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I am almost completely new to electronics. Recently I was reading about an 'analog multiplexer' I know what a multiplexer does, and how it works, but is there such a thing as a 'digital multiplexer' that would just give +5v when a channel was on?

Pardon if my question is malformed, I just started in on electronics two days ago.

REPHRASE:
OK, so the difference is that a 'digital' multiplexer will just output +5/0, and an 'analog' multiplexer will just pass the input signal with as little distortion as possible ?

A.R.
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  • Use popular electronics distributors' parametric search tools to learn more: DigiKey. – tyblu Feb 19 '11 at 02:04
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    Please consider accepting one of the other responses. Mine was just a quick reaction, others have since done far nicer work here. – JustJeff Jun 01 '12 at 14:23
  • Yeah, you'll note that you also provided an answer many months ahead of everyone else. I'm happy with it, even if Fred isn't. Meanwhile, I picked someone else as requested. Cheers! – A.R. Jun 04 '12 at 14:32

3 Answers3

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A big difference between analog and digital multiplexers is that the former can be used bidirectionally, while the latter are unidirectional. That means that analog multiplexers can also be used as demultiplexer, while for digital you need separate ICs for that. There are also several types, depending on the number of in/outputs:

multiplexers (non-exhaustive)
8-input: 74HC151, 74HC251,
dual 4-input: 74HC153, 74HC253,
quad 2-input: 74HC157, 74HC257.

demultiplexers (non-exhaustive)
3-to-8 inverting: 74HC138,
3-to-8 non-inverting: 74HC238,
dual 2-to-4: 74HC139.

FarO
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stevenvh
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    More generally, "multiplexers" may either connect the input to the output, or they may feed a selected input to a some type of amplifying/regenerating circuit. So-called "digital multiplexers" are of the latter type, but analog multiplexers of that type (typically containing an op amp whose non-inverting input is tied to the selected channel) also exist. It may be more accurate to refer to the devices that connect a pin to one of several others as electrically-controlled one-of-N switches, rather than multiplexers, though the term "multiplexer" seems common. – supercat Aug 08 '12 at 19:22
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You can probably use an analog multiplexer as a digital one.

In actuality, in many cases you can use some types of "digital" multiplexor as an analog one, too. People build extremely high performance radios using digital bus switches as commutating RF mixers.

There are basically two ways to do a multiplexor - one is as a choice made of passive switches, basically MOSFET "pass gates" which ideally alter the signal as little as possible when passing through them.

The other is to build a multiplexor using logic gates (or PLD LUT's) to implement a truth table. This would most likely have the usual logic-gate property of non-linear amplification, ie, an input which just barely qualifies as a 1 or a 0 becomes an output which is quite strongly and unambiguously that value, restoring noise margins.

Chris Stratton
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Yes, analog and digital multiplexers have been around since the dawn of SSI. http://en.wikipedia.org/wiki/Multiplexer. Typically the analog variety are integrated with an A/D device.

kenny
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