Amplifying High-Impedence Sensors ? Photodiode Example

AN951 Amplifying High-Impedance Sensors - Photodiode Example and C reduce the output noise. The output noiseR N N Author: Kumen Blake and Steven Bible is also kept low by not over-compensating the feedback Microchip Technology Inc. loop. INTRODUCTION C F Embedded systems contain many types of sensors, used to sense events in the real world. They can be R R F N classified according to the role they play in the analog V OUT conditioning circuitry, such as voltage source, current I C S - Csource, resistive and capacitive (frequency-based). S N This application note discusses the analog conditioning + circuit used for high-impedance sensors that act like current sensors. FIGURE 1: Transimpedance Amplifier Current sensors connect to a transimpedance amplifier Equivalent Circuit. which converts current to voltage. The design approach illustrated in this application note, using op Step 1: DC amps, is broken down into four design steps: DC, stability compensation, closed-loop gain and noise This design step sets the DC gain and bias point to reduction. ensure proper operation. It also addresses DC offsets. A design using a PIN photodiode (light detector) illus- High-impedance sensors have a finite output resis- trates the principles discussed. Measurement results tance. The source's output resistance is negligible are provided to support the theory presented. in mostbecause it is significantly larger than R F The last sections of this application note contain transimpedance amplifier applications. supplemental information. The References section As explained earlier, the DC output voltage due to the contains references to pertinent literature, while source current will be: Appendix A: "Op Amp Stability Analysis" covers op amp stability analysis. Finally, Appendix B: "Set-up V = I R OUT S F Overview" discusses the tools used to measure the design. The op amp will contribute a DC offset voltage to the DESIGN output: Figure 1 shows the equivalent circuit of a transimped- V + I R = output offset voltage OS B F repre-ance amplifier and a high-impedance source. I S sents the output current of the source. C is the sum of S Where: the source's output capacitance and the op amp's input , with the help of the op amp, convertscapacitance. R = op amp's input offset voltageVF OS I to a voltage. = op amp's input bias currentIS B At low frequencies, the op amp's inverting input is Select the feedback resistor value to give a high gain to must flowforced to be at ground potential and I S . Usually, this gain is high enough to use most of theI S. This combination of effects creates anthrough R F is at its extremeop amp's output voltage swing when I Soutput voltage of I R . S F values.