[sdiy] Common-Mode and/or Differential Input protection

[Justin Owen]

I hope nobody minds me pushing this out a little further for a little longer...

http://www.sdiy.org/juz/euro_input_detail_01.pdf

Config A in the link above has parallel Schottky's across the -v and +v terminals and 100K from the +v to GND (which I hope I've placed and understood properly?) based on the advice of Harry B and Neil J - thanks again to you both.

With this advice I went back and re-read the protection chapters in Jung's Op Amp Applications and Op Amp Cookbook (*love* this book).

The parallel Schottky's are recommended as protection for Differential Input breakdown but both chapters also describe a method for Common-Mode breakdown that uses two Schottky's in serial - one from GND to the junction of the Input and Feedback resistors and a second from the same junction to V+. I've added this in Config B in the link above.

In general, but particularly in a modular environment where there's a few common (as opposed to 's@!t happens') fault-causing scenarios (15V gear into 12V gear, Outputs/Inputs into Inputs/Outputs and - with particular regard to the Common-Mode method as I have read it - plugging powered gear into non-powered gear)...

...is there a case for a) using both of these methods, b) choosing one over the other, c) combining them effectively or d) none and or all of the above*. I can see some oddities and potential problems with this combination in its current form - what do you lot have to say about this?

Thanks again,

Justin

*you'll be required to explain your choice of this option - thanks ;)