Op-Amp question

[edofbrighton]

Hello,
I have an op-am circuit which doesn't work. The circuit is a simple differential op-amp circuit, which is supposed to get the diffence in the two outputs from a wheatstone bridge.
Instead the output is just the postive power supply voltage. Does anyone know why this might be happening? I've used the circuit before to do exactly the same thing and it worked. The pins all appear to be connected properly, so I really have no idea why it's not working. I thought i might be because it is now on PCB, but i tried it on bread and it didn't work on that either.

pin 2-ve
pin3 +ve
pin4 ground
pin 7 power supply
pin6 out

it is a rail-to-rail op-amp and there is no feedback.
data sheet: https://www.electro-tech-online.com/custompdfs/2011/02/0900766b8078002b.pdf

thanks ed

 

[ljcox]

I suggest that you post a circuit diagram.

It is impossible to comment otherwise.

 

[edofbrighton]

**broken link removed**

 

[BrownOut]

You've connected the opamp as a simple comparator. The gain is so high that a fraction of microvolt will cause the output to slam to the supply rail. You need an amplifier with feedback that sets the gain to the required value. You can use an instrumentation amplifier or a wheatstone bridge that's built around the opamp. But you can't use the opamp this way.

Look at the differential amplifiers starting on P. 73 of this. ( also P. 67 )

 

[edofbrighton]

I can't believe i did that.

thanks for your help.

 

[ljcox]

Note that on the pages indicated by Brownout above, the resistors R0 must be a close to equal as possible. The same applies to those marked R1.

This will maximise the Common Mode Rejection Ratio (CMRR).

I would buy 10 of each value, measure them and select the 2 that are closest in value.

Also note that you may have to provide off set adjustment.