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OPA857: Regarding connection of photodiode to IN

Hardware Intern
Prodigy 40 points
Part Number: OPA857
Other Parts Discussed in Thread: OPA855, OPA859, OPA856

Tool/software:

Im connecting a silicon detector photodiode to the TIA , i have been told to connect cathode to TIA's inverting input and anode to ground , but in the datasheet of the OPA857 it says that the anode should be connected to IN and cathode to positive supply . 
what's the right way to connect the diode ? and can i gnd cathode instead of supplying it with 3.3v?

  • 0
    TI__Mastermind 29595 points

    Hello,

      Yes that is correct. For the OPA857, the device will only function properly when you bias the photodiode with the cathode connected to a positive supply, and the anode connected to IN pin of the OPA857. These connections make sure that the photodiode sources an output current that results in the OPA857 output swinging down below the reference voltage = (5 / 9) × 3.3 V = 1.83 V. For your second question, you can not ground the cathode. The reason is because the OPA857 requires a photodiode to be connected to the positive bias voltage because the output voltage can only swing down from the reference voltage (1.85 V for a 3.3-V supply) to ground. If you ground the cathode, the PD will forward bias (when it is suppose to only work properly when reverse biased) due to the reference voltage will be higher than the Vbias. 

      Do you need the opposite configuration? If so, we would suggest switching to the other OPA85x family of devices such as the OPA855, OPA856, and OPA859 depending on your GBW and noise needs.

    Thank you,

    Sima