For the photodiode circuit to work, the engineer has to meet some basic requirements: the circuit must be stable; it must have enough gain to present a full range to the ADC; the offset voltage, or DC error, must be small, and not drift over temperature; the AC response must be fast enough, yet without excessive ringing; and there cannot be excessive noise that masks the signal. Like many analog problems, the designer must make a series of trade-offs. When the feedback resistor RF is large enough to give a full output, the noise will also increase. But the signal increases directly with resistor value while the noise increases as the square root of the resistor value. Therefore, the designer must try to get as much gain as needed. It is even possible to gain the response larger than the ADC needs, and then attenuate it, which also attenuates the noise. Photodiode circuits are very demanding for an amplifier. The chosen part must have low offset voltage and low bias current to reduce DC errors. Additionally, low voltage noise and current noise is desirable to ensure sufficient SNR (signal to noise ratio). For passing a fast pulse signal, a wideband amplifier is needed, but that very same wide bandwidth also increases the noise. Designers sometimes pursue solutions in the optical domain. By magnifying the light with a lens, they can make the feedback resistor RF smaller. That lowers the noise, reduces DC errors, and will tend to make the circuit more stable.