This application note reviews the features of Microchip’s MCP6491 low input bias current op amps, the characteristics and operation modes of photodiodes, then it focuses on designing photodiode amplifier circuitry, and several key design points are discussed in order to improve the circuit’s performance. The noise analysis of a photodiode amplifier and the design technique of a low pass filter are also discussed. Finally, the PCB techniques that help reduce the current leakage are briefly introduced as well.