80Views7Replies

Author Options:

Operational Amplifier input voltage affects other input? Answered

Hello,


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?

Discussions

0
None
steveastrouk

4 years ago

these things have very high gains, without feedback, they just won't work here.

0
None
steveastrouk

4 years ago

out of interest, why did you choose a specialist video amp ?

0
None
steveastrouk

4 years ago

you were using feed back resistors weren't you?

0
None
irishjim68

4 years ago

The OpAmps are working. However, you need to have separate resistors on the inputs, and not be connecting the inputs to the same voltage divider. Also, you will need a feedback resistor from the output to the inverting (-) input. The following page gives a good tutorial on basic OpAmp theory and operation. Connecting the inverting input directly to supply voltage is a good way to burn up the chip too.

http://www.electronics-tutorials.ws/opamp/opamp_1.html

0
None
irishjim68irishjim68

Answer 4 years ago

Here is a good schematic of a basic OpAmp circuit:

350px-Opamp-differential.png
0
None
steveastrouk

4 years ago

check the common mode input range for the amp. Does it include the supply rails?

0
None
steveastrouksteveastrouk

Answer 4 years ago

you need to really read up on op-amps. You can't learn their intricacies on the breadboard.