Common Mode Voltage Range with Negative Feedback
But does the common mode voltage range mean anything if one of the inputs is grounded and there is negative feedback? Negative feedback guarantees that now both my positive and negative input will always be at ground.


Not all op-amps allow rail-to-rail inputs. For non-rail-to-rail op-amps, having the input pins at ground when the negative supply rail is at ground would violate the input common mode range spec. 

Now the case of the PA85 is a bit mystifying. Here's the common mode range spec in the datasheet:



This is not clear, but almost certainly this does not mean that the common mode range extends to 15 V below the negative rail, although the datasheet might be interpreted to say that. It most probably means the minimum input voltage is 15 V above the negative rail. 

So in order for this op-amp to operate with its inputs at ground, the negative rail must in fact be at -15 V or lower.

Edit

This excerpt from Apex Technologies App Note 1 shows that when they say the common mode range is \$\pm V_s-15\ {\rm V}\$, they do mean that the input voltage must be at least 15 V above the negative rail, not no more than 15 V below the negative rail: