One very important consideration is the input bias current of the op amp. The input bias current directly adds to I_PD and creates an undesired error. The input bias current typically determines the minimum measurable I_PD within a given error tolerance. For example, a 1pA input bias current yields a 20% error when measuring a 5pA photodiode current. A 1% error target requires a 50fA input bias current maximum specification. Input bias current is strongly temperature dependent, and CMOS amplifier input bias current typically doubles for every 10°C increase of temperature. The OPA928 input bias current is tested to be less than 20fA at 85°C. The ultra-low input bias current of the OPA928 enables accurate, extremely low I_PD measurements across a wide temperature range. For information on how to maintain the specified input bias performance, see Section 7.4.