I add the .asc

 

If it were me, having separately established that U3 could generate a Vg voltage as required I would just replace U3 and its associated components with a standard voltage source. The simpler the simulation the less the chance of time-step errors and the quicker it will run.

 

If it were me, having separately established that U3 could generate a Vg voltage as required I would just replace U3 and its associated components with a standard voltage source. The simpler the simulation the less the chance of time-step errors and the quicker it will run.

Thanks a lot for info! On the other hand I really am astonished that spice couldnt handle this simple thing and since i am a complete newbie with LTspice I think I have commited an error somewhere....

 

Hi. You may be having a problem with offset voltages at the inputs of the AD623. The datasheet says (depending on which version you have) that the offset voltage is TYPICALLY 25mV. It could be as high as 200mV. With a gain of 50, that means the output will be 1.25 volts! You should definitely connect the REF pin to ground, IMHO. If any of the inputs need to go more negative than 150mV, you'll need a negative supply, or find a better amplifier.

Also, what amplifiers are you using for IC1? Those inputs are not balanced, so they will contribute to the offset.

Happy hunting.