[sdiy] dsPIC CV input protection

[Simon Brouwer]

Seb Francis schreef:
> 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)
Looking at the equivalent schematic in the data sheet the inputs are 
MOSFET gates. So any significant current would only flow through 
protection diodes (not shown).

With this buffer circuit, and input voltage range of +/- 15V, the input 
current will never even come close to that 5mA.

-- 
Vriendelijke groet, 
Simon Brouwer. 

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