[sdiy] dsPIC CV input protection

[Seb Francis]

So, same question for the CV inputs.  The dsPIC ADC again has input 
range 0 to 3.3V (with 0.3V margin to absmax figures)

I'm designing for a CV range of -5V to 5V (which can be changed by using 
different resistors if desired).

I'm using a standard inverting summer opamp configuration, and I guess I 
need some 3.3V diodes back to back in parallel with the feedback 
resistor to clip any out of range CVs.  But what to use to get precisely 
3.3V?  Zeners aren't going to work as they will conduct forward biased 
as well.  Zeners in series with normal diodes in opposite direction?  
Can I rely on this to give me precisely 3.3V (I'd like to keep the full 
input range if possible).  And again, looking for minimum component count.

Apologies for the bandwidth asking what are probably very obvious 
questions, but I'm trying not to make any mistakes since I'm going 
direct to prototype PCB with this design ... hard to breadboard with 
those little 80pin parts!

Seb