Optimisation of photodiode sensor circuit design
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 1: Simple transimpedance amplifier circuit.
Figure 2: Typical photodiode transfer function.
Current flows from cathode to anode when light strikes the photodiodes 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 applications 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 can remove the initial DC offset through software calibration, AC-coupling, or a combination of both, but having large offset errors decreases the systems dynamic range. Fortunately, there is a wide selection of op amps with input offset voltage in the hundreds or even tens of microvolts.
The next important DC specification is the op amps input-leakage current. Any current that goes into the input of the op amp, or anywhere else other than through the feedback resistor, results in measurement errors. There are no op amps with zero input bias current, but some CMOS or JFET-input op amps get close. For example, the AD8615 has maximum input bias current of 1pA at room temperature. The classic AD549 has a maximum input bias current of 60fA that is guaranteed and production tested. The input bias current of FET-input amplifiers increases exponentially as temperature rises. Many op amps include specifications at 85C or 125C, but for those that do not, a good approximation is that the current will double for every ten degrees of temperature increase.
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