[sdiy] dsPIC CV input protection

[Jason Proctor]

caution: newbie chiming in...

you say that there's a comparator inside the PIC which flips at 0.6v 
or thereabouts? why not implement that comparator yourself and then 
just feed the PIC 0v or 3v (or 2v, or 1v..)?

then you could use a standard digital input protection scheme similar 
to wot Ken uses in all of just about all of his CGS designs.

then divide down the +15 or whatever from the comparator to 3v and off you go.

hth

>Ingo Debus wrote:
>>
>>Am 15.11.2008 um 14:06 schrieb Seb Francis:
>>
>>>and also ones with rail-to-tail input too, although this isn't 
>>>necessary for an inverting configuration as the inputs are always 
>>>at 0V
>>
>>Only if the opamp is not overdriven. And wasn't overdriving the 
>>opamp the point of your protection scheme?
>>
>
>Well, yes, but while the input CV is within the 'working' range the 
>opamp input will be at 0V.  Anyway rail-to-rail input opamps are 
>talking about common-mode input voltage range which like you say 
>isn't going to be case when the opamp is overdriven.
>
>I assume that as long as the input voltage doesn't exceed the absmax 
>specs then it'll be ok.
>
>This is the circuit in question:
>http://burnit.co.uk/sdiy/stuff/cv-buffer.png
>
>This particular opamp has absmax input voltage specs of -0.3V to 
>3.3V with this supply.
>
>If the current going into the inverting input of the opamp was 0, 
>then theoretically these specs would be exceeded with input CVs 
><-12V or >11V.  But I'm not sure this would actually happen as the 
>current flowing into the opamp input would mean the voltage present 
>there would be less.
>
>Is this correct thinking that the opamp input will just draw more 
>current until the voltage ends up within the absmax specs anyway? 
>(the absmax input current is 5mA)
>
>Seb
>
>
>
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