Improving Photodiode Circuit Performance: Understanding Slow Rise Time Issues

In summary, the conversation discusses a photodiode circuit and its slow rise time. One person suggests using a smaller feedback resistance and reverse biasing the diode to increase the speed. The circuit in question has a 1M feedback resistor and no biasing for the photodiode. The person also suggests considering the input capacitance of the opamp and PCB parasitic trace capacitance in the circuit's speed.
  • #1
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Hi,

I have a photodiode circuit (attached), where a photodiode is hooked up to a current to voltage opamp connected to a voltage divider. This circuit works as intended: when there is light, Vo ~ 3.3V, and when there is no light, Vo ~ 0.3V. However, when I measured the rise time of this circuit, I found out that it takes ~130ms to switch, which is much slower than I anticipated. The photodiode's spec says that its rise and fall time is 200us, and although I am not in the same operating condition, I expect the delay to be in the same order of magnitude, give or take. Therefore, if any1 has any idea on this, please let me know.

Thanks.

[opamp] http://focus.ti.com/lit/ds/symlink/opa657.pdf
[photodiode] http://catalog.osram-os.com/catalogue/catalogue.do;jsessionid=D791D4FBE4D533287FAB36C83ADE1303?act=downloadFile&favOid=020000030000c448000100b6
 

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  • #2
Is that 1M for the feedback resistor - that's rather large.
You haven't show the bias supply for the photodiode, are you just using the opamp input to bias it?
 
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  • #3
Yes, that's 1M ohm. The reason why it's made large is because I wanted the output voltage BEFORE the voltage divider to be 5V (saturating the opamp) and then step it down to 3.3V AFTER the voltage divider. In fact, based on my calculation, the current generated by the photodiode is ~5uA. So to get 5V, I did, 5V = 5u*1M.

It should be ok even if the Rf is large because it will simply saturate the opamp and get 5V as mentioned before.

As for biasing, you are correct and I didn't bias the photodiode. I didn't realize you have to. I saw a proposed design from Sharp that used a simliar circuit so I thought it was fine. It's on page 3 Figure 6 of the link below

http://vorlon.case.edu/~flm/eecs245/Datasheets/Sharp%20photodevices.pdf
 
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  • #4
To speed up your photodetector I --> V circuit, you need to reverse bias the diode (to lower its capacitance) and lower the feedback resistance (to speed up the associated RC time constant). No circuit with a 1Meg resistor in it is going to be fast. Don't forget to include the input capacitance of the opamp and the PCB parasitic trace capacitance in your RC calculations.
 
  • #5
Thanks for your suggestion. I will try them out.
 

1. What is a slow photodiode circuit?

A slow photodiode circuit is a type of electronic circuit that utilizes a photodiode, which is a semiconductor device that converts light into an electrical current, to detect and measure light intensity. The circuit is designed to have a slow response time, typically ranging from milliseconds to seconds, making it suitable for applications that require low-speed detection and measurement of light.

2. How does a slow photodiode circuit work?

A slow photodiode circuit works by converting light energy into an electrical current through the photodiode. The current is then amplified and converted into a measurable output, such as voltage or current, by the circuit components. The slow response time of the circuit is achieved by using components with low bandwidth, such as resistors and capacitors, to limit the speed of the signal.

3. What are the applications of a slow photodiode circuit?

Slow photodiode circuits are commonly used in applications that require low-speed detection and measurement of light, such as in photometry, colorimetry, and spectroscopy. They are also used in scientific experiments and research to study light-sensitive processes, such as photosynthesis, bioluminescence, and photoreception in organisms.

4. What are the advantages of using a slow photodiode circuit?

The main advantage of using a slow photodiode circuit is its ability to accurately detect and measure low levels of light with a slow response time. This makes it suitable for applications that require precise and stable measurements, such as in scientific research. Slow photodiode circuits are also relatively simple and inexpensive to construct compared to other types of light detection circuits.

5. What are the limitations of a slow photodiode circuit?

One of the main limitations of a slow photodiode circuit is its slow response time, which makes it unsuitable for applications that require fast detection and measurement of light. The circuit is also sensitive to external factors, such as temperature and ambient light, which can affect the accuracy of the measurements. Additionally, the circuit may require frequent calibration to maintain its accuracy over time.

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