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

[Justin Owen]

Hooray!

This is one of those topics that keeps coming up here on SDIY so I've uploaded a few schems based on the 'standards' and the conversations that have gone on here in the past.

Hopefully your answers and comments will be helpful to more people than just me.

1) Input: http://www.sdiy.org/juz/euro_input_01.pdf

4* variations here: AC/DC Coupled and Inverting/Non-Inverting.

Q: My understanding is the 100R at the socket protects the Zeners from a short when plugging a jack into the socket - is 100R big enough and if not - should I adjust the impedance setting resistor down accordingly?

The back to back Zeners protect against over voltage at approx. +/-20V

Q: Input impedance is set to 100K on the non-inverting config via a 100K to ground. This should be to ground right? Not in series with the input?

Q: On the non-inverting input - is the 100R/100K combo redundant? Can I just use a 100K at the input to look after the zeners and set the input impedance?...

Q:...and if Yes to the above - were would the AC coupling cap go? Between Input and 100K (formerly 100R) or between Zeners and inverting input?


2) Output: http://www.sdiy.org/juz/euro_output_01.pdf

4* variations here: AC/DC Coupled and Inverting/Non-Inverting.

The voltage follower uses a 10K/1K in the feedback loop to set the output impedance to 1K.

Q: The 100R on the output is to protect the Zeners, again - is 100R big enough?

Q: Again, on the inverting config can the 100R/100K combo be combined into a single 100K at the input?

Q: Again, if yes, where does this put the AC coupling cap?



3) Summing: http://www.sdiy.org/juz/euro_summing_01.pdf

Only one config here - it's the one everyone seems to agree on for the most part. However...

Q: Is there a case for a buffer before the attenuation Pot? If I'm protecting my I/O from unknowns - is it odd to assume that suddenly that input will be prepped to 1K impedance no matter what?

The outputs are negative DC blocked and Zener capped to 4V7 (or whatever) to provide approx. 0-5V CV.


So - how does that all look? What's wrong, what looks odd, what would yu add/change/omit.

Thanks heaps.

Justin