Can some explain the difference between Digtal and Analog Circuits?

[Lord Banshee]

Ok i am a 3rd year EE student and just started really getting into my EE major classes but i am confused on the whole what is Digital Circuit and Analog Circuits.

This is what i assumed:

Analog is a bunch of hardware such as capacitor,transistors, inductors, resistor op-amps that meet a SINGLE requirement and the only what to change the design is with new hardware.

Digital is a bunch of IC chips that do a similar function but has the ability to changes is parameters.

If i am correct why use analog at all?

Can anyone bring some light to my long night

Thanks,
Chris

 

[FrustratedUser]

Basically a digital circuit has two states, 0 and 1, with which it operates. An analog circuit has infinite number of states.

A digital circuit (chip, gate) is made of analog components such as transistors, diodes and in older logic families even resistors.
Thats where the name TTL comes from...

RTL - Resistor Transistor Logic. The first family of logic circuits.
DTL - Diode Transistor Logic.
TTL - Transistor Transistor Logic.


A digital circuit can also be made out of electro mechanical components such as relays.

 

[BrownTown]

your a third year EE student and you still don't know the difference between Analog and Digital?

In circuits you are often concerned with finding the transform function of the circuit. For a analog circuit this is a continuous function which maps an infinte domain of inputs to an infinite range of outputs (assuming everything is perfect). Digital transform take discrete inputs with a finite number of states and transform them to a finite number of output states. So for example a 4bit digital circuit can only have 2^4=16 different outputs, and logic gates is used to deterimine these outputs. In the digital circuit there is a layer of abstraction over the actual physical properties which maps them to boolean s tates of 0 and 1. In a digital circuit you are worried about the actuall pysical response of the circuit. So a simple analog circuit could jsut be an amplifier that multiplies the voltage by 2. So obviously there are an infine number of possible outputs due to the infinte number of possible inputs

 

[BEL6772]

Or, to hear one of my old professors tell it, there's no such thing as a digital circuit. Everything is analog. Some analog circuit are designed to emulate the digital ideal, though

 

[dmens]

Yeah, everything is analog... digital is just a circuit that is engineered such that a system variable can be used to distinguish nodes into binary states (almost always voltage, but there are cool people who can do it with current). Sometimes the differential between binary states is pretty wide, such as plain CMOS logic, static and domino, pretty much rail to rail. Or it can be razor thin, such as low-voltage swing logic, or small-signal register files, or DRAM's... those are topologies that have analog guts, but can be presented as a nice black-box to a digital environment.

 

[Lord Banshee]

Thanks guys

and BrownTown lol. Yeah well none of my classes have really said what the difference in the two are so i was wondering. I still have a ton of EE classes left, this year was my first year in taking REAL EE classes.

 

[djhuber82]

Analog circuits operate on analog signals, which are real physical quantities such as voltage or current. Digital circuits operate on digital signals, which are abstractions of real physical quantities (i.e. 0-0.5V = "0", 0.5-1.0V = "1"). Digital circuits are great as long as you are only driving or recieving from other digital circuits, but the real world is analog and if you want to interface with it then you NEED analog circuits. To drive a speaker or antenna you must create an analog signal. To recieve from an antenna or sensor you must process an analog signal.

As an aside, there are ways to create/process analog signals with (mostly) digital circuits (i.e. sigma-delta converters, class D amplifiers), but these architectures are usually limited in speed or accuracy and only find use in certain applications. In any case, the signal of interest is still analog, but these circuits are able to decompose it into a discrete-valued waveform.

 

[harrkev]

Here is the way that I look at it:

Take the Bible and make a photo copy of it. Is it still the same bible? Copy it to pink paper with purple ink. Is it still the same bible? Make a copy of a copy. Same question.

Take the Mona List and make a photo copy of it. Is it still the same Mona Lisa?

Once you can answer these questions, you are well on your way to understanding the difference between digital and analog.

NOTE: Digital does not HAVE to refer to binary. Digital could also refer to trinary (the stuff of thesis papers, but never seen in real life). It could refer to any number of discrete levels. The trick to digital, though, is that there ARE discrete levels.

