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OPT101 dark current

Olivier Rousseau
Prodigy 30 points

Other Parts Discussed in Thread: OPT101

Hi, I want to measure little amount of light power with the opt101. I tried the basic circuit like in datasheet with 9V on pin 1, ground connected on pins 3 and 8, 0.1uF decoupling capacitor on pin 1 and I shorted pins 4 and 5 together. It works, but instead of getting around 7.5mV of dark current, I get around 70mV when I measure in absence of light. I tried 0.01uF capacitor instead but same result. Also, I tried another chip but same result.

I don't mind having a 70mV offset but I just want to know if its normal and if I'm gonna get the right signal out of it.

Thank you

  • 0
    TI__Expert 6930 points

    Hi Olivier,

    Thank you for the question regarding OPT101.  It is not normal to measure 70mV when operated as you have indicated...the dark output is fully production tested and will be within the range of 5mV to 10mV.  I can think of a few things to consider...

    How are you measuring the output?  A simple meter connected between pins 4/5 and pin 8 should suffice.  

    How light tight is your setup?  In other words how do you ensure the device is in the dark?  Are there any LED's that may be within range of the OPT?  Are there any computer displays or meter or power supply displays that may be emitting any visible light or do you completely eliminate any and all sources of light?  Perhaps you can cover the OPT setup that you have with some sort of shroud...it can be something as simple as a dark piece of cloth like a sweater or jacket, ensuring that it is in the dark. 

    If there is any impedance from pin 2, like ~100k Ohms, this would gain up the 7.5mV into the range you are mentioning, so please check to see if there is a leakage path from pin 2 to ground...one easy way to ensure that there is not is to take a device and carefully support the pin 2 near where it goes into the clear plastic and while supporting it, bend the pin up in the air so it will not contact your board when you reinstall the device.  Just be careful when you bend the pin as if you are too rough with it the wire bond inside can become damaged, that is why I mention to support the pin near the plastic body with a pair of needle nosed pliers...please let me know if either of those suggestions eliminated the issue...

    Lastly does the device seem to respond properly to light?  For example the output should increase with increasing light.  If you are monitoring the output with an oscilloscope you would expect to see a fully rectified 120Hz signal if you were primarily exposed to an incandescant bulb...

  • 0 in reply to Edward Mullins
    Prodigy 30 points

    After trying everything you mentioned, I found out that my scope probe was set on 10X haha. Now I got a 9 mV dark output, thank you.

  • 0 in reply to Olivier Rousseau
    TI__Expert 6930 points

    Hi Olivier,

    That is good news...pretty funny as well...I am the designer of the OPT101 and am always curious to know what people are using it for...any chance you'd tell me a little about what is your application?

  • 0 in reply to Edward Mullins
    Prodigy 30 points

    Sure, I am trying to design a pyrometer and the OPT101 will be used as the sensor in a specific wavelength (around 700nm, 800nm). I'm still in the first phase and I have to determine at which temperature a blackbody will emit enough energy for the sensor to see it or if it will do at least.