The LMH32404 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 TIA stage.

If the photodiode produces a DC output current resulting from ambient light, the output of the level-shift buffer stage is offset from the reference voltage V_REF. The ALC loop detects this offset and produces an opposing DC current to compensate for the differential offset voltage at its input. The loop has a high-pass cutoff frequency of 400 kHz. The ambient light cancellation loop is disabled when the amplifier is placed in low-power 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 LMH32404 dynamic performance.

The ambient light cancellation loop is active (depending on IDC_EN configuration) when one or more of the channels is in standby mode. The ambient light cancellation loop is disabled when the amplifier is placed in low-power mode. When the LMH32404 is brought out of low-power operation the ambient light cancellation loop requires several time constants to settle. The time constant is based on the 400-kHz cutoff frequency of the loop. When in standby mode, the ALC loop is still active depending on IDC_EN configuration.