In performing TINA simulations of a TIA design using the OPA855, I noticed what seems to be an impossibly low noise performance. In my understanding, the input-referred current noise cannot be lower than the current noise of the opamp. However, the simulated results show much lower noise (e.g. 450fA/rtHz for a 100kohm TIA). I have made a measurement of this circuit to confirm my expectations, and the measurement agrees with a noise floor of 3pA/rtHz. Perhaps I am not reading the simulated results correctly (the unit states V/rtHz which always confuses me).
Simulated response:
Measured noise:
