The LMH32401 device has an integrated DC cancellation loop that cancels and voltage offsets from incidental ambient light. The ALC mode only works when the PD is sinking the photocurrent. The DC cancellation loop is enabled by setting IDC_EN low. Incident ambient light on a photodiode produces a DC current resulting in an offset voltage at the output of the LMH32401's TIA stage. The Functional Block Diagram shows how the ALC loop senses the low-frequency DC offset at the output of the TIA stage and compares it against an internal reference voltage (V_REF). The ALC loop then outputs an opposing DC current (I_DC) to compensate for the differential offset voltage at its input. The loop has a high-pass cutoff frequency of 100 kHz. The ambient light cancellation loop is disabled when the amplifier is placed in power-down mode.

The shot noise current introduced by the DC cancellation loop increases the overall amplifier noise; so, if the ambient light level is negligible, then disable the loop to improve SNR. The cancellation loop helps save PCB space and system costs by eliminating the need for external AC coupling passive components. Additionally, the extra trace inductance and PCB capacitance introduced by using external AC coupling components degrades the LMH32401 device dynamic performance.