Electronic – Can you feed in a voltage higher than your supply to the input of an opamp


I have a 28V digital signal. 28V being logic 1 and 0V being logic 0.

Can I feed the 28V into an opamp (Vcc @5V) to level shift it ? Or will I have to create a voltage divider to bring the 28V < Vcc ?

Best Answer

Direct feed isn't going to work well for you.

You'll find that the common-mode input range of an opamp is quite often less than the supply voltage rails (less than VCC, greater than VEE) . Some parts (those with rail-to-rail inputs) can accept signals at the same potential as the supply lines.

Usually there are p-n junctions between the opamp inputs and the rails (sometimes deliberate, often parasitic) which can forward-bias and fry if a stiff signal higher than the postiive supply or lower the than negative supply is applied.

Current-limiting the source will cause the voltage at the input to clamp a diode drop above or below the rail (depending on polarity) but there's no guarantee the rest of the opamp will behave nicely in this condition.