Controlling for ambient temperature changes in a transimpedance amplifier

Question

I'll start by saying I'm new to electronics, and have only minor working knowledge. I am designing a transimpedance amplifier to measure the opacity of a surface that is changing over time. The resulting voltage is sent into an ADC for me to record. My circuit is below:

I am seeing a lot of voltage variation due to the ambient temperature of the room (I've put a temperature sensor next to my TIA, and noted a strong correlation between a rise in temperature and a decrease in voltage). This variation is relatively large, and causes my measurements to have long cyclic behaviour (due to room heating turning on and off). FYI the ambient temperature changes by ~0.5std. Below is an example of the relationship I'm seeing (Blue is temperature, green is voltage):

(Blue is temperature, green is voltage)

My questions are:

1. Is the variation in output caused by

   • the LED warming up, causing a decrease in light output?
   • the feedback resistor changing due to temperature changes?
   • both? maybe the photodiode?

2. What are some possible solutions to correct for this, assuming I'll never have perfectly stable ambient temperature?

   1. I could add a thermsistor to the diagram, and use an input on the ADC to record the variation in temperature and use software to correct the op-amp output voltage. This makes sense to me, but I've can't seem to find ppl doing this after some searching.
   2. Measure temperature and use a calibration curve to "correct" the reading back to 20C - is this going to be robust though?
   3. Some electronic solution? Not sure what that looks like.
   4. A PID controller that tries to stabilize the LED output (assuming it's the LED that is causing the variation).

Thanks for any suggestions!

Answer 1

The temperature coefficient of resistors tends to be pretty low, compared to what (I understand) you are seeing, typically tens or hundreds of PPM (parts per million), but check the datasheet for your resistors to be sure. Consider using a constant current source to drive your LED, rather than a voltage source through a resistor. With only 100 ohms I suspect you'll have a pretty strong dependence on supply voltage.

Also check the datasheet for your photodiode. Some have a much different temperature coefficient when operated in photovoltaic mode rather than by controlling a current (the photodiode will produce some voltage when it sees light).

Your ideas about feedback to control the LED current also has merit, but you might also consider instead making an identical copy of the circuit. On the copy, don't have the light pass through the translucent material, but just directly illuminate the photodiode. Connect this circuit to another ADC input and use it as a reference for comparison. If the components are matched reasonably well, this will cancel out a lot of the temperature and/or voltage dependence you typically see.

Also, the ADS1115 has an internal voltage reference, so the ADC should give very stable readings when presented with a constant voltage. However, if your supply voltage varies, then any signal derived from your supply voltage also varies. While this circuit may not benefit from it a great deal, many sensor circuits are ratiometric, providing a signal that is proportional to the power supply voltages. In that case you want your ADC to use the supply rail as a reference, rather than a fixed voltage.

Answer 2

Suspect the diodes first. Suspect the photodiode first. I don't think it's the LED, because I would expect the voltage across the LED to go down with increasing temperature, causing the current to go up.

The leakage current in the photodiode is probably the culprit. Basically, as the temperature goes up, the photodiode will become more conductive. While this shouldn't make a difference (because the op-amp is holding the diode voltage to zero), I suspect that it's still allowing carriers to recombine in the diode, instead of getting out where they can affect the diode's terminal voltage.

So you really want to bias the photodiode into photoconductive mode. By doing this, you'll make sure that the carriers get swept out of the junction before they have a chance to recombine.

You probably want to do this by running the anode of the photodiode at some negative voltage -- I'd get a charge pump chip and run it at -5V or -10V. In the absence of a charge pump chip you can use a 555 and some diodes.

Actually -- if you have easy access, try it first with a 9V battery. If it works, then make your charge pump.

Answer 3

Two things I'd add to what's already been said in a couple of good answers:

• Even though the TLC27L2 says "rail-to-rail IO" on the front page of the datasheet, you should always look carefully at the specs to see what is meant by this claim. This chart from the datasheet shows that even though "rail-to-rail output" is claimed, you really can't count on output below about 500 mV.

  

  (However the temperature dependence of this minimum voltage limit is opposite what you are seeing). You might also see something like the input offset voltage having much stronger temperature dependence when the inputs are near the voltage rails than at the mid-supply voltage where \$V_{OS}\$ is specified.

  Try adding a negative supply rail and see if it changes the behavior.

• You haven't shared anything about your optical/mechanical setup, but often the optical coupling to a photodiode can be highly temperature sensitive, due to simple mechanical motion as components of the system expand or contract.

  If you can, try heating the optical system separately from the electrical circuit and vice versa to help narrow down where the problem is.