure.

The distinction between pulsing DC and genuine AC is another topic.

#### MrChips

Joined Oct 2, 2009
19,160
Simple.

A digital signal is analog.

An analog signal is not digital.

#### MrChips

Joined Oct 2, 2009
19,160
To be more precise, an analog signal is a continuum of voltages (or current) from -∞ to +∞.

A digital waveform is an analog signal with discrete values.

Even more precisely, what identifies a signal as being digital is not the signal itself, but the drivers and receivers that create the signal and respond to the signal. In other words, it is the application of the signal that defines if the signal is considered to be digital.

Here is a classic example.
A transistor curve tracer displays the I-V characteristics of a transistor in fixed steps of base current. Internally, there is a circuit that steps the base current in discrete steps. This is a digital waveform. It is still an analog waveform of a digital nature.

#### bwilliams60

Joined Nov 18, 2012
1,156
Thank you for the replies. Pretty much what I was thinking. I find it amazing though how many sites and Youtubers explain it as an AC Voltage or sine wave. I even found an "automotive expert" from a College Explain it that way. I wonder why the comments were shut off?

Joined Jul 18, 2013
18,872
I find it amazing though how many sites and Youtubers explain it as an AC Voltage or sine wave.
AC. to me, comes down to the simple explanation of what the words in AC mean.
"Alternating current is an electric current which periodically reverses direction, in contrast to direct current which flows only in one direction."
Max.

#### MrChips

Joined Oct 2, 2009
19,160
AC. to me, comes down to the simple explanation of what the words in AC mean.
"Alternating current is an electric current which periodically reverses direction, in contrast to direct current which flows only in one direction."
Max.
That is what AC means in power electronics.
In signal electronics, AC means anything that is above 0Hz.

Joined Jul 18, 2013
18,872
That is what AC means in power electronics.
In signal electronics, AC means anything that is above 0Hz.
Maybe in the latter context is should be AV then?.
To avoid confusion.
Max.

#### MrChips

Joined Oct 2, 2009
19,160
This is always confusing to newbies without the knowhow.
They can never quite understand the function of the AC-DC switch on the input channel of an oscilloscope. They usually assume that you have to set the switch to AC when viewing an AC sinewave.

#### Papabravo

Joined Feb 24, 2006
12,301
Maybe in the latter context is should be AV then?.
To avoid confusion.
Max.
...avoid confusion? A dubious assertion.

#### crutschow

Joined Mar 14, 2008
23,158
Maybe in the latter context is should be AV then?.
To avoid confusion.
Max.
But that still means the signal would not be "AV" if it were biased with a DC voltage (doesn't technically alternate voltage polarity).

I think confusion can be avoided if the archaic mains power definition of AC is modified to say that it includes any signal that has a varying component (Fourier components other than just DC).
That's the definition I use when dealing with any AC signals and avoids having to use the awkward "AC riding on DC" signal description.

That does way with the oddity of calling a signal AC when it has a zero DC bias, but not when it has a bias greater than its peak value (where the current does not technically reverse direction).

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