 

[Sohcan]

Funny story: Not too long ago, I was at lunch at a Chinese restaurant with people from work. When we got our fortune cookies, one of my coworkers got a fortune that read (I'm not kidding) "Digital circuits are made of analog components". We figure that a disgruntled, laid-off analog IC engineer is now writing fortunes for fortune cookies.

 

[Loki726]

A digital signal is just an interpretation of an analog signal. Say you have a system of transistors that operates between 0 and 5 volts. Say the switching voltage for your logic gates is 2.5 volts. So anything over 2.5 volts will cause your transistors to switch, and anything under 2.5 will not. If this were the case, you would define anything over 2.5 volts to be a logical 1 and anything under 2.5 to be a logical zero.

This would be an ideal case. In reality, transistors do not simply switch from off to on instantaniously. There will always be some region say from 2-3 volts where it will be unclear whether the transistor is on or off. So in real systems, anything under 2 volts might be considered a zero and anything over 3 volts might be considered a 1 and anything from 2-3 volts would be undefined.

One of the reasons that digital circuits have become so much more popular than analog circuits is because they don't carry noise throughout the entire system. Say I'm trying to pass an analog signal through a multistage amplifier.


5----5x2+U ----------- (5x2+U)x2+U ------- ((5x2+U)x2+U)x2+U
-------[-------------------[-----------------------[--------------------

So the original signal is 5 volts, and each transistor amplifier has a gain of two and noise of U. As the signal propagates through the analog circuit, the noise from each transistor is amplified.

Now say we want to pass a signal through a digital circuit. The difference here is that instead of the output being tied to the input, the output is either tied to ground, or 5v, depending on whether the digital gate is off or on. For this simple example, assume that each gate is simply a transmission gate.

5v ------------- 5v+U ---- 5v----------5v+U ------- 5v------------- 5v+U -------- 5v
-------------------------[-------------------------[--------------------------------[-------------------

Here, the signal starts out at 5v and gains U noise before it reaches the first gate. If we assume that U is not enough to change the logic state of the input (ie it is less than 2v), then the first gate will consider it to be a logical 1 and connect its output to 5v. The same thing happens at every gate stage.

As long as you make sure that the noise in between each gate is not enough to change the logic state of the system, then noise will not propagate through an enitre system. So you can have a signal travel through millions of gates without it being affected significantly by noise. That would be almost impossible with even 100 analog gates...

 

[Loki726]

Originally posted by: Sohcan
Funny story: Not too long ago, I was at lunch at a Chinese restaurant with people from work. When we got our fortune cookies, one of my coworkers got a fortune that read (I'm not kidding) "Digital circuits are made of analog components". We figure that a disgruntled, laid-off analog IC engineer is now writing fortunes for fortune cookies.

Don't be so quick to write off analog IC engineering as dead. Remember that although it is a lot easier to work with digital systems, those systems have to have some interface with the real world. There has to be some analog device that captures a signal, filters and samples it before it goes into the digital system, and another device that takes a digital signal and synthesizes an analog signal from it.

As digital technology is advancing faster and gets more media attention than analog technology, there is a shortage of good analog engineers as well as students studying analog.

Digital technology might be the wave of the future, but it can only advance as fast as the analog technology that supports it.

 

[herm0016]

the human brain is analog, a computer is digital. which is more adaptable? analog has a lot of power once we figure out how to make it adapte to its enviroment, which has been done on a small scale. small 6 legged circtuit boards with just a few components dont move for the first few seconds but start to walk and run after that, its really neat to watch them sort themselves out and get there legs orginized.

 

[Sohcan]

Originally posted by: Loki726

Originally posted by: Sohcan
Funny story: Not too long ago, I was at lunch at a Chinese restaurant with people from work. When we got our fortune cookies, one of my coworkers got a fortune that read (I'm not kidding) "Digital circuits are made of analog components". We figure that a disgruntled, laid-off analog IC engineer is now writing fortunes for fortune cookies.

Don't be so quick to write off analog IC engineering as dead.

??? I never said that analog IC is dead. On the contrary, I do mixed signal design in another area that requires a lot of analog components, microprocessor clock system design. However, the fortune cookie was extremely unusual, no?

