Introduction to Photosensitive Integrated Circuits
This article continues our introductory discussion of light-sensitive components that produce higher output-current amplitude than photodiodes.
The boiled-down conclusion of the previous article on phototransistors is the following: if speed or linearity is an important performance spec in your application, you’ll probably opt for a photodiode over a phototransistor. If speed and linearity are not particularly important and a phototransistor’s inherent gain allows you to achieve significant reductions in cost, complexity, or board space, you should consider a phototransistor.
It turns out, though, that this is not an either/or decision. There’s a third option. I refer to this group of components as “photosensitive integrated circuits.” When I use this term, I’m referring primarily to devices that Hamamatsu calls “photo IC diodes,” but it also encapsulates any other component that integrates a photodiode and an amplifier into the same package.
Photodiode Plus TIA
There’s not a lot to say about this first type of photosensitive integrated circuit. In this age of exceedingly complex mixed-signal ICs, it’s no surprise that engineers figured out a way to combine a photodiode and a transimpedance amplifier (TIA) into a single component.
The best way to learn about these components is to read the datasheets of whatever parts appeal to you. For example, the OPT301 from Texas Instruments has a TIA gain of 120 dB, a bandwidth of 4 kHz, and a zero-bias-mode photodiode that is sensitive to visible, UV, and infrared light.
Figure 1. This diagram shows the internal architecture of the OPT301 photosensitive IC.
The MLX75305 from Melexis appears to employ photoconductive mode, and it incorporates additional output circuitry.
Figure 2. This is the internal architecture of the Melexis MLX75305 “light-to-voltage SensorEyeC.” That name is just a bit too clever for my liking.
An example of a more exotic part in this category is the ADN3010-11 from Analog Devices. It has a germanium photodiode, incorporates a limiting amplifier in addition to a transimpedance amplifier, and is intended for optical data transmission at speeds up to 11.3 Gbps.
Figure 4. The ADN3010-11 is designed for on/off light detection and produces a differential output.
Photo IC Diodes
As mentioned above, I’m using Hamamatsu’s terminology here. A photo IC diode differs from both a phototransistor-based amplifier and a photodiode-plus-TIA component in that it does not convert the photocurrent to a voltage.
The output of a photo IC diode is a current, and this current can be used in essentially the same way as the photocurrent from a normal photodiode. The difference is that the current is much larger, because the device incorporates a high-gain current amplifier. Thus, photodiodes overcome the primary complaint about photodiodes—that they produce extremely small photocurrents—without forcing the designer to switch over to a phototransistor.
The following diagram shows the internal structure and the circuit implementation of a Hamamatsu photo IC diode.
Figure 5. Note that the photodiode in this device has a reverse bias and therefore is operating in photoconductive mode. The diagram was taken from this Hamamatsu app note.
As you can see in the diagram, the photo IC diode gives you a gain of 1300 A/A, and Hamamatsu’s higher-gain parts provide 30,000 A/A. Increasing photocurrent amplitude by a factor of 30,000 will make the output signal much easier to work with.
Another benefit of photo IC diodes is their ability to include a second photodiode that can compensate for offsets introduced by sensitivity to wavelengths in the near-infrared region. By subtracting a signal generated from a photodiode that responds only to near IR, the device provides a spectral response that is restricted primarily to visible wavelengths.
Figure 6. This diagram, from the S10604-200CT datasheet, shows a second photodiode that allows the device to automatically compensate for near-IR sensitivity.
Generating a Voltage Signal
As shown in the two preceding diagrams, you don’t need a TIA to generate a voltage signal from the photo IC diode’s output current. The amplifier produces a current signal that can be effectively converted into a usable voltage signal with nothing more than a resistor. Though I’m not opposed to designing TIAs, it’s hard to argue with the simplicity and convenience of a single resistor.
The output capacitor shown in the diagrams (connected by dashed lines) isn’t essential, but it’s recommended as a means of suppressing undesirable high-frequency components in the output signal.
My impression is that photodiode-plus-TIA components are not very popular, and I admit that I wouldn’t be inclined to use one over a discrete diode with a custom-designed TIA.
I find the photo IC diodes much more appealing. In this article, I focused on the Hamamatsu devices, but there must be at least a few other companies that make them. The only other manufacturer that I came across as I was writing this article was ROHM, which sells a similar device and calls it an “analog current output type ambient light sensor IC.” If you have any experience with photo IC diodes or can recommend other vendors, feel free to share your knowledge in the comments section.