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Photodiode is one of the most commonly used sensor types in many optical measurements. Applications such as absorption and emission spectra, color measurements, turbidity, and gas detection rely on photodiodes for precise optical measurements. Photodiodes generate an electric current proportional to the amount of light reaching the active region. Most measurement applications require transresistance amplifiers to convert photodiode current into output voltage.
Figure 1 shows the schematic diagram of the circuit.
Figure 1 simple transresistance amplifier circuit
The circuit’s photodiode operates in photovoltaic mode, where the operational amplifier maintains a voltage of 0 V on the photodiode. This is the most common configuration in precision applications. The voltage and current curve of photodiodes is very similar to that of conventional diodes, but the entire curve of photodiodes shifts up or down as the light level changes. Figure 2a shows a typical photodiode transfer function. Figure 2b is an enlarged version of the transfer function, showing that photodiodes output a small amount of current even in the absence of light. This dark current increases as the reverse voltage on the photodiode increases. Most manufacturers give the dark current of the photodiode at a reverse voltage of 10 mV.
Figure 2. Typical photodiode transfer function
When the light strikes the active region of the photodiode, the current flows from the cathode to the anode. Ideally, all photodiode current flows through the feedback resistor in figure 1, producing a value equal to the photodiode current times the feedback voltage of the feedback resistor. The circuit is simple in principle, but some problems must be solved if the system is to perform optimally.
The first challenge is to select the operational amplifier required for the dc specification matching application. For most applications, low input offset voltage is the most important specification. The input maladjusted voltage at the output end of the amplifier will increase the total error of the system. In photodiode amplifiers, it also produces other errors. There is an input misaligned voltage on the photodiode, which generates more dark current and further increases the misaligned error of the system. Initial dc misalignment is eliminated by software calibration, ac coupling — or both — but large misalignment errors can narrow the dynamic range of the system. Fortunately, with input misaligned voltages in the hundreds or even tens of MMV range, there are plenty of operational amplifiers to choose from. The second most important dc specification is the input leakage current of the operational amplifier. Measurement errors occur when the current enters the input end of the operational amplifier, or anywhere other than the feedback resistance. No operational amplifiers with zero input bias current exist, but some CMOS or JFET input operational amplifiers are very close to this value. The input bias current of the FET input amplifier increases exponentially with temperature. Many operational amplifiers are available at 85°C or 125°C. But if not, a better approximation would be that the current doubles for every ten degree increase in temperature.
Another challenge is to design and layout the circuit to minimize the external leakage current path — which can affect the performance of low-input bias current operational amplifiers. The most common external drain path is the printed circuit board itself. For example, figure 3 shows a possible layout of the photodiode amplifier in figure 1. The pink wiring represents the +5 V power rail that powers the amplifier and transmits the power to the rest of the circuit board. If the resistance between the + 5v wiring and the wiring carrying the photodiode current is equal to 5 G(shown in figure 3 as RL), then 1 nA current will flow from the + 5v wiring into the amplifier. Obviously, this is contrary to the objective of carefully selecting a 1 pA operational amplifier in the application. One way to minimize the path of external drain current is to increase the resistance between the conductor carrying the photodiode current and any other conductor. This may be as convenient as adding a large routing exclusion zone around the walkway to increase the distance from other walkways. In some extreme applications, engineers cancel PCB wiring altogether, exposing photodiode leads to air and connecting them directly to the input pins of the op amp.
Photodiode layout with leakage current path
Another way to prevent external leakage is to place a protective cable next to the one carrying the photodiode current and ensure that both cables are driven to the same voltage. Figure 4 shows the protection wiring around a network with photodiode current. The leakage current generated by +5 V wiring then flows into the guard wiring through RL, not into the amplifier. In this circuit, the voltage difference between the protection circuit and the input circuit is only related to the input misaligned voltage of the op amplifier — which is another reason to choose a low input misaligned voltage amplifier.
Use protective wiring to reduce external leakage current