[sdiy] dsPIC CV input protection

[harrybissell at wowway.com]

I don't know what kind of A/D input it is... some are
switched capacitor in which case adding series resistance can
lead to serious errors... :^)

H^) harry



On Sat, 15 Nov 2008 02:47:57 +0000, Seb Francis wrote
> 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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> >
> >
> 
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Harry Bissell & Nora Abdullah 4eva