The three largest contributors to output offset, also known as DC errors, are the amplifier’s bias current, offset voltage, and the diode’s dark current. Dark current will only exist in the diode if the circuit has a reverse bias on the diode to reduce its capacitance. There is no dark current with zero volts across the diode. The amplifier’s input bias current enters or leaves the input pins. The positive pin is grounded, so it does not cause a problem. The current into or out of the negative pin will create an error voltage by reacting across the large feedback resistor RF. At room temperature, the offset voltage gain caused by the diode’s resistance can be ignored. That means that the amplifier's offset voltage will appear at the output. This is why the chosen amplifier should have low input offset voltage. For offset errors that happen at high temperatures, see the following slide. A negative bias on the diode will reduce its capacitance and make the circuit faster, but it will also create an error due to a dark current that flows in any diode with a reverse bias. Dark current is a DC error term. Possible solutions are to eliminate or reduce the negative bias on the diode if possible if the circuit is still fast enough; reduce the feedback resistor if at all possible; choose an amplifier with low input bias current and low offset voltage, CMOS and JFET op amps have the lowest bias current. Try using a low-bias bipolar part. It will have greater bias current, but that current will not double every 10°C. Also try a circuit that does dark current cancellation to allow bias to be retained across the photodiode for speed, while eliminating the error. Or try a circuit that does input bias current cancellation. Putting a resistor of equal value in series with the positive pin helps, but it should be shunted with a capacitor to reduce noise.