Position sensitive devices (PSD) - Page 1
Forum section: electronics / projects

author: jimon
author_rank: Regular Contributor
timestamp: June 15, 2016, 08:48:11 pm
content: There is very little information about actual practical usage of this devices on internet, so I'll try to collect my little experience here.So for intro: PSD is a sensor that can measure position of a light spot on its surface, and it's fully analogue and kinda precise: for example S3932 is a 1D PSD with working area of 1x12mm, and positional error is +-60 micrometers with resolution of 0.3 micrometers! Mainly this devices are used in academia labs for laser alignments and it will look something like this:So what is PSD electrically? Well it's a diodebut with two (or more) anodes, position is determinated by difference of current between anodes.If you will google how to use PSD you will probably find something like this (old Ontrak diagram):And 2D PSD version (from Teledyne Judson):As we can see the basic usage is quite simple and straightforwardThe only troubles is analogue division part which is a bit tricky to get, Analog Devices have a list of their analogue multipliers/dividers herehttp://www.analog.com/en/products/linear-products/analog-multipliers-dividers.htmlbut the selection is quite limited and some chips are just obscure or expensive.... Will continue ...

author: jimon
author_rank: Regular Contributor
timestamp: June 15, 2016, 08:51:30 pm
content: What I was curious is that Hamamatsu is selling"Signal processing circuit for 1-D PSD C3683-02"with only two chips on it (and one clearly says DO NOT TOUCH)They have 18 pins device in ceramic package, which kinda limits options only toAD538which is "Real-Time Analog Computational Unit (ACU)" and its transfer function is V = Vy(Vz/Vx)^m.Just look on this beast, feel the smell of military-ish productThe only problem with it - 53.44$ per piece / 39.8$ for 2.5k ondigikey.Sure it's a nice thing to use if you have money for it, but if we want to optimize cost we should look for cheaper solution.One very straightforward thing to use here - one IC with 4 op-amps in it, and cheaper divider solution, the most straightforward isAD633(around 5.77/piece 5.38/1k) orAD734(around 29.7$/piece or 20.0$/1k).... Will continue ...

author: jimon
author_rank: Regular Contributor
timestamp: June 15, 2016, 08:57:21 pm
content: So currently I made a prototype with S3932 + OPA4134 + AD734 on a breadboard and it seems to work just fine (a little bit noisy in low light though).It still two IC solution and we have one op-amp for spare! Though one huge minus of AD734 - it's only available in DIP packageSo I ordered AD633 to try it out as well, but it requires additional op-amp to make division circuit, will see if it worksBecause I'm a noob-ish I have some questions:- Will 100nf caps be enough to decouple both IC's?- Any tips what to improve to make the schematics look better in KiCad?- Any obvious problems that I might encounter with all that?

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 16, 2016, 02:15:59 am
content: Any reason to not just digitize the signals and do the calculations digitally?

author: jacobfeder
author_rank: Contributor
timestamp: June 16, 2016, 04:41:30 am
content: Funny I was very recently looking into using one of these devices but decided to change directions on the project. I think I'm with NiHaoMike — either feed the values directly into an ADC after your transimpedance amp or just after the addition/subtraction operations. No need to do an analog division.Here is the device I was going to use (this one is 4 sector for full 2D nulling):http://www.edmundoptics.com/testing-detection/detectors/segmented-photodiodes/84616/EO didn't provide a (useful) datasheet for the diode but I managed to find it from the original manufacturer:http://www.osioptoelectronics.com/Libraries/Product-Data-Sheets/Quadrant-Bi-Cell-Photodiode.sflb.ashxAnd the ADC:http://www.ti.com/lit/ds/symlink/adc124s021.pdfI also found a really great AD article about pcb layout for photodiodes. Attached.

author: jimon
author_rank: Regular Contributor
timestamp: June 16, 2016, 07:31:41 am
content: NiHaoMike, jacobfeder I think digital is fine, though depending on application you might need to get better ADC: let's say S3932 have 12mm surface with 0.3 micrometers precision, which will be 12'000 / 0.3 = 40'000. 40'000 count ADC is 16 bit, which is ok. I guess 12bit will be enough for most application thoughI'm trying out analogue approach mostly for fun/experience, and partially because I want to try to use PSD and audio pickup head from analogue source, apparently it's not "nice" to digitize signals in modern audio equipment, because people want to use their 10k$ DAC's

author: DTJ
author_rank: Super Contributor
timestamp: June 16, 2016, 11:49:52 am
content: I worked on one many years ago for tracking the position of a beam steering lens. It was a 2D system and we used a piece of OF mechanically linked to the moving mechanical parts to project a dot onto the 2D photodiode. I vaguely remember using op-amps to do the division. It worked ok.edit - I also remember seeing a 1d sensor used for atomized particle sizing.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 16, 2016, 11:56:57 am
content: Nowadays you could probably do it cheaper with an image sensor and software processing.