AD-FMCLIDAR1-EBZ:About the princple of the LT6016 Feedback circuit

Hi,I've been studying the AD-FMCLIDAR1-EBZ AFE Board recently.I don't understand the function and the principle of the LT6016 circuit.

If the light intensity is low enough, for example, the apd current will be less than 100nA,and then the TIA output will be less than 7.5mV. Under this condition,the tia output measured value is 1V.

Does this phenomenon means that the 1V reference  voltage bypasses all light with low intensity?Does it means only when the Iapd*Rf>1V, the effective voltage value can be sampled by the ADC?

I wonder how  the LT6016 circiut works?

Thank you for your answers~