I plan on using a 74HC4067 to add have more analog & digital inputs on my SBC.
The datasheet talks about having built-in ESD. The analog inputs would be fed 1.8V. The digital ones would be fed 3.3V.
I have a nice article on how to protect digital inputs. It mentions current limiting, filtering and external clipping diode.
My interpretation is I don't need the clipping diode because it's already provided by the multiplexer. Additionally, it seems to me that the filtering could be done "after" the multiplexer (so on it's output side), which means I need only 1 filter instead of 16. And I'm not too sure about the current limiting part.
(My use case involves long wires, sometimes beyond 20m).
Can I skip the clamping diodes?
Can I have the filter and the current limit after the multiplexer?
(16 inputs of the MUX go to one analog input of the Arduino… so here the question is what protection does the mux need, and as an extra, what does the Arduino need)
Best Answer
long wires act as antennas. ESD is one problem, EMC is another. Then there is lightning. The degree of protection really depends on the environment.
The ESD diodes on ics only provide a small amount of protection and are inadequate for wires that go out into the real world. Putting protection after the mux is useless.
For a commercial design, we determine the expected environment for our device and apply the appropriate standards that guide what level of protection is required. For a hobby project, we don't need to go to that length, but is good practice to follow.
We also need to specify what the expected signals are - frequency range, impedance voltage, current and so on as this guides us as to what techniques we can use for protection. Minimal protection would be a RC filter and maybe TVS diodes, varistors etc. As well, the type of wiring - shielded? etc.
As well, hardware is only part of the solution. Your software needs to filter adc values, debounce/filter digital inputs to ensure the rest of your code responds to valid inputs. The old maxim 'garbage in/garbage out' applies. None of this is particularly difficult (usually!) but if considered during the design phase makes the likelihood of a successful project much greater.
Or you can simply ignore all of this. If so you can expect the following: