The goal is to design a circuit that can accurately measure light intensity with little temperature dependency.

Common application uses photodiodes to do so.

Photodiodes can be measured either in Photovoltaic mode or in photoconductive mode.

The caveat, is that photodiodes have strong temperature dependency almost 1% for 10°C, and I need to correct for it.

An idea I had going through some schematics would be to design a differential measurement between two identical photodiodes in photovoltaic mode.

The photodiodes would be on the same PCB and on top of the photodiodes a piece of translucent, diffusive material would be glued and then covered with a dark casing which is open in one side.

The light would come from one side, hit the first photodiode and then hit the second one with a lower intensity as it's being diffused through the material.

An amplification circuitry would compare the difference between the two photodiodes and the overall circuit calibrated with a known light source.

Would this principle work, or is there issue I overlooked?

Perhaps a simpler solution is possible?

There is a schematic out there that works in photoconductive mode and correct for dark current, but there is seems to be other factors that contribute to the temperature dependency, perhaps they are negligible?

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  • \$\begingroup\$ It'd be a wise choice to link to the PD in question. \$\endgroup\$ – Andy aka Nov 23 '18 at 18:34
  • \$\begingroup\$ The PD hasn't been selected yet, might depend on the findings of the question or further research. \$\endgroup\$ – Damien Nov 23 '18 at 18:42
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    \$\begingroup\$ So, your caveat stated is for all PDs yes? If so then please link at least one typical device that states this. \$\endgroup\$ – Andy aka Nov 23 '18 at 18:49
  • \$\begingroup\$ Yes it's for all the PDs. It's not always stated on the datasheet but here is one example: TEMD7100X01 \$\endgroup\$ – Damien Nov 26 '18 at 3:39
  • \$\begingroup\$ Note on this device we can see dark current contribute to about 6% drift over 10°C if there is an illumination of 1sun. \$\endgroup\$ – Damien Nov 26 '18 at 10:27

I don't see how subtracting one temperature-dependent signal from a smaller temperature-dependent signal does much of value. Perhaps you could explain, using equations rather than words, how you think this could work.

The "known light source" is probably going to turn out to be more difficult that simply measuring and compensating for temperature directly. 1% for 10°C is not a very strong temperature dependency, so you should not need much accuracy in your temperature measurement/compensation circuit.

The output of LEDs, for example, vary with temperature and time. Photodiodes are at least pretty much time invariant.

Edit: I don't see anything wrong with this, but I might be missing some subtlety of PD behavior. Obviously it will depend on the photodiodes being matched in characteristics and at the same temperature. The dark current and shunt resistance compensations should track.

I'm not sure what the point is of giving the dark reference PD any light at all. If you give it much light the bias voltages will remain matched but will change from almost exactly 5V to a bit less.

  • \$\begingroup\$ I have a secondary calibrated reference absolute light source so that is not the issue. Also it's not an led application. And I need to be on the 0.1% range per 10°C drift. It's common practice to measure differential between two of the same sensor held in different conditions to reduce uncertainties. On the schematic, Since the two photodiode has same dark photo current, which is the primary cause of the temperature dependency, it is simply being canceled. \$\endgroup\$ – Damien Nov 23 '18 at 18:23
  • \$\begingroup\$ I would have assumed that in your proposed PV mode there will be ~0V across the PD so dark current will not be a factor. The schematic is for photoconductive mode, of course. \$\endgroup\$ – Spehro Pefhany Nov 23 '18 at 18:25
  • \$\begingroup\$ The question is open to both implementation as you can see the schematic it's photoconductive mode. PV mode has recombination and shunt variations temperature dependency which is also spectral dependent so would be a bit more complicated \$\endgroup\$ – Damien Nov 23 '18 at 18:29
  • \$\begingroup\$ Put your photodiode in an oven, or at least a have a control loop keep it at a constant temperature. (ie a cooler, or TEC) Then your device is always at the same temperature and there is no TC correction required. \$\endgroup\$ – D Duck Nov 24 '18 at 0:03
  • \$\begingroup\$ It's a possibility but I can't in my particular design due to the cost it would imply. (would be fine for 1 sensor but I have hundreds of them). @DDuck \$\endgroup\$ – Damien Nov 26 '18 at 10:11

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