[sdiy] dsPIC CV input protection

Mattias Rickardsson mr at analogue.org
Sat Nov 15 13:10:24 CET 2008

Would it be a bad idea to skip the OP-amp buffer and have a passive
input attenuation, with two ordinary diodes (connected in reverse to
ground and dsPICsupply) added as protection?
(Assuming that the dsPIC survives input voltages reaching one PN
voltage drop outside the desired input range)

I have no experience with dsPIC, but with the many A/D inputs of
today's microcontrollers it is nice to have as minimalistic input
circuitry as possible to reduce outboard component count. :-)


2008/11/15 Tom Wiltshire <tom at electricdruid.net>:
> On 15 Nov 2008, at 03:28, Ian Fritz wrote:
>> Another approach to consider: amplify your CV so its full range just
>> saturates an opamp.  Then get to 3.3 with a voltage divider.  For my SoS
>> wind controller I used LM324 opamps powered at 6.4 V to get a 0-5V swing,
>> which went directly to PIC inputs.
> I've been designing a PCB for my PIC-based VCLFO, and that's the approach I
> took too.
> I've got a standard op-amp inverting summer powered from +/-15V. You can
> throw whatever you like at that and saturate it if you want to. I then
> followed that with a precision half-wave rectifier stage with the feedback
> resistor set up to reduce the full scale output to something safe for the
> PIC (0-5V in my case). Negative CVs are chopped off by the rectifier.
> It uses two op-amps, so it isn't a *very* low component count, but there
> aren't many other components beyond the IC, and it seems to make a robust
> solution.
> T.
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