# Photodiode falltime problem

Discussion in 'LEDs and Optoelectronics' started by pooya_BA, Mar 24, 2021.

1. ### pooya_BA

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0
Mar 24, 2021
Hello
I have a problem with the photodiode. I bought the SFH 2704 photodiode and connected it directly to the oscilloscope circuit without an electronic circuit.
The fall time of this very, very long piece was about 380 microseconds, while in the information of this piece, the fall time was 67 nanoseconds.
What do you think is the problem? And how can I solve this problem?

The diagram below shows the output of the photodiode with a pulse of 160 microseconds.
This pulse is taken with a commercial PDA10A2 photodiode and is exactly equal to the pulse, meaning that the light source does not have a short time.

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2. ### Nanren888

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146
Nov 8, 2015
What was the circuit that the PDA10a2 was in?
If the scope input is high impedance, then the charge on the diode junction has no path to go anywhere.
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I vaguely remember circuits designed to keep the diode voltage at zero to minimse the effects of any junction capacitance. Probably short of that sort of thing, you might just provide a discharge path, loosing a little sensitivity for speed.
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Any app notes or reference designs for that part?

https://www.eotech.com/194/applicat...es-are-different-in-photodetector-measurement

Last edited: Mar 24, 2021
3. ### Harald KappModeratorModerator

11,644
2,691
Nov 17, 2011
Look at the test conditions in the datasheet:

The times given are measured with a load impedance of 50 Ω. Your scope would have 1 MΩ or even 10 MΩ, depending on the probe.
If your scope has that option, set it to 50 Ω impedance and use a direct BNC cable, no probe.

As @Nanren888 stated: usually photodiodes are operated with a load impedance near 0 Ω by using a transconductance amplifier. See e.g. the discussion here.

4. ### pooya_BA

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Mar 24, 2021
Thankful
I do not know much about PDA circuitry. This is a commercial photodiode and not much is available.

5. ### pooya_BA

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Mar 24, 2021
Thank you very much

6. ### bertusModerator

2,022
756
Nov 8, 2019
Hello,

FYI, there is a long thread on All About Circuits from the same user:
Circuit design of a photodiode
There is already a lot of information given.
We do not have to invent the wheel twice.

Bertus

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4
0
Mar 24, 2021
Thank you

8. ### kpatz

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Feb 24, 2014
There's two ways to use a photodiode. One is photovoltaic mode, where it acts like a tiny solar panel, generating a voltage when light hits it. The diode will have a slower response when used in this manner.

The other way is to reverse bias the diode, and detect current that flows when light hits it. This requires more circuitry, but allows the diode to react much faster as the capacitance is no longer a factor.

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