Operational Amplifier input voltage affects other input? Answered
I have created a simple circuit where 2 10k resistors make a voltage divider. Supply voltage is 12.5V and if I measure in between the 2 resistors, I'm getting 6.3V. Great!
Now I connect that 6.3V to the +input of an OpAmp. Result: the value is still 6.3V. Great!
(The OpAmp is also fed with 12.5V)
Now I connect the -input of the OpAmp to the supply voltage (12.5V).
The voltage in between both resistors suddenly changes from 6.3V to 10.1V. NOT Great!
Now I connect the -input of the OpAmp to ground (0V).
The voltage in between both resistors changes from 6.3V to 2.4V. ?!
The output of the OpAmp is not connected to anything.
I thought that OpAmp inputs, in theory, have infinite resistance?
The datasheet of this OpAmp (LT1253) sais the input resistance should be 10Mohm
I also tested this same thing with the LT1497 and the results are the same.
When the -input of the OpAmp was connected to 12.5V, I measured the current going into that pin, 39mA!!
(Only measured on the LT1253)
What's the problem here? Are both OpAmps broken? Or is there something about the way OpAmps work, that I clearly don't understand?