Problem with the Howland current source design

Hi inabh ,

Good day. We'll try to look into this. To give you a background about stability, you can refer to this document: Stability KWIK Lecture.

Thanks and regards,
Gilbeys

This is the opamp response when it is enabled with a complex load connected to its output.

Green- Sinewave input

Yellow- Opamp output

Hi inabh ,

As an initial check, I test and verify that the model matches the datasheet. It appears that the LTspice model has a more stable characteristic compared to the actual data in the datasheet, which is why it seems stable in the simulation. 

By connecting your complex load to your actual prototype board, does it have any cables in it? Cables do introduce instability in some aspects, then we need to add this to the analysis.

Also, which part of the circuit is the complex load you are referring to?

The offset voltage was causing some DC current at the output. The DC resistance of the complex load is around 8k ohms. The DC voltage across the load was causing positive feedback to increase, resulting in instability. Adding a 10uF capacitor in the positive feedback solved the problem.