Bertrand wrote, "
I tried to connect directly vref to GND but it did not work.?"
I don't know what you mean by "it did not work."? Did the simulation abort?? Did it run but the voltages looked wrong?
With your 2-stage circuit in "issue_with_Vground2.asc", after changing Vref to Ground, I simulated it with this change:
There is a "burp" in both output signals in the first 20 us while the power sources are ramping up, but that's normal and should be expected.? You can skip it by starting to save data (.tran statement) after it's gone.? The second output (OUT12) has some stuff going on but that is part of the transient response of that filter.
The "Startup" option frequently helps when LTspice has trouble solving the initial operating point.? You can tell when that happens because it aborts before there is any plot window on the screen.? The options "Startup" and "UIC" have some similarities for helping get past that DC operating point, but "Startup" is usually much better than "UIC".
Adding the Alternate Solver sometimes helps too.? But in this case, with "Startup", the Normal Solver works better.
Andy
