[sdiy] dsPIC CV input protection

[Seb Francis]

P.S. It did occur to me to put a pair of schottky diodes at the dsPIC 
like with the digital inputs, but in this case the opamp would push lots 
of current through them, and I can't use a series resistor as I need to 
keep the impedance low for the ADC.

Seb Francis wrote:
> 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
>
>
>
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