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

[Harry Bissell]

The added shottky diodes would keep the input within +/- .3 volt of your power supply. That's good. The other
'feature' is that if your power supply rails are not up, the input voltage will try and power your circuit
through the 100K input resistor.

You should make the 100K resistor from the non-inverting input to ground 50K instead of 100K. The inverting input
sees the input resistor and feedback resistor as being in parallel in terms of input bias current. if there were
MORE resistors (summer) the value goes even lower, you calculate the parallel resistance of whatever resistors
you have in the inverting input section. BTW just as discussed earlier, if the bias currents are low the resistor
value (or even presence)are moot point (not really needed...)

Whether you need any of the diodes depends on the actual opamp, need to check the data sheet and see if they specify
anything.

imho the shottky from the inverting input to ground should go to the negative rail...

Also, you can get (reverse) leakage current through the shottky diodes increasing with temperature. This might allow some
supply noise to get through, and you will also have capacitance through them to the rails. This does not happen to the shottky
diodes between the inv and non-inv inputs as there is no voltage potential between them in normal operation.

My vote would be check the opamp data sheet, and if you just need to limit the current into the input pins for protection, go with
plan "a".

H^) harry


----- Original Message -----
From: Justin Owen <juzowen at gmail.com>
To: SDIY List <synth-diy at dropmix.xs4all.nl>
Sent: Sat, 16 Jul 2011 11:41:29 -0400 (EDT)
Subject: [sdiy] Common-Mode and/or Differential Input protection

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 ;)

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-- 
Harry Bissell & Nora Abdullah 4eva