 

[Lord Banshee]

Talking about Analog signals are we talking about sine waves and such?

So is Analog circuit used only with AC voltage/current, and whats a good example of a analog circuit, DSP's are right.

 

[duragezic]

A digital signal can be modeled after a sine wave, but but a digital signal can only take a finite number of values (y-axis values if you were plotting a sine wave) and the number of possible values it can take is limited by the number of bits holding the value. So if you were using only 3bits to convert an analog signal like a sine wave into a digital you could have 2^3=8 different "levels". So it would look sort of staggered like steps as the sine wave rose and fell. If you were to increase the bits the difference between steps would decrease and it would look more smooth and curvy. If you were to let the bits go to infinity it would become a perfect sine wave, indistinguishable from an analog one, but of course this is not possible.

 

[sao123]

Originally posted by: BEL6772
Or, to hear one of my old professors tell it, there's no such thing as a digital circuit. Everything is analog. Some analog circuit are designed to emulate the digital ideal, though

QFT...

 

[Lord Banshee]

Originally posted by: duragezic
A digital signal can be modeled after a sine wave, but but a digital signal can only take a finite number of values (y-axis values if you were plotting a sine wave) and the number of possible values it can take is limited by the number of bits holding the value. So if you were using only 3bits to convert an analog signal like a sine wave into a digital you could have 2^3=8 different "levels". So it would look sort of staggered like steps as the sine wave rose and fell. If you were to increase the bits the difference between steps would decrease and it would look more smooth and curvy. If you were to let the bits go to infinity it would become a perfect sine wave, indistinguishable from an analog one, but of course this is not possible.

Yes we've talked about this in signals. So if you did not want to work with Discrette Digital Signals you will use analog circuits?

 

[ebaycj]

I'm really surprised that you are a 3rd year EE, I learned about this stuff during the second semester of my first year.

As an engineer, you should not think of IC's as "black boxes" you should think of them as "massively shrunken breadboards". (I guess this assumes you have taken a hands-on circuits class, and know what a breadboard is and what it is used for). A lot of the components are the same (or functionally the same) as the discrete components you could put in a breadboard.

The true difference between analog and digital circuits is not the actual componentry, but the electrical signals flowing through the components.

 

[Lord Banshee]

Yep third year and yes i know what a breadboard is lol...

My first two years had no EE class all Math Science Humanities and such, Last semster i took my first EE class Circuit 1. this semster i am taking Digital Logic and Discrete Time Signals as true EE classes. I had to put my self out there to ask this question, yeah i know, but i did not have a clear view so far what the true difference was. ( i have two more years left have tons to learn )

Just wondering what class did you take your first year it pointed this out for you?

lol i did not seem to let you think that i thought of IC only as black boxes.

 

[kcthomas]

simplest explanation: analog = sin waves, digital = square waves

i would say the most fundamental circuit of analog design is the amplifier. first analog circuit i ever built: speak into a microphone, amplify the signal, output through speaker.

 

[BrownTown]

uh, sine waves and square waves arent exactly a good determinant of analog verse digital. The discrete/continuous difference is the real divide. As im sure you know you often deal with hitting annalog circuits with square waves in order to obtain the transfer function. Also, square waves can be represent as the sum of sine waves (just like everything else). Also, you often use digital circuits to work on sinusoidal inputs. So if you are using a DSP chip and are taking audio as an input then you are working on digitized sine waves.

 

[Yeraze]

What's the difference between Analog & Digital? Why, it's which side of the DAC you put them on, of course

Seriously tho, Brown is right. it's the Discretization that's the real trick. In the analog world you deal with "real continuous" signals, while in the digital works you deal with discretized versions of them. While in the analog world you typically deal with capacitance, inductance, and EM field effects (Physical properties), in the digital world you typically worry with quantization errors and nyquist rates (Mathematical properties).

 

[stelleg151]

Digital= finite formal system(binary). Analog = (theoretically)infinite set. In circuits, I think binary will begin to lose steam at some point(probably very far away) as more biologically based "computers" based on analog "circuitry" will become useful. Digital is useful mostly for understandability/programability, as well as algorithmic compression.