This shows the two main blocks of the LT3571: the boost converter and the current mirror. The components circled with light brown comprise the basic elements of a boost converter – the inductor, the switch, the steering diode and the error amplifier. For an APD bias current range of 250nA to 2.5mA, the current mirror on the MON pin sees 20% of this, or 50nA to 0.5mA. This current, passing through a 10k resistor, gives a voltage range of 500µV to 5V which can easily be measured with an ADC.