Op amp substractor output offset

[ayashisunyday]

Hi there,

I'm facing an issue with an op amp substractor circuit. I've used the 4 same resistor values.



The idea is to remove a constant 2.5V offset from the input signal. The offset pin is connected to a voltage follower fed with exactly 2.5V.

When the input signal is around 2.5V (2.5 - 2.56V), the ouput is always 60mV then with higher values, the substractor seems to work in a linear way. As the input signal variation is in a very small range, this prevent me to isolate the first 60mV.

I've tried to change the resistor values. Simulation shows that with 100k resistors the output when feeding 2.5V is lowered to 6mV but while trying it in the real world, the circuit get crazy and the output stuck to +5V.

What would be the best way to do the substractor operation with the best accuracy even with small Offset / Input deltas?

Thanks for your help and comments!
Aya

 

[keith1200rs]

You will get closer to GND with higher value resistors. Also, a resistor from output to GND will help, although then you will have a problem getting close to the positive rail. Most rail-rail opamps are "almost rail-rail" opamps.

Another option is to work with a slight, deliberate, positive offset, say 100mV. That depends on what you are doing with the signal afterwards.

Keith

 

[Venkadesh_M]

The best option is to change the opamp IC, Which is having very small output offset voltage...
The second way is to bias the opamp with dual mode power supply in your case +5V and -5V................

 

[ayashisunyday]

Humm, you're right! I can deal with a small output offset as long as I can have a linear output.

I'll give it a try with an Offset voltage of 2.4V instead of 2.5V.

Thank you very much!
Aya