[sdiy] The annual I/O impedance/protection thread! (revised)

[Neil Johnson]

Inputs
> http://www.sdiy.org/juz/euro_input_03.pdf

Harry wrote:
> I'd suggest this... use only inverting inputs

That's probably the best compromise, and means you can effectively drop
the zeners and the 100R resistors.

Note that for the AC-coupled case you need to either use a non-polarised 
capacitor, or think about what the DC operating point of the op-amp -ve 
pin will be to determine which way round the polarized electrolytic goes.

Outputs
> http://www.sdiy.org/juz/euro_output_03.pdf

Again, I think the zeners are probably of little use.  Where you have a 
DC output you're going to be concerned with absolute value, so best to 
put the protection resistor inside the feedback loop, with a small 
capacitor as well.  And for the non-inverting circuits add anti-parallel 
Schottky diodes across the op-amp inputs and a resistor to the +ve input.

In the AC output versions I'd just put the series protection resistor 
after the feedback tap, then series capacitor (watch the polarity!) and 
high-ish drain resistor.

Summing
> http://www.sdiy.org/juz/euro_summing_03.pdf

1/ Drop the 100R and the two 9V1 zeners per above.

2/ R1 will appreciably load the input pot to give a non-linear control 
curve.  This might not be an issue for you (designers choice) but it 
irks me.

3/ Your manual CV pot reference voltage is negative, but unless your 
external CVs are also going to be negative that just doesn't make any 
sense.  True, turn the pot to the right and the op-amp's output will go 
up...

4/ ...But this won't be the case for the external CV: as it goes from 0 
to 5V the internal CV will go down/negative.  Is that what you intended?

5/ Depending on what the internal load looks like that 1N4001 could 
behave in a rather non-linear fashion.  And assuming enough current 
passes through it to turn it on, it will drop about 1V from the output 
of the op-amp.

6/ When the 5V1 zener turns on its going to provide a nice low-impedance 
path to ground to pull lots of current out of the op-amp, which will 
overheat and possibly melt.  Not quite desirable.

Cheers,
Neil
-- 
www.njohnson.co.uk