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opa 656

Tharindu Janaraj
Prodigy 40 points

Other Parts Discussed in Thread: OPA656, TINA-TI

i need to amplify the avalanchi photo diode current signal. i used the opa656 current to voltage amplifier to amplify. but output signal contain high noise so please tell to me how to remove the noise. thank you.

  • 0
    TI__Genius 15975 points
    Hi Tharindu,

    Would it be possible for you to attach a schematic of your OPA656 current to voltage amplifier circuit? Also, what is the avalanche photo diode part number you are using? Have you tried to measure the output noise with no signal applied and tried to correlate it with your OPA656 circuit in TINA-TI?

    Best Regards,
    Rohit
  • 0 in reply to Tharindu Janaraj
    Prodigy 40 points
    This is the schematic of the circuit we have used. First one is the trans impedance amplifier and the second is voltage amplifier, Both are working individually but together they do not work. There is a lot of noise when it work together. The frequency of the output signal is always 50Hz one regardless of the input signal frequency. Thank you.
  • 0 in reply to Tharindu Janaraj
    TI__Genius 15975 points
    Hi Tharindu,

    You are missing a series resistor in-between the TIA output stage and the voltage amplifier negative input, which mainly sets the inverting gain of the voltage amplifier.

    Also, the 2M ohm resistor in the feedback contributes to a very high 4kTRf noise at both the TIA and voltage amplifier output. In-order to reduce the output noise, try to see if you can reduce the Rf value on the TIA stage to 100k or less, and subsequently for the voltage amplifier stage.

    If you are noticing a constant output signal of 50Hz irrespective of the input signal frequency, then the first OPA656 TIA stage is probably oscillating. I would recommend you to simulate your circuit in TINA-TI to make sure the circuit simulates correctly without any oscillation and then proceed with the hardware implementation.

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Prodigy 40 points
    Dear Rohit,
    Thank you for your kind cooperation. We have tried to simulate it from TINA. But we cant find AD500-8 avalanche photo diode component from the component list. Could you please help us in this . How to add AD500-8 photo diode for the simulation? Its data sheet is attached here.
    Thank you.
  • 0 in reply to Tharindu Janaraj
    TI__Genius 15975 points

    Hi Tharindu,

    You can use the below TINA-TI circuit for simulating the photo diode component.

    In the below TINA-TI circuit, you could set the R3 as 0-ohms and R1 as 2Mohm the way you have shown to get a very high gain output of the second stage. However, I would not recommend doing that because the part would be amplifying a lot of noise and would eventually rail the output. Also, the cascaded gain will be so high that it is practically not feasible.

    I have used a more reasonable Rf and Rg of 250-ohms on the second stage which sets its gain at -1. You could play around Rf and Rg to get a higher gain value if needed.

    Best Regards,

    Rohit

    OPA656_TIA_VoltageFollower.TSC