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OPA857EVM: What is the highest output from this board?

Jimmy Du
Prodigy 20 points
Part Number: OPA857EVM
Other Parts Discussed in Thread: OPA857

Hello,

I have tried a few different photodiodes with this board but I have only got ~30mV as my highest output. I'm woundering what the highest output is for this board. Is it depending on my photodiode and LED/laser combo or is there a limit?

With kind regards,

Jimmy

  • 0
    TI__Genius 15975 points
    Hi Jimmy,

    You should be able to swing from +1.9V to +0.6V minimum output on the OPA857EVM. Have you made sure the photodiode is connected to the positive-bias voltage? It is important to note that the OPA857 output can only swing down from the reference voltage of ~1.85V to lower value towards GND, which mainly requires a positive bias on the photodiode.

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Prodigy 20 points

    Hi Rohit,
    I have connected the photodiode to the positive-bias voltage.
    I'm currently using this photodiode: optodiode.com/.../ODD-15W.pdf
    With these inputs on the board:
    VAPD: +50V
    Vcc: +3.3V
    Less than 1W from the LED onto the photodiode.
    When I test the board I've the photodiode 5mm from my LED just to see the maximum output I can get.
    Do you have any tips on what I can do to increase the output?
    Kind Regards,
    Jimmy

  • 0 in reply to Jimmy Du
    TI__Genius 15975 points

    Hi Jimmy,

    In-order to configure the OPA857EVM as a TIA, I think you need to make the R4 as 0-ohms. Can you confirm the R4 as shown below is 0-ohms in your EVM? By default, the EVM ships with an R4 of 20-kohm resistor. If you connect a photo-diode at the input for a default OPA857EVM, there will be a significant voltage drop at the input for any amount of input current and thus, results in reduced output swing.

    The other possibility for reduced output swing might be the Laser is not tuned at the right wavelength as the photo-diode responsivity. Please make sure the laser wavelength is set to 620nm required by the photo-diode.

    Best Regards,

    Rohit