OPT101 Photodiode Transimpedance Amp circuit

Discussion in 'The Projects Forum' started by Jerry Gunn, Mar 8, 2015.

  1. Jerry Gunn

    Thread Starter New Member

    Mar 8, 2015
    3
    0
    Does anyone know the maximum external resistor value that can be used with the OPT101? I am designing a low cost differential light sensor to detect starlight. I am currently not getting enough gain using an external 20M resistor. The circuit uses two OPT101 with outputs run into an INA118 for more gain. The data sheet shows 50M. I think I need closer to 1G ohm.

    I want to avoid avalanche diodes due to their high cost and HV power supply. The circuit works on brighter stars like Sirius, but stars like Polaris, which are 8 times times dimmer do not work well.

    Any suggestions would be appreciated.

    Jerry
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    You can try a higher feedback resistor but, at some point, dark current and offset errors will dominate the output (read the first paragraph on page 10 of the data sheet).
     
  3. Jerry Gunn

    Thread Starter New Member

    Mar 8, 2015
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    0
    Thanks. I read that and agree. Since frequency bandwidth is not an issue, is there any way to use this device in an integrating circuit. Even 10 seconds would be fine. I'm not coming up with any ideas to do this.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    Instead of using a feedback resistor in the OPT101, you can use a small feedback capacitor in an integrate-and-dump circuit. (The internal 8pF capacitor may be adequate for that depending upon the signal level).
    You put a switch across the capacitor to periodically reset (dump) it to 0V, For the switch you want one with a low charge injection, such as one of these. (Select one with a 2pC or less Qinjection which would give a 1/4 V offset on an 8pF capacitor when reset.)
     
  5. Jerry Gunn

    Thread Starter New Member

    Mar 8, 2015
    3
    0
    I looked at the specs on the switch and your idea may work. I will give it a try.

    Thanks very much, Jerry
     
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