Luminometer - UV sensor correct signal processing - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: May 19, 2021, 03:19:27 pm
content: Hi,a client needs to replicate an existing luminometer to detect photo-emissions and customise it as part of a one-off unit they are building for their lab.The system detects very weak UV emissions when two reagents are mixed (usually urine and an antibiotic). The sensor to used in various commercial units that do the similar job is this:https://www.hamamatsu.com/resources/pdf/ssd/s1227_series_kspd1036e.pdfUsually the sample to be tested is put in a completely dark chamber and left there for 10 to 30 seconds and a relative measurement is taken.What I could not find information about (and the sensor manufacturer cannot advise because it is application specific) is how to read the signal.Do I consider the peak reading? Or do I integrate across time? Or do some other type of processing?Thank you

author: penfold
author_rank: Frequent Contributor
timestamp: May 19, 2021, 07:20:37 pm
content: It depends a little on the exact nature of the emission and its total power but the incident photons will mostly form something like a DC level + (a lot of) shot noise (because it's the sum of a lot of discrete events with a reasonably random distribution in time) unless you're in the game of single-photon counting in which case its a different kettle of fish.A typical way to read it out would be to feed it through a trans-impedance amp which has a fairly low bandwidth (1kHz maybe) and enough gain to give you some useful signal, maybe with a further voltage gain stage. The first opamp needs to have low current noise and bias/offset current (LMP7721 jumps to mind though). Being able to add a known offset to the signal before the ADC can be very useful to keep the photo-diode noise/leakage just outside of the ADC "window" to stop it encroaching on your dynamic range.Integrating the signal analoguely is possible, but it doesn't really give you many benefits over a TIA in terms of noise.The thermal stability of the diode is a consideration, as is being able to sense its temperature... also being able to shut off the bias voltage for when the enclosure gets opened.

author: Marco
author_rank: Super Contributor
timestamp: May 19, 2021, 09:28:07 pm
content: Max bias voltage is 5V and it's not like the TIA is going to be able to push a lot of current through it either, why worry about disconnecting the bias?

author: penfold
author_rank: Frequent Contributor
timestamp: May 19, 2021, 10:50:48 pm
content: Quote from: Marco on May 19, 2021, 09:28:07 pmIt avoids unduly saturating the TIA, some op-amps don't like it... not something toworryabout too much,

author: profdc9
author_rank: Frequent Contributor
timestamp: May 20, 2021, 02:45:56 am
content: It may be useful to use a chopper wheel and lock-in detection if you are trying to detect very small signals, depending on the duration of the luminescence. There may be 1/f noise you could remove this way.

author: jpanhalt
author_rank: Super Contributor
timestamp: May 20, 2021, 09:39:18 am
content: Quote from: ricko_uk on May 19, 2021, 03:19:27 pmIf you are doing quantitative analysis, generally the signal is integrated as the actual formation of the emitting molecules is a relatively slow chemical reaction. The rate for which will probably vary with temperature and concentrations. However, the quantum yield (photons emitted per reaction) will not vary that much.Many years ago, a technique for quantitation was developed based on chemiluminescence from dioxetanes. With that substrate, emission can be inhibited by pH while the underlying chemical reaction takes place. Then changing the pH gives a flash of light in a very short time (acid-base reactions are usually very fast). In that case, one might use peak emission, but integration will still work.

author: ricko_uk
author_rank: Super Contributor
timestamp: May 20, 2021, 01:28:00 pm
content: Thank you all for the very useful details!!To decrease the 1/f noise, generally I know instrumentation amplifiers are quite good at that. Would they not be suitable because not low enough "low current noise and bias/offset current" like the LMP7721?Thank you

author: Marco
author_rank: Super Contributor
timestamp: May 20, 2021, 02:12:44 pm
content: Instrumentation amplifiers will get you nothing.A simple low noise cmos/jfet opamp as a TIA will probably do you fine. That's what Hamatsurecommendsand I think used in some of their own amplifiers.One step up from that is using low noise JFET (BF862 in the old days, dunno what is the modern replacement) as a common source amplifier followed by an opamp as the amplifier for your TIA.

author: penfold
author_rank: Frequent Contributor
timestamp: May 20, 2021, 02:53:28 pm
content: Quote from: ricko_uk on May 20, 2021, 01:28:00 pmIt depends a little bit on what you mean by instrumentation amplifier, I think it's a bit of a generic term but to me generally applies to a voltage amplifier with differential input, low noise, high CMRR, low bias and high input impedance but generally best suited to measuring small voltages from slow-changing voltage-generating sensors such as thermocouples or pH probes. Less so for a photodiode which isn't really well suited to operating in voltage mode.The trans-impedance method using an opamp, allows this current to be detected without influencing the reverse bias and is generally a bit more controlled. I seem to recall most of the applications being focused around higher speed because also by controlling the reverse bias can help mitigate the RC time constant that (in the above paragraph) using a high-value resistor to get high sensitivity would introduce... however, they're good for slower speed applications also.1/f noise... it may or may not be an issue for you. That's kinda up for you to decide unless you happen to know a little more about the intensity of light from the experiment. The LMP7721 does have a fairly high 1/f corner frequency, other amps may vary, but definitely worth keeping in mind. The chopper wheel is a great suggestion... but depends a bit on the optical geometry of the setup, how the sample is positioned and whether introducing a sufficient distance to fit in a wheel would actually reduce your incident intensity too far.

author: mjs
author_rank: Regular Contributor
timestamp: May 20, 2021, 03:41:57 pm
content: One off and no experience in low level photodiode circuits ?https://www.ti.com/lit/ds/symlink/ivc102.pdf,zero bias and perhaps an external additional C0G or PPS integration capacitor if there's a 'lot' of light.

author: Someone
author_rank: Super Contributor
timestamp: May 21, 2021, 12:45:27 am
content: Quote from: mjs on May 20, 2021, 03:41:57 pmThere are a range of integrated "AFEs", but hamamatsu go the extra mile and offer everything full integrated in packages like their S9295Depending on the wavelengths involved, there are also a range of consumer volume products pushing small/cheap integrated light intensity sensors.

author: ricko_uk
author_rank: Super Contributor
timestamp: May 21, 2021, 10:04:34 pm
content: Thank you all for the additional infos!@Someone, you mention "there are also a range of consumer volume products pushing small/cheap integrated light intensity sensors.". Could you please point me to some of them, whether from Hamamatsu or other suppliers? I tried Hamamatsu but couldn't find any.Thank you