Multiplexers (and demultiplexers)

Some thoughts on Multiplexers

In a previous article on Patchbays and the author’s own dilemma at having to switch different components in an audio system using XLR and TRS connectors, it made sense to investigate real world examples of what other enthusiasts are doing to complete their kit.

MUX/DEMUX CD4067 and Quad bilateral switching

It comes as no surprise that the Texas Instruments CD4067 and CD4051, respectively 16 and 8 channel MUX/DEMUX chips are used quite extensively.  Both chips are inexpensive and the 16 channel surface mount chip comes readily available on a breakout board for Arduino at about R45.00.

The of the first of a series, the CD4016 is a quad bilateral switch and has been around for eons.  The known problem here is the high Ron spec, typically in the region of 300 Ohms at Vdd of +15V. This to many designers is significant and will lead to signal degradation.  Then we have the tried and trusted version, the CD4066, a direct replacement for the 4016 but with a lower Ron.  This makes the 4066 the more commonly used.

The bilateral switch IC should always be used at maximum supply rating, possibly 15V to get the lowest Ron. In most spec sheets 10kΩ is listed as the circuit or switch load, referring to input impedance of the next stage. The CD4066 retails for about R4.00.

To multiplex or not?

The MUX is a God send. The 4067, an equivalent to a 16:1 rotary selector can be used back to back with another 4067 to route any one of 16 inputs to any one of 16 outputs. Using an Arduino to control this chip is simple and mapping can be done on a circuit board, visual display through high impedance LED and drivers. There are many circuits on the net displaying the versatility of the 4067. Of course one could use 5 mechanical switches to control the mapping, 1-4 = A-D and one for Inhibit or Enable. Using ten switches or only 8 if you tie the inhibit to Lo (Hi switches off the output devices).

Block Diagram of CD4067
Block Diagram of CD4067

Below is a simple layout of the possibilities, routing a signal from channel input one to channel output  one.  The 4066 could be used to bridge two outputs together.  The buffer offsets line losses. Vdd and Vss should be calculated to give best Ron and not affect the accuracy of the input to output waveform.  The circuit below shows the permutations which could be used and does not necessarily follow good design practices.  e.g. when paralleling outputs careful design consideration must be given to loading and mismatching.

Multiplexing - routing audio channel with 4067 and 4066
Routing audio channels with the 4067 and 4066

For an entry level audio signal router the 4067 and 4066 can form a powerful relationship.  Focus here would be on overall cost.  Like most projects the hardware, the cabinet and finish will prove to be the most expensive.

For an Arduino setup,  here is a good link from a South African supplier, the 74HC6067-MOD.

In a follow up we will investigate switching between some pro gear.  As this is not my real job, time is restricted.

Further Reading:

Wicked Switches – thetonegod

Switches and Multiplexers – TI.com

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