Possible Interference among biased photodiodes - Page 1
Forum section: electronics / beginners

author: LoveLaika
author_rank: Frequent Contributor
timestamp: January 17, 2025, 04:46:16 pm
content: For a photodiode to properly work, there must be a bias voltage. However, when you bias multiple photodiodes with the same bias voltage, there's a chance that they can interfere with one another through the current that they generate. I'm looking at a solution of trying to bias them separately through the use of linear regulators, but could they still interfere with one another via the shared common ground node?Take the example schematic mockup below. Rather than biasing the photodiodes together, they each have their own bias source via a linear regulator. Each regulator takes 8 volts as an input and outputs 5 volts that bias the respective photodiode. Assuming that the regulator can handle the current of the photodiode when active, I don't believe the bias voltage will change. However, as it is now, could one photodiode still interfere with the other when active?

author: RoGeorge
author_rank: Super Contributor
timestamp: January 17, 2025, 04:57:52 pm
content: If you add a series resistor with each photodiode, they do not interfere with each other. The series resistor is way larger than the internal resistance of the power supply. Any signal produced by an illuminated photodiode would go to the power supply, and the power supply will sink any current variations.One regulator for all (not for each) should be enough.What exactly are the photodides detecting, and how many are in total?

author: Andree Henkel
author_rank: Regular Contributor
timestamp: January 17, 2025, 05:18:06 pm
content: best is one regulator, and for each diode individually one series resistor somewhere 10R..47R + suitable sized cap, somewhere 100n...10µLook at the diode datasheets there should be plots showing dependency on bias voltageyou can calulate the variation in bias voltage by multiplying the series resistor with the photocurrent

author: tggzzz
author_rank: Super Contributor
timestamp: January 17, 2025, 05:42:57 pm
content: Quote from: LoveLaika on January 17, 2025, 04:46:16 pmUnanswerable unless you specify the voltages and currents and speeds.There may be a "common shared" node, but attaching "ground" to that indicates a lack of understanding. Start by considering what "ground" would mean in the context of battery powered equipment or airborne equipment. Then move onto understanding EMI and EMC and conductors that act as antennas.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: January 17, 2025, 05:47:08 pm
content: Thanks for your reply. In my example, they'd be detecting light, say from two different possible sources as part of some makeshift alarm system. This is all just a proof of concept.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: January 17, 2025, 05:59:50 pm
content: Quote from: tggzzz on January 17, 2025, 05:42:57 pmIn the case of a battery, it'd be the return path to the power source, labeled as ground, considered to be the voltage reference (0 volts) to other points in the circuit. Perhaps this could have been more elegant, but this seems to sum it up. Assuming no other points are at a lower potential than ground in the circuit, there (theoretically) shouldn't be a way for the current flow induced by the photodiode to go to each other. I ask this question in case there might be some other scenario that I haven't thought of and wanted to get advice from others wiser than I am.

author: tggzzz
author_rank: Super Contributor
timestamp: January 17, 2025, 06:12:43 pm
content: Quote from: LoveLaika on January 17, 2025, 05:47:08 pmSo mA, Hz.You need to understand how transimpedance amplifiers are used in applications like this. There are many references easily found by a quick search. TAoE is, as usual, excellent.You might also need to understand how to deal with a reverse biassed PIN photodiode's "dark current" and ambient light.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 18, 2025, 01:12:08 pm
content: Thats a an unusual way to set up your PDs.In photoconductive mode the diode current will usually be measured with a transimpedance amp.(There are many design tools for these amplifiers on line, eg.https://tools.analog.com/en/photodiode)You can bias as many PDs as you like without having any interaction with a single regulated voltage.The currents are very small and will not create discernable voltage drops on a pcb trace or a wire.