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Photodiode is one of the most common sensor types used in many optical measurements. Applications such as absorption and emission spectroscopy, color measurement, turbidity, and gas detection rely on photodiodes for precision optical measurements. Photodiodes produce an electric current proportional to the amount of light hitting the active region. Transresistance amplifiers are required for most measurement applications to convert photodiode current to output voltage. Figure 1 shows the schematic diagram of the circuit.

 

Figure 1. Simple transresistance amplifier circuit

The photodiode of the circuit operates in photovoltaic mode, where the operational amplifier maintains a voltage of 0 V across the photodiode. This is the most common configuration in precision applications. The voltage and current curve of a photodiode is very similar to that of a conventional diode, but the whole curve of the former will shift up or down as the light level changes. Figure 2A shows a typical photodiode transfer function. Figure 2b shows an enlarged version of the transfer function, showing that the photodiode produces a small amount of current even in the absence of light. This dark current will rise as the reverse voltage on the photodiode increases. Most manufacturers give the dark current of photodiodes at a reverse voltage of 10 mV.

 

FIG. 2 Typical photodiode transfer function

After light hits the active region of the photodiode, current flows from the cathode to the anode. Ideally, all of the photodiode current flows through the feedback resistor in Figure 1, producing a value equal to the photodiode current multiplied by the feedback voltage of the feedback resistor. The circuit is simple in principle, but some difficult problems must be solved if the system is to have the best performance.

Dc considerations

The first challenge is to select operational amplifiers whose DC specifications match the application requirements. Low input offset voltage is the most important specification for most applications. There is input misalignment voltage at the output end of the amplifier, which will increase the total error of the system. In photodiode amplifiers, it can also produce other errors. There is an input offset voltage on the photodiode, which generates more dark current and further increases the system offset error. Eliminate the initial DC misalignment by software calibration, AC coupling, or both, but large misalignment errors reduce the dynamic range of the system. Fortunately, there are plenty of operational amplifiers to choose from in the hundreds or even tens of mV range of input offset voltages. The second most important DC specification is the input leakage current of the operational amplifier. Current entering the operational amplifier input, or anywhere other than the feedback resistor, can cause measurement errors. Operational amplifiers with zero input bias current do not exist, but some CMOS or JFET input operational amplifiers come very close to this value. The input bias current of the FET input amplifier increases exponentially with increasing temperature. Many operational amplifiers are available at 85°C or 125°C; But if it is not provided, a good approximation is that the current doubles for every ten degree increase in temperature.

 

Another challenge is to design and lay out the circuits so as to minimize external leakage current paths — leakage currents that can affect low input bias current op amp performance. The most common external leakage 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 supply rail that supplies power to the amplifier and to the rest of the board. If the resistance between the +5 V wiring and the wiring with photodiode current is equal to 5 G(represented as RL in Figure 3), 1 nA current will flow from the +5 V wiring into the amplifier.Obviously, this is contrary to the goal of careful selection of 1 pA operational amplifiers in applications. 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 could be as convenient as adding a large routing exclusion zone around a route to increase distance from other routes. In extreme applications, some engineers may eliminate PCB wiring altogether, exposing the photodiode leads to the air and connecting them directly to the op-amp input pins.

 

FIG. 3 Layout of photodiode with leakage current path

Another way to prevent external leakage current is to place a guard wire next to the wire carrying photodiode current and ensure that both wires 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 protection wiring through the RL rather than into the amplifier. In this circuit, the differential pressure between the guard line and the input line is related only to the operational amplifier’s input offset voltage — another reason why low input offset amplifiers are used.

 

Figure 4. Use protective wiring to reduce external leakage current