Need help selecting photodetector

The ISL29125 is a color meter that must be read by a computer.
The TSL235 is light to frequency.
I know where you can get boards with these parts mounted on them.

**broken link removed**

A great part has very low input current and low input capacitance with 500MHz gain BW product.
https://www.digikey.com/product-detail/en/LTC6268IS6#TRPBF/LTC6268IS6#TRPBF-ND/5030414

There are even better ones with 4GHz GBW and same high input impedance.

The AMS302 Panasonic part in #19 is a 5mm radial Photo sensor that runs off 5V with sensitivity down to a few LUX for a slow LUX meter. Sensitivity and speed are tradeoff choices with R load.

spec said:

Hy run,

Here is a circuit for an opto receiver:

Click to expand...

Very nice, thanks!
What changes would I have to do to allow it to detect the intensity of the light, as it should give different outputs if the light is brighter/weaker?
I should have specified this earlier, sorry.

Hi run.
Sorry the circuit took so long, but my missus gets unreasonable and expects me to do non-technical things from time to time, so ended up shopping in Bristol most of yesterday.

The present Trans Conductance Amplifier (TCA) does output a voltage directly proportional to light intensity (power) but, by design only over a relatively small window as the objective is to ignore the analogue side of the light and produce a clean, noise free, and fast digital signal. This means that if the present (TCA) were modified to cover a wider range of light power, the data integrity and speed would be compromised. I will give your requirement some thought though. I suspect there is a solution.
spec

Hi again run- only a question:
(1) Do you need to monitor the light received power all the time while data is being received
(2) or would it be acceptable if you could switch to either datamode or light received power measurement mode.

I think I know your answer (1), but thought I would ask just in case.

spec

spec said:

Hi run.
Sorry the circuit took so long, but my missus gets unreasonable and expects me to do non-technical things from time to time, so ended up shopping in Bristol most of yesterday.

Click to expand...

See further proof women are a bad idea! Shame they are not more like Oscilloscopes and then you could switch them on only when needed.

spec said:

Hi again run- only a question:
(1) Do you need to monitor the light received power all the time while data is being received
(2) or would it be acceptable if you could switch to either datamode or light received power measurement mode.

I think I know your answer (1), but thought I would ask just in case.

spec

Click to expand...

Not 100% certain what you mean, but I want to measure different light intensities to monitor and compare them (with two or more detectors) at the same time (at the same freq as the uv pulses from the other circuit).

runarh. you seem to be avoiding specs. Is there a reason?

Tony Stewart said:

runarh. you seem to be avoiding specs. Is there a reason?

Click to expand...

Just a rookie, sorry

Hi run,

Here is a circuit which has a digital data output (inverted) of between 0V and 12V and a linear DC voltage output of between 0V and 12V indicating the peak light power received within a data pulse. This is a high impedance output so it should be monitored by a high impedance voltmeter.

The circuit would need to be calibrated by adjusting the value of R3. I have also included an opto receiving LED bias voltage input rather than have a fixed value so that you can experiment with the bias voltage (not to exceed -50V). The circuit is essentially the same as the first circuit, except the transimpedance amplifier now has an extended output range and the light power is captured by a positive peak detector.

I suggest that the best approach would be to first to build the circuitry around N1 and use that to characterise the fibre optic comms link before considering the rest of the circuit functions.

spec

ERRATA
(1) Change label from RX LED BIAS (-50V MAX) to RX OPTO DIODE BIAS (-50V MAX)
(2) Change C2 to 1uF solid capacitor
(3) Change R9 to 1M Ohm
(4) Connect a 1K Ohm resistor between RX OPTO DIODE BIAS (-50V MAX) input and opto receiving diode anode

runarh said:

Very nice, thanks!
What changes would I have to do to allow it to detect the intensity of the light, as it should give different outputs if the light is brighter/weaker?
I should have specified this earlier, sorry.

Click to expand...

To detect intensity requires a a peak detector not included which can introduce distortion if not buffered from the signal. Generally a verty high gain limiter(comparator) is used for front end and thus intensity cannot be derived from the output. You would need a linear wideband amp as the front end to measure it's analog peak, perhaps with very high GBW such as my previous post suggestion, depending on your data rate.

Since Comparators don't use feedback and are thus inherently stable ( if supply and Vref is clean, and thus are designed with much higher slew rates than Op Amps like OPA192

We still dont have a spec for your slew rate, data rate or signal level. (big hint)

Hello Tony,

Tony Stewart said:

To detect intensity requires a a peak detector not included

Click to expand...

N3 is a peak detector

Tony Stewart said:

which can introduce distortion if not buffered from the signal.

Click to expand...

