The LMH34400 has an integrated DC cancellation loop that can used to cancel any voltage offsets resulting from ambient light. 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, then the output of the level-shift buffer stage is offset from the reference voltage VREF. 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 LMH34400 dynamic performance.