I believe I understand why we need to use a high voltage in Vin. I ran some simulations on the Isolated Amplifier structure using as much of the same operating conditions as in the lab. I used the same values for the operating parameters as mentioned in Giulio’s email, see below:
Vdd = 3.3V
Vcc = 1.8V
Ibias SF = -250uA
IbiasSH = -200uA
IbiasPRE = -250uA
VbiasPRECascN = 0.500V
VbiasSHCascN = 1.500V
I used a high voltage in the Vin input, about 1V, and the result of the simulation is attached. It looks very similar to the response you have been seeing, like the first result, attached in this message for convenience. Also, when I change the bias voltage of the preamplifier cascade device to 1.5V from 0.5V the output almost disappears as observed experimentally although 1.5V is the recommended operating bias voltage as this ensures that the input transistor of the preamplifier is working in saturation. Since we do not have any injection capacitor inside the chip it makes testing the Isolated Amplifier in the usual way somewhat difficult. The actual results so far tell us that the electronics in the isolated amplifier is working to an extent, but we are not able to have ideal setup conditions which in turn means we are at the moment unable to fully characterise the isolated amplifiers. I will try to pop into the lab tomorrow and work with Giulio and Gary to find a way to fully characterise the isolated amplifiers.
Regarding the laser injection of the active pixels, its hard to say without knowing in more detail about the laser and the test setup. Personally I would have liked Structure 1 to be tested as it is almost a replica of Pimms with NWELL diodes and Pimms was proven to work in the same type of substrate.
Its also important to remember that there are NO ESD protection and we can easily damage the devices inside. So extra attention needs to be paid while handling.