This is just s meaningless unqualified statement about 101 circuit design. The OPA192 has a very high impedance input so the peak detector is isolated from the signal.

Tony Stewart said:

Generally a verty high gain limiter(comparator) is used for front end and thus intensity cannot be derived from the output.

Click to expand...

Not correct with this diode which produces a current. A current to voltage converter is used to get a workable voltage signal. Feeding the current into a virtual earth point means that no parasitic capacitances need to be charged up and down. Thus speed is optimized and distortion is minimized.

Tony Stewart said:

You would need a linear wideband amp as the front end to measure it's analog peak, perhaps with very high GBW such as my previous post suggestion, depending on your data rate.

Click to expand...

You keep on about specifications but you do not seem to use specifications yourself. The OP has stated that his data rate is 1K baud

Tony Stewart said:

Since Comparators don't use feedback and are thus inherently stable ( if supply and Vref is clean, and thus are designed with much higher slew rates than Op Amps like OPA192

Click to expand...

This is another generalization without any figures. It is perfectly acceptable to use an op amp as a peak detector, in fact in the main they are. And in this case the opa192 has a minimum source and sink current of 65mA, a gain bandwidth of 10MHz and a slew rate of 20V/us rate. It also has a minute indecision window around 7uV, so it is no slouch. I do agree that a comparator would be faster but that is unnecessary in this application. Also, finding a fast comparator to to handle a 12V peak signal would be a challenge.

It is not true that a comparator cannot oscillate, especially in a peak detector circuit where there is feedback. In fact, some of the early comparators, which were not frequency tailored in their linear region, oscillated well with no feedback and were buggers to tame. By the same token, a TTL circuit will oscillate if you keep it in the linear region for too long by having a data edge over a few tens of nanoseconds.

Tony Stewart said:

We still don't have a spec for your slew rate, data rate or signal level. (big hint)

Click to expand...

Hardly any circuits have specification included with the circuits posted on ETO. But at least I give you the data sheets so you can work out the performance of the circuit for yourself. But you will have to get all the facts from the data posted in the thread to see what the OPs requirement are. You will also have to make an attempt to see how the circuit works. If you don't understand, or need clarification, just ask like most other members.

The OP has stated that he already has a transmitter and detector diode, so suggesting other optical components, especially with hundreds of megabyte data rate is invidious. This is another example where you do not comply with your own stated ethos of getting the specification right. Also I think you should be more constructive and less negative. If I took the same attitude as some people on ETO, I could spend most of my time ripping circuits to pieces. Having said that, you will see many instances where helpful members have pointed out an error or suggested a specific improvement, and I have thanked them.

Finally, if you have a good circuit for this application, and I don't mean a load of general assumptions and references to other circuits, I mean a complete self contained circuit with all suporting data, please post it. I for one, would be genuinely interested.

spec

I deserved the criticism for overlooking the thin thread #9 on 1KHz data rate.

When I say include a spec. this is not something the technical advisor creates from a client's paper napkin block diagram requirements, which so far are not existent.

From what I understand the OP is using a chemical fluorescent medium stimulated by pulsed UV at 1kHz and hoping to accurately measure the energy converted to Green from the UV stimulation in an undefined volume and gap with undefined flow of the medium. The OP has suggested a requirement for measuring amplitude but not much else, such as phosphor remanance , which might require peak and average response or wavelength which might require optical filters , nor has physical design been defined such as distance and lost from optical<> electrical interface, background EMI, temperature , AMBIENT LIGHT etc etc
These physical, functional and environment requirements are crucial to define a spec. Try to define all the requirements in the beginning or so someone else can summarize a spec if you do not have this experience.

Attention to details is the secret to getting it right the 1st time.

I might ask for more details on the experiment so as to ensure errors are minimized in the design. e.g.

Does accumulated UV power increase Green phosphor emission level until saturated? Or just duty cycle? What losses can be allowed in detection? What background light is present? Does background light need to be blocked or filtered or measured by Sample and Hold between UV ON/OFF.

Here is a simplified requirement example made with my simple assumptions, which may be wrong From this a detailed spec can be made WITHOUT any need for specific choices of parts. After this the design can start quickly.

Objective: Design an Electra-Fluoescent efficacy Detector with UV in and Green light out.

Input : 1W UV with x Lumens @160deg beamwidth (or y? deg ) and z distance to target PD (beamwidth may be defined as full beam at half power)
pulse rate = 1kHz with <10% rise time.
Output: Light in true green band (515~525nmD )low level fluorescent output in fluid medium at room temp. non-corrosive,
Electrical output: Detector diameter = x mm, Sensitivity = [uA/uW] or calibrated 1V @ 100Lux for example. Use linear peak amplitude measurement on Optical to Green output during ON and OFF levels for UV input.


