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OPA818DRGEVM: How to use as a transimpedance amplifier?

ArTzy
Genius 17595 points
Part Number: OPA818DRGEVM
Other Parts Discussed in Thread: OPA818, LMH32401

Tool/software:

Hi Experts,

We'd like to ask assistance on this query from customer:

We have OPA818DRGEVM evaluation board and would like to use it as a transimpedance amplifier. The inverting terminal is grounded. However, it seems that connecting the photodiode directly to the noninverting terminal won't work (through LTSpice simulations). We would prefer it if we do not have to de-solder the resistor R3 grounding the inverting terminal.

Could you please guide us as to how to use it and why is the inverting terminal grounded.

Thank you.

Regards,
Archie A.

  • 0
    TI__Mastermind 28865 points

    Hello Archie,

      Apologize for the delay here.

      You are correct, as a TIA, the input must be at the inverting terminal, otherwise it would not function properly as a TIA. The reason why is the input current source from the photodiode is routed through the feedback resistor since the input to the amplifier is high impedance. This converts the current to a voltage at the output of the amplifier with RF acting as the gain resistor. The closed-loop feedback keeps the voltages at input to output in check to function in its linear mode (if input/output ranges are not violated). 

      Unfortunately, the only way to connect the photodiode is by de-soldering the R3 resistor and replacing R4 resistor with a 0ohm resistor, and R2 with the correct transimpedance gain resistor. 

      We do have a different amplifier with an EVM that includes an optional photodiode connection: https://www.ti.com/tool/LMH32401RGTEVM. The LMH32401 is different from the OPA818 where it is an integrated TIA with selectable gain and ambient light cancelation. Depending on our requirements, it could work if you need an easier TIA setup for measurements. 

    Thank you,

    Sima