[sdiy] dsPIC CV input protection

Seb Francis seb at burnit.co.uk
Sat Nov 15 16:52:46 CET 2008


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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