In the ideal world, one simply calculates the APD bias voltage, selects a few external component values and the design is complete. Unfortunately, the real world is not so simplistic. The APD bias voltage changes with temperature. This change in voltage affects the gain of the APD and thus its sensitivity. To overcome this, most circuits are designed with a slight positive temperature coefficient (approximately 100ppm per degrees Celsius) for the APD bias voltage. The LT3571 includes functionality that simplifies compensation circuitry. For example, for the voltage at the CTRL pin > 1.2V, the internal 1.2V reference is used and the voltage at the APD pin is fixed. However for V_CTRL < 1V, V_APD scales with V_CTRL. For a lower cost, non-microcontroller based compensation scheme, an NTC resistor can be used to offset the positive temperature coefficient of the APD reverse bias circuit. However, note the NTC has a nonlinear response with temperature and thus the correction does not yield a linear response.