Is the following movement right?
・When I'll make IRED increased, output is saturated.
・When more IRED was increased, a reversed corrugation was observed.
Why will it be a reversed corrugation?
As the TIA first becomes saturated, the ADC will also saturate resulting in a flat line clipped at full scale as you observed. As you continue to increase the input to the TIA, it will significantly distort the output of the TIA which will also significantly distort the output of the bandpass filter into the integrator. Since the timing of the integrator sequence is dependent on being optimally timed to a correct bandpass response, this distortion in the bandpass response can lead to a reversal in the signal being presented to the ADC. So as you continue to push the TIA further and further into saturation, the ADC will begin to decrease as a result.