I still prefer you consider putting the OE interface inside the probe unless you intend to use low loss black clad fiber OR else in a dark environment.
Please define in LUX or Lumens or Candella the optical power range you expect or some reference we can relate to.
e.g. Glowing green slime with a 10Watt bulb at a distance of 10 cm. just kidding.

spec said:

Hello Tony,

spec

Click to expand...

Hello Spec,
Yes I am aware of the parasitic positive feedback potential for spurious resonance in high speed comparators.

I may have assumed too much. I expected ultrafast fluorescence spectroscopy from the question with UV stimulated green light, which is performed in nano or fempto seconds.

Hy Tony,

Once again I am with you about defining things, but it is information that I do not know. The circuit I have posted is to establish some of theses characteristics. This is what I wrote:

'The circuit would need to be calibrated by adjusting the value of R3. I have also included an opto receiving LED bias voltage input rather than have a fixed value so that you can experiment with the bias voltage (not to exceed -50V). The circuit is essentially the same as the first circuit, except the transimpedance amplifier now has an extended output range and the light power is captured by a positive peak detector.

I suggest that the best approach would be to first to build the circuitry around N1 and use that to characterise the fibre optic comms link before considering the rest of the circuit functions.'

Sometimes you have to do some investigations before a design can be finalized, otherwise nothing would ever get done.

I do not agree that the power should be monitored in the head. That would be very difficult to do and also it would degrade the signal to noise because some of the opto diode current would be consumed by the monitoring circuit.

spec

Tony Stewart said:

Hello Spec,
Yes I am aware of the parasitic positive feedback potential for spurious resonance in high speed comparators.

I may have assumed too much. I expected ultrafast fluorescence spectroscopy from the question with UV stimulated green light, which is performed in nano or fempto seconds.

Click to expand...

I appreciate that you are considering a very high speed opto link and what you say is no doubt the way to go for that sort of link. I simply do not have the experience. The opto stuff I worked on was handled by experts in that field and we just got a nice voltage or, as in this case a current- that is the trouble of working in a big organization- you don't do the whole job.

spec

The solution I was thinking of for in-situ probe detection ( OE conversion) is the same used in camera photo detectors with a CIE eye-corrected flat response and is fairly accurate with a dark current well under 1uA.

They are cheap and readily available at D-K. I used many of them in a design of an optical light meter for testing LEDs of any visible colour straight into a built-in portable cheap DMM.

I suggested this approach on the assumption that the user might not know how to design the optical interface with an optical splitter between emitter and detector for reflected fibre endoscopic detection. With this approach a better defined volume of medium. With an in-situ emitter<>detector, a controlled volume of medium for a transmission method can be conducted. This avoids the Lambertian Response of an endoscopic method of unknown volume of medium.

The circuit is trivial with a low power 5V three terminal low-current regulator, a defined resistor or switchable set of values and a multi-meter (DMM) or ADC data collection system.
The package is the same as the 5mm LEDs.




I would choose a 10 ms UV pulse with whatever duty cycle is desired and then use the Analog output for the experimental data with an accuracy of < 1% after calibration with a known light source.

If one does not need optical corrected intensity vs wavelength , then any old Sharp/Vishay PD will do with 0.5uA/uW.

I tend to think of overall requirements, not just what an inexperienced OP asks for.

Tony Stewart said:

The solution I was thinking of for in-situ probe detection ( OE conversion) is the same used in camera photo detectors with a CIE eye-corrected flat response and is fairly accurate with a dark current well under 1uA.

They are cheap and readily available at D-K. I used many of them in a design of an optical light meter for testing LEDs of any visible colour straight into a built-in portable cheap DMM.

I suggested this approach on the assumption that the user might not know how to design the optical interface with an optical splitter between emitter and detector for reflected fibre endoscopic detection. With this approach a better defined volume of medium. With an in-situ emitter<>detector, a controlled volume of medium for a transmission method can be conducted. This avoids the Lambertian Response of an endoscopic method of unknown volume of medium.

The circuit is trivial with a low power 5V three terminal low-current regulator, a defined resistor or switchable set of values and a multi-meter (DMM) or ADC data collection system.
The package is the same as the 5mm LEDs.
View attachment 98669

View attachment 98667

I would choose a 10 ms UV pulse with whatever duty cycle is desired and then use the Analog output for the experimental data with an accuracy of < 1% after calibration with a known light source.

If one does not need optical corrected intensity vs wavelength , then any old Sharp/Vishay PD will do with 0.5uA/uW.

I tend to think of overall requirements, not just what an inexperienced OP asks for.

Click to expand...

Nice info Tony- I learned something today.

spec