Dear all,
I'm just testing the idea of ac-coupling an instrumentation amplifier (hear I tested both AD620 & AD8220) by a feedback from output to reference pin. The circuit and output are attached. In the test experiment, non-inverting input (IN+) is connected to VGND and inverting input (IN-) is derived by a small amplitude pulse around VGND (blue line). The pulse is generated by a 555 IC then buffered and damped by an LM358 IC. V_CM is O.C. Output and input are plotted red and blue respectively, with the aim of Arduino serial plotter. VCC is connected to 5V battery.
The problem: as it was expected, the rectangular pulse input is amplified and high passed exactly with the 1/(RF*CF) HPF (I imported data for different frequencies to Matlab and checked the spectrum). The problem is the output envelope which looks like a first order circuit charge and decharge and I can not stop it from oscillation.
I have simulated the circuit in PSpice and no such effect exists there.
The problem get better but not solved if I replace AD8220 by AD620.
I have tested different ideas to solve this problem but non of them was successful. Any idea on the cause of the problem and its possible solution will be greatly helpful for me!
Thanks is advance