Timeline for Help with basic transimpedance amplifier circuit
Current License: CC BY-SA 3.0
19 events
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| May 8, 2017 at 3:42 | comment | added | DepressedRobot | I'm sorry, I posted the pic because I thought maybe I did some horrible connection. Here the link to the schematic (basically from wikipedia): img4.file-upload.com/i/00235/sg3pkb1a7giw.png Anyways, as you pointed out I shouldn't get positive values with that configuration..how come I'm actually reading something? How would I be able to tell if the thing is a phototransistor? It looks rather plain and simple to me (pretty much like a normal led...maybe its that?)... | |
| May 8, 2017 at 3:06 | comment | added | Kevin White | You need to learn to translate the physical arrangement in a schematic it is not easy to se what you have from a photo. It will help you as well as us to understand what you have. It looks like you just have the photodiode to +5 with the resistor to ground. I suspect it is a phototransistor, not photodiode that would account for the poor performance when used in photovoltaic mode. | |
| May 8, 2017 at 2:56 | comment | added | DepressedRobot | Ugh, you are right of course...the output should swing to negative..something the opAmp shouldn't be able to do in this connection. I have no idea what I'm reading then...0-1 when not illuminated, 200 (arduino values, so about 1 V) when illuminated. Here a link to the circuit: img1.file-upload.com/i/00235/84y2mtkv3hi5.png | |
| May 8, 2017 at 1:29 | comment | added | Kevin White | How does it work if you have the cathode to +5V? That would inject current into the opamp and would result in negative voltage out - since you have no negative supply that won't work and an Arduino can't take a negative input anyway. I don't think it is working how you think it is. Can you post a schematic? | |
| May 7, 2017 at 22:40 | comment | added | DepressedRobot | Before that, I had the photodiode hooked up in photoconductive mode with a 5kohm resistor, without any amplification. It worked fine most of the time, but last time I checked I got HUGE noise in my signal and have no idea why exactly, everything was like usual but I got heavy fluctuations in my ADC readings (instead of values around 250, for example, I got values between 200 and 300). That's when I decided that I should go into noise management (leading to the op-amp amplification on sensor end), because I can't allow the device to be so unpredictable. Thanks for the link, I'm gonna read it! | |
| May 7, 2017 at 22:37 | comment | added | DepressedRobot | @jonk At the moment things are hooked up in photoconductive mode, the cathode of the photodiode to +5 V and the anode to the inverting input of the op-amp. Truth is, after that adjustment it is working really fine and should do it for my purposes. I'm not expecting particular problems with noise, but I think it could be a good idea to take noise management into account anyways. | |
| May 7, 2017 at 6:20 | comment | added | jonk | @DepressedRobot Do a little reading here: hamamatsu.com/resources/pdf/ssd/e02_handbook_si_photodiode.pdf and also starting about 16 pages down at here: depts.washington.edu/mictech/optics/me557/detector.pdf | |
| May 7, 2017 at 6:02 | comment | added | jonk | @DepressedRobot I've little idea exactly how things are hooked up, to be honest. I get it that you are looking at ADC counts, though. Operate your diodes however works for you. Your needs are close to DC and well into the 1/f noise section. You might want to modulate to get out of that particular noise section, but I don't think you are ready to worry about that. I don't have a cap suggestion. But are you really having trouble with noise? Sounds like you are getting a nice large number with your light and a nice low number without it. Isn't that good enough? | |
| May 7, 2017 at 1:49 | comment | added | DepressedRobot | small update: I decided to just connect the diode in photoconductive mode...and the response now is much, much, much stronger. With the 1Mohm resistor I get nearly instant saturation at ambient light (about 900 on arduino), so I switched to a 5kOhm which gives about 2 at ambient light, but about 900 when the flashlight is shined on it. I think I might even go lower. A quick additional question: What capacitor would you advice to have as little noise as possible? I want to measure the angle of a moving pendulum (period about 900 ms), so what cutoff frecuency would be advisable? | |
