Patent ID: 8283620

Claim:
A lock in charge amplifier circuit configured to be coupled to an output of a photodetector which generates input charge in response to illumination by an illumination source, the lock in charge amplifier circuit comprising: a switch network, a controller, and a charge amplifier, said charge amplifier further comprising a feedback capacitor connected between an input of the charge amplifier and an output of the charge amplifier; wherein said controller is configured to provide a control signal to switch the switch network between at least first and second states and to switch on the illumination source during only one of said first and second states; said first and second states being operable to apply a first input charge obtained over a first period of time and a second input charge obtained over a second period of time to said feedback capacitor such that after operation of said first and second states a difference between the first and second input charges is stored on the feedback capacitor, the charge amplifier configured to output an output voltage dependent upon the charge stored on the feedback capacitor; and wherein said first state is operable to apply said first input charge directly to a first plate of said feedback capacitor and said second state is operable to apply said second input charge directly to a second plate of said feedback capacitor.