Consider a '555 Timer IC', Is it a Analog or Digital Device ? Or consider a more complex IC as a Microprocessor. I know the difference between a analog and Digital signal. But what is the difference between Analog and Digital Devices ?


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    \$\begingroup\$ Digital is perverted analog. \$\endgroup\$ – Matt Young Apr 7 '16 at 14:22
  • \$\begingroup\$ Primarily, a digital device is like an analog one, except that the transistors are intended to be driven fully on or fully off, not somewhere in between. \$\endgroup\$ – Simon B Apr 7 '16 at 14:25
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    \$\begingroup\$ In edge cases like the 555 it depends which marketing team is tasked with selling it. \$\endgroup\$ – The Photon Apr 7 '16 at 14:52
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    \$\begingroup\$ @MattYoung , to be precise, digital is analog with a bipolar disorder. Mixed signals components suffer from an identity crisis. \$\endgroup\$ – Sredni Vashtar Apr 7 '16 at 16:13
  • \$\begingroup\$ Reminded of this not-too-closely-related tangent: someone once got funding for iterative feeding of analog signals through logic chips, with mildy interesting results (much less interesting than that fluffy hype piece seems to be trying to assert, frankly). \$\endgroup\$ – junkyardsparkle Apr 7 '16 at 20:31

Everything is analog / analogue !

Digital is a simplification of analog, instead of that a voltage can have any value between 0 and 5 V, for example 1.23456789 V (Analog) we only consider 2 states: 0 V and call it "0" and 5 V and call that "1".

That is regarding signals and you already knew this.

You can do something similar with circuits and devices. In essence, all circuits and devices are Analog, yes even that CPU in your PC. The voltages inside it are not "0" or "1" they're 0.002338 V or 1.23328 V. But to make things easier we make an abstraction and say it's either zero or one.

However the core functionality of that CPU or any other digital device can be described fully when considering only the zeros and the ones. So in my opinion, if you can fully describe the functionality of a device with only ones and zeros, it must be digital !

Coming back to the 555 timer. Note that part of its functionality is to charge a capacitor, that voltage can have any value between a lower and an upper threshold. I call that analog behaviour. Inside the 555 there is a flip-flop which has two states, you could call that digital.

So the 555 is more of a mixture, mostly analog but with a bit of digital.

Most CPUs and MicroControllers are considered digital although some CPUs have voltage regulators and/or a PLL (with an Analog oscillator) inside and/or an ADC which is very Analog indeed. And yet, no-one calls CPUs and MicroControllers analog devices (but secretly they are !)

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    \$\begingroup\$ Indeed everything is analogue +1 \$\endgroup\$ – Andy aka Apr 7 '16 at 14:31
  • \$\begingroup\$ The 555 is indeed a "mixed signal" device. You can think of voltage comparators as a way of interfacing analog to digital, and indeed, the 555 has two of them (pins 2 and 6). Similarly, an open-collector "switch" (pin 7) is one way of interfacing digital to analog. Pin 5 is also an analog pin, but pins 3 (output) and 4 (input) are strictly digital. \$\endgroup\$ – Dave Tweed Apr 7 '16 at 15:07
  • \$\begingroup\$ FET devices printed in real chips do vary b/w analog and digital. Analog devices will generally be much larger with tighter tolerances; digital will be as small as possible. \$\endgroup\$ – jbord39 Apr 7 '16 at 16:16
  • \$\begingroup\$ @jbord39 digital will be as small as possible In digital also many transistor widths are used, for example for powerfull (high fanout) output buffers/drivers. Analog designs also need to be made as small as possible for cost reasons. I design analog circuits using FETs, these FETs are designed to make digital circuits. They happen to work for Analog also ;-) \$\endgroup\$ – Bimpelrekkie Apr 7 '16 at 17:54
  • \$\begingroup\$ Yes, they work for analog but it is not typically done that way. Using the smallest FET (1 finger, 2 fins) for analog applications would be ridiculous as the process variations at this small of a size are huge. Instead analog FET's are typically layed out in a multi-fingered fashion, distributed over a larger area. The idea here is that any local variation will cancel out to create a more robust FET. FET's for critical analog applications that I have encountered were typically at least 100x larger than the digital FET's minimum size. \$\endgroup\$ – jbord39 Apr 7 '16 at 18:16

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