Photodiode is one of the most commonly used sensor types in many optical measurements.。Such as absorption and emission spectra、Color measurement、Turbidity、Gas detection and other applications depend on photodiodes for precise optical measurement.。Photodiode generates current proportional to the amount of light illuminated to the active region。Transimpedance amplifiers are required for most measurement applications.，In order to convert photodiode current to output voltage。chart1Schematic diagram of display circuit。

chart1 Simple Transimpedance Amplifier Circuit

The photodiode of the circuit operates in photovoltaic mode，The Operational Amplifier keeps the voltage on the photodiode at zero.0 V。This is the most common configuration in precision applications.。The voltage-current curve of photodiode is very similar to that of conventional diode.，But the whole curve of the former will move up or down with the change of illumination level.。chart2aDisplay typical photodiode transfer function。chart2bIt’s an enlarged graph of the transfer function.，Show that even in the absence of light，Photodiodes also produce a small amount of current.。This dark current increases with the increase of the reverse voltage on the photodiode.。The reverse voltage of most manufacturers is10 mVThe dark current of photodiode is given under the premise。

chart2 Typical Photodiode Transfer Function

After illumination reaches the active region of the photodiode，Current flows from cathode to anode。Ideally，All photodiode currents flow through the diagram1Feedback resistance in，Generating a value equal to the photodiode current multiplied by the feedback voltage of the feedback resistance。The circuit is simple in principle.，However, some problems must be solved if the system is to have the best performance.。

DC considerations

The first difficulty is to select an operational amplifier that matches DC specifications for application requirements.。For most applications，Low input offset voltage is the most important specification。Input offset voltage exists at the output end of amplifier，The offset voltage will increase the total system error.；In photodiode amplifiers，It also produces other errors.。Input offset voltage on photodiode，Generating more dark currents，Further Increasing System Misalignment Error。Through software calibration、AC coupling——Or both.——Elimination of initial DC offset，But the larger misalignment error will narrow the dynamic range of the system.。Fortunately,，Input offset voltage in several hundredmVEven dozens ofmVWithin the scope of，There are a large number of operational amplifiers to choose from.。The second important DC specification is the input leakage current of the operational amplifier.。Current enters the input of operational amplifier，Or go anywhere other than the feedback resistor，There will be measurement errors.。Operational amplifiers with zero input bias currents do not exist，But someCMOSorJFETThe input operational amplifier is very close to this value.。FETThe input bias current of the input amplifier increases exponentially with the increase of temperature.。Many operational amplifiers provide85°Cor125°CSpecifications below；But if not provided，A better approximation isTen degrees per rise in temperature，The current doubles.。

Another challenge is to design circuits and lay them out.，So as to minimize the external leakage current path——Leakage Current Affects the Performance of Low Input Bias Current Operational Amplifier。The most commonly used external leakage current path is the printed circuit board itself.。for example，chart3Display map1A Feasible Layout of Medium Photodiode Amplifier。Pink line representation+5 VPower supply rail，Power the amplifier and transfer the power to the rest of the circuit board。If in+5 VThe resistance between the line and the line carrying the photodiode current is equal to5 G(chart3Medium toRLExpress)，that1 nACurrent will flow from+5 VWire-in amplifier。Obviously，This is carefully selected with application1 pAOperational Amplifier Target Violation。One way to minimize the external leakage current path is to increase the resistance between the line carrying the photodiode current and any other line.。This may be as convenient as adding a larger route forbidden area around the route to increase the distance between other routes.。In some extreme applications，Some engineers will cancel them altogether.PCBRouting，Exposure of photodiode lead to air and direct connection with input pin of operational amplifier。

chart3 Photodiode Layout with Leakage Current Path

Another way to prevent external leakage current is to lay a protective line beside the line carrying the photodiode current.，And ensure that both lines are driven to the same voltage。chart4Display protective routes around the network carrying photodiode currents。+5 VLeakage current generated by routing is then passed throughRLInlet protection line，Not an inflow amplifier。In this circuit，The differential pressure between the protective and input routes is only related to the input offset voltage of the operational amplifier.——This is another reason why low input offset voltage amplifiers are chosen.。

chart4 Reducing external leakage current by using protective wiring

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