| May 7, 2017 at 1:19 | comment | added | jonk | @WhatRoughBeast It was some of the most fun in my life, though, working on closed loop control of rapid thermal processing of wafers as well as general FAB stuff. I was observing photon flocking (related to Bose-Einstein condensates), which is another source of noise very few electronics folks have experience with. | |
| May 7, 2017 at 1:15 | comment | added | WhatRoughBeast | @jonk - Ah. My apologies. I keep forgetting about pesky real-world conditions. It's so much easier to assume op amps are ideal. | |
| May 7, 2017 at 1:12 | comment | added | jonk | @WhatRoughBeast They sure had to be accounted for in what I did. We might have \$500\:\mu\textrm{V}\$ across the poor thing. I even had to worry about the bulk package impedances (those supposedly infinite impedance IC epoxy packages) and instead got Hamamatsu to ship me detector dies -- which they told me was the first time they'd ever permitted that. Had fun with wire bonding. Was working at almost \$100\:\textrm{fA}\$ full scale levels. | |
| May 7, 2017 at 1:07 | comment | added | WhatRoughBeast | @jonk - You should keep in mind that photodiodes in photovoltaic mode don't have dark currents. | |
| May 7, 2017 at 0:39 | comment | added | DepressedRobot | I noticed, however, in previous tests that it reacts much less to light than other photodiodes I have. Those other photodiodes I already have included in my prototype, and the one I'm using here is for testing purposes. If it helps, I have a phototransistor with part number here (ST-KL3B) which yields similar results, but higher values (about 150, connected either way). I was using those phototransistors first, but decided to use photodiodes because I read that the response is more linear. | |
| May 7, 2017 at 0:36 | comment | added | DepressedRobot | Yes I understand the concept of offset and dark current. One question: Are you referring to the original picture I posted or to the recent comments? As Kevin White pointed out, it was first wrongly connected (I'm ashamed to say so). What I'm getting now is 0 values if not illuminated, and small values if illuminated with one bright white led, so it always reacts with positive rising values to light, but the magnitud of the reaction (20 or 40 on arduino scale) depends on the cathode orientation. I sadly have no part number for that diode and no part number is written on it. | |
| May 7, 2017 at 0:15 | comment | added | jonk | @DepressedRobot As a physics student I assume you understand the idea of \$y=m\left(\cdot x + b\right)\$. There will be leakage current (bias and offset) that must be accounted as part of your results. A small offset voltage will also exist across the diode, with associated dark current. That's \$b\$. Of course, \$m\$ is your resistor value, whatever it is. Also, if the current is positive, the output will move negatively in response to more light. But it's possible that \$b\$ might cause it to start out slightly positive without light. What is the photodiode part number? | |
| May 6, 2017 at 23:56 | comment | added | DepressedRobot | This seems quite weird to me, I was expecting bigger values, especially with 1Mohm connected to it. Also, it is weird to get a light-depending value independant on the cathode position. Is it actually amplifying anything? Can I just adjust the resistor value accordingly to get the full span? Will I not get into trouble when trying to measure the voltage on such a high impedance source (with arduino)? The light intensity I would use ranges from directly illuminated by 7 bright leds to nothing (depending on the angle between the polarizers.) | |
| May 6, 2017 at 23:51 | comment | added | DepressedRobot | thank you very much for the quick response. I just checked and it was really wired up wrong (first I was using another OP-Amp (the LM358P)where the GND was directly next to the non inverting output and wired it this way and didn't change the circuit correctly to the new opamp). Quite embarrassing. I did now connect it the right way, and first I thought it wasn't working (was showing pure zeroes), but then I changed the resistor to a 1Mohm resistor, and it shows small values when flashlight directly shined on it. (About 20 with cathode to non-inverting and about 45 when cathode to inverting | |
| May 6, 2017 at 23:34 | history | answered | Kevin White | CC BY-SA 3.0 |
