Photodiodes generate a current proportional to the light that strikes their active area. Most measurement applications involve using a transimpedance amplifier to convert the photodiode current into an output voltage. Figure 1 shows a simplified schematic of what the circuit could look like.
Figure 1. Simple Transimpedance Amplifier Circuit.
This circuit operates the photodiode in photovoltaic mode, where the op amp keeps the voltage across the photodiode at 0 V. This is the most common configuration for precision applications. The photodiode's voltage vs. current curve is very similar to that of a 'regular’ diode, with the exception that the entire curve will shift up or down as the light level changes. Figure 2a shows a typical photodiode transfer function. Figure 2b is a zoomed-in view of the transfer function, and it shows how a photodiode outputs a small current even if there is no light present. This ‘dark current’ increases with increasing reverse voltage across the photodiode. Most manufacturers specify photodiode dark current with a reverse voltage of 10mV.
Figure 2. Typical Photodiode Transfer Function
Current flows from cathode to anode when light strikes the photodiode’s active area. Ideally, all of the photodiode current flows through the feedback resistor of Figure 1, generating an output voltage equal to the photodiode current multiplied by the feedback resistor. The circuit is conceptually simple, but there are a few challenges you must address to get the best possible performance from your system.
The first challenge is to select an op amp with DC specifications that match your application’s requirements. Most precision applications will have low input offset voltage at the top of the list. The input offset voltage appears at the output of the amplifier, contributing to the overall system error, but in a photodiode amplifier, it generates additional error. The input offset voltage appears across the photodiode and causes increased dark current, which further increases the system offset error. You