[sdiy] Another protecting inputs question...

Seb Francis seb at burnit.co.uk
Thu Nov 20 02:01:33 CET 2008

Hopefully I've solved this already, but I couldn't get my head round the 
maths, even though it looks right... so running it past you guys :)

This is the audio input buffer for the CODEC:

The CODEC itself ("OUT") is easily enough protected as it can handle 
from -0.7 to 5.7V ..
With the resistor divider R85 & R86 the +ve limit will never be exceeded 
with the max swing of opamp U14:A
And the diode D14 protects against -ve voltages.

My question is about protecting the CV input of the VCA U10:A that is 
used for the feedback return gain.
The input EC- has abs max +/-0.6V
I want a linear control range of +/-0.35V

So my solution was the 2 diodes D12 and D13.  I'm pretty sure this gives 
enough protection, but I couldn't quite work out if they are going to 
noticeably affect the linearity of the control from -0.35V to +0.35V.  
After a while of looking at datasheet graphs and scratching my head I 
thought I'd email for 2nd opinions!


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