Problem: Wheatstone bridge + InAmp

[PhysZe]

Hi, everyone

I'm having a little bit of a problem with my set-up (in the image below), and I could really use some of your help!

I designed a wheatstone bridge for a NTC (R(@25ºC)=470k) using 3x 505k resistors, and then created an intrumentation amplifier using the OpAmp TL082-n for the gain part plus the OpAmp in the PIC24 for the differential part. The problem is that, while the bridge is unbalanced at a voltage value less than 0.65V it works fine, but above that, the value at the output of the InAmp stays fixed at 1.30V.

I tried to find where the problem arises and it seems to be in the TL082-n, which doesn't vary its output values after the stated 0.65V - the outputs of both OpAmps of the TL082-n are: 2.70V (for the resistor-only bridge part) and 1.40V (for the NTC bridge part).

Do you know where I got it wrong?

Thank you very much!

Best regards,

José

 

[PhysZe]

plus the OpAmp in the PIC24 for the differential part

The I/O of the PIC OpAmp are:
+IN - pin 5
-IN - pin 6
OUT - pin7

Thanks!

 

[JimB]

The problem is that, while the bridge is unbalanced at a voltage value less than 0.65V it works fine, but above that, the value at the output of the InAmp stays fixed at 1.30V.

I think that you have come up against a limitation of the TL082.
I dont think that it is expressly stated in the datasheet, but the op-amp is not rail to rail and does not work with input voltages which are close to the supply rail voltages.
Try something like an LM358, or maybe an MCP6042 which is designed for rail to rail operation from a low supply voltage (6v max).

JimB

 

[PhysZe]

Ahh, now I see it! I was trying to use some spare Tl082 I had from previous projects, but I have to leave them in the drawer until they're useful... Thank you very much for your help, as well as for the tip on more suitable components!

 

[PhysZe]

*Upgrade*

Hi!

So, to follow up on this matter, I've put my hands on a LM358 (still waiting for the MCP6042) and used it in the place of the Tl082. Still I have a strange output signal... I can't understand the clipping (@ 1.612V) nor the reduced gain (*0.27) nor the 1.2V offset.

If I try to change the 100k resistors of the differential part for 1k, the gain gets negative (eq.: y=-0.31*x-0.52)

I can and will try with the MCP6042 once I recieve it, but I'm afraid that the problem might be somewhere else in the circuit.

Thanks, once again for your help!