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OPA818: Stability when Calibrating OPA818 as a TIA

steve smith1
Intellectual 310 points
Part Number: OPA818

Tool/software:

Hi I have an OPA818 application with OPA818 configured as a 9M TIA with a small amount of capacitive feedback to give -3dB transimpedance at 1MHz. For testing I drive it fr om a signal generator via a 9M resistor so it becomes a voltage amplifier of gain x-1 (or noise gain x2).  Simulations show it works, bench testing shows operation is stable. However according to the datasheet you need additional compensation for gains <7, does this apply to the configuration that I describe ? If so please advise. Thanks Steve

  • 0
    TI__Mastermind 18996 points

    Hello Steve,

    Great question!  We have a section of the TI Analog Design Journal which touches on this briefly, which I will link here: "Easily improve the performance of analog circuits with decompensated amplifiers"

    In short, your equivalent input capacitance and your feedback capacitance affect your gain response and provide a high frequency gain (different from the noise gain or signal gain): the high frequency gain is 1 + (CEQ/CF).  As you have simulated your circuit and tested the response on bench, I imagine your combination of equivalent and feedback capacitances provide an equivalent high frequency gain, raising the crossover point for the 'noise gain' to cross the open-loop gain.

    Best,

    Alec

  • 0
    TI__Intellectual 1025 points

    Hi

    Thanks Alec for that explanation. If you want to understand the circuit operation and stability more, please have a look at the Photodiode Circuit Design Tool that we have developed.

    https://dev.ti.com/gallery/view/CircuitDesignStudio/Analog_Circuit_Design_Studio/ver/1.0.0/

    You can provide all the parameters as per your design needs and hit "Design Circuit" button. If your circuit has stability issues, there will be warning in the "Suggestions" tab on right bottom of the screen. 

    Thanks

    Aditya Gosavi