Photodiode circuit design - Page 1
Forum section: electronics / beginners

author: stcoso
author_rank: Regular Contributor
timestamp: March 29, 2019, 11:37:46 am
content: Hi.. I'm trying to design a little photodiode amplifier with some parts that i have at hand... i attached some schematic.I have a problem... the amplifier works opposite as i would expect... Basically the output voltage goes down when i shine some light on it.I'm using a BPV10 pin photodiode and a ada4857 op amp.Thank you in advance.

author: StillTrying
author_rank: Super Contributor
timestamp: March 29, 2019, 11:53:48 am
content: PD is in the Power Down direction.

author: stcoso
author_rank: Regular Contributor
timestamp: March 29, 2019, 11:56:36 am
content: Oh f**k

author: janoc
author_rank: Super Contributor
timestamp: March 29, 2019, 02:53:36 pm
content: Quote from: stcoso on March 29, 2019, 11:37:46 amUm, and what exactly are you expecting to happen?The photodiode will generate current into theinvertinginput of the amplifier (making it "positive"), so the opamp will use its output to bring that node through the feedback resistor back to matching the non-inverting input (the inverting input behaves as a virtual ground here). The output voltage will go down because that's what is needed to compensate the photodiode current out.

author: mikerj
author_rank: Super Contributor
timestamp: March 29, 2019, 03:30:07 pm
content: Quote from: janoc on March 29, 2019, 02:53:36 pmThe photodiode in the schematic is reverse biased (since the non-inverting input is biased at +5v) and operates in photoconductive mode. More light will cause the diode to sink more current, and the output voltage of the op-amp will increase to keep the inverting input at the same voltage as the non-inverting.As StillTrying mentioned, the power down pin is connected to the wrong rail.

author: janoc
author_rank: Super Contributor
timestamp: March 29, 2019, 09:14:13 pm
content: Quote from: mikerj on March 29, 2019, 03:30:07 pmThat would mean the opamp is powered down - I somehow don't expect a powered down opamp to keep working as an actual opamp. The datasheet only mentions the output not being in high impedance mode.You are right about the photodiode - I have missed the reverse biasing because what he has there is a normal current-to-voltage converter configuration (transimpedance amplifier). But in that case the diode should have been reversed and cathode connected not to ground but to +5V (middle of that voltage divider) in order to not have external bias on it.

author: StillTrying
author_rank: Super Contributor
timestamp: March 30, 2019, 12:01:24 pm
content: Quote from: janoc on March 29, 2019, 09:14:13 pmIt's not! It's allowing the photo current topull downthe voltage at -IN and (FB,OUT). When powered the (FB,OUT) voltage willRiseto cancel the photo current's pull down of -IN.

author: stcoso
author_rank: Regular Contributor
timestamp: March 30, 2019, 05:15:33 pm
content: Ok, so.. i've attached a new schematic. I've added a second stage (same op amp) and while it works ok (40ns rise time) I'm still not happy enough.Do you guys think i can go a little bit faster with this same components?

author: StillTrying
author_rank: Super Contributor
timestamp: March 30, 2019, 06:28:09 pm
content: 40ns is good. What are you using for the light source?The fastest 5mm LED received light rise and falls times I've got is 70 - 75ns.https://www.eevblog.com/forum/chat/20w-halogen-bulb-viewed-by-a-photodiode/msg1751210/#msg1751210

author: stcoso
author_rank: Regular Contributor
timestamp: March 30, 2019, 06:59:16 pm
content: I'm using a 5mm red LED driven by a signal generator for testing.

author: StillTrying
author_rank: Super Contributor
timestamp: March 30, 2019, 07:40:00 pm
content: I've found the light shape from reds to be slower and curved, and super bright green and blue the fastest and squarest.I won't be trying to get the fastest speeds out of the green and blues because I've got a 100W white COB waiting for >>100W testing!

author: jlmoon
author_rank: Unknown
timestamp: March 30, 2019, 07:55:52 pm
content: Just a curious question, how come you're setting the gain on 2nd op amp to 6.8? Wouldn't it be ok to set that to unity or 1? Using the 2nd amp as a follower or buffer only?

author: stcoso
author_rank: Regular Contributor
timestamp: March 31, 2019, 12:23:00 am
content: Quote from: StillTrying on March 30, 2019, 07:40:00 pmI've tried also with some green led and found them slower. I used an IR led and it was the fastest.I'm trying to build this little probe because i need it to measure a UV source (20W 365nm) that i've already built and rise/fall time are really important.Isupposewhite LED are really bad (microseconds?) at turning on/off because of phosphorescence (turn off especially).

author: stcoso
author_rank: Regular Contributor
timestamp: March 31, 2019, 12:35:56 am
content: Quote from: jlmoon on March 30, 2019, 07:55:52 pmShort and thruthful answer: basically i don't know what i'm doing... but, as i (maybe wrongfully) understood to achieve higher bandwidth without compromising stability and noise i needed lower gain in the first stage.

author: StillTrying
author_rank: Super Contributor
timestamp: March 31, 2019, 01:13:38 am
content: "white LED are really bad (microseconds?) at turning on/off because of phosphorescence (turn off especially)."As long as they're not warm white, white LEDs are faster than that ~160ns.The last 2 pics in my link above reply#8 are RedGreenWhite5mm.gif andUV+Blue LED.gif The UV edges are fast."Short and thruthful answer: basically i don't know what i'm doing"Join the club!

author: jlmoon
author_rank: Unknown
timestamp: March 31, 2019, 04:34:41 am
content: Join our club!

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2019, 08:40:15 am
content: Quote from: stcoso on March 31, 2019, 12:23:00 amI've tested a variety of LEDs and had mixed results.White is interesting. The initial turn off is very fast. There is some afterglow, but it's very weak, several orders of magnitude dimmer than when driven at the rated current, so it's not a problem, Even the warm white Cree CMA3090 I tested was fast enough for use in a 1µs strobe application.Some phosphor converted LEDs are slow. The amber one I tested was extremely slow. I can't remember the exact part number. It was a Lumiled/LUXEON brand.https://www.lumileds.com/uploads/161/DS58-pdfhttps://www.luxeonstar.com/assets/downloads/ds62.pdfHow bright is the source shining on the photodiode? If it's bright, you need a large revere bias voltage to prevent saturation, as well as speed it up. At very high intensities, no amplification is required. As you can see from the above schematic, I connected the photodiode directly to the 50Ohm input of an oscilloscope and applied a bias voltage to it.

author: stcoso
author_rank: Regular Contributor
timestamp: March 31, 2019, 09:28:07 am
content: My source is 6/7W of radiated power over a 25cm^2 area but it's 365nm. BPV10 sensitivity at that wavelength is about one tenth of the sensitivity at 900nm. So I decided to dick around with op-amps

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2019, 09:47:00 am
content: Quote from: stcoso on March 31, 2019, 09:28:07 amThat's quite intense, don't look at the beam and consider UV googles.Yes the sensitivity of photodiodes declines fairly linearly past the their peak response. One reason is because past the a peak sensitivity the current is proportional to the number of photons per second and UV photons have much more energy, than IR photons, so the current vs radiant power is much less. Another is not all photons hitting the photodiode result in current flow, some ware absorbed and reflected, which also gets worst, with increasing energy.At high intensities photodiodes start to saturate. An increase in intensity no longer causes the current to increase and there's a storage time i.e. a delay in it turning off, after the light source is removed.