# Common Mode Voltage Range with Negative Feedback

In section 3.1 of AN21 from Apex Microtechnology it mentions to make sure to accomodate the common mode voltage requirements. 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. This means I could have an input voltage greater than the supply voltages - since no matter what the input voltage is, the negative and positive inputs are always 0V (which is less than the positive supply voltage). Does this mean for example in the case shown below, I could apply an input voltage of greater than 430V since negative feedback ensures the input terminals of the op-amp will always be 0? • They are merely asking you to make sure that the (-) and (+) inputs stay within the common mode range. This means the points right at the opamp pins. They don't mean voltages elsewhere in the circuit. (Of course, most opamps also need both inputs near each other, too.)
– jonk
Nov 22 '18 at 3:14

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: • And so the input voltage ('Vin' in the PA85 diagram) could exceed the rail voltages as long as the common mode range is obeyed at the positive and negative inputs of the op-amp ? Nov 22 '18 at 6:52
• @VanGo, I don't understand what you said. The minimum common mode voltage is 15 V above the lower rail and the maximum common mode voltage is 15 V below the upper rail. So the input voltage can't exceed a rail voltage without violating the common mode range limits. Nov 22 '18 at 7:15
• I mean the Vin that's on the very left of figure 2 of the original post, like before the input resistor Nov 22 '18 at 7:25
• @VanGo, the op-amp is only affected by what voltages are at its pins. But if you want to operate it with higher voltages in the circuit elsewhere, you need to be careful about what could happen during unusual operating conditions, power up and power down, etc, to be sure excess voltages never appear at the actual pins. Nov 24 '18 at 23:25