Embedded systems contain many types of sensors, used to sense events in the real world. They can be classified according to the role they play in the analog conditioning circuitry, such as voltage source, current source, resistive and capacitive (frequency-based). This application note discusses the analog conditioning circuit used for high-impedance sensors that act like current sensors. The design approach illustrated in this application note, using op amps, is broken down into four design steps: DC, stability compensation, closed-loop gain and noise reduction. A design using a PIN photodiode (light detector) illustrates the principles discussed.