I am working on a low noise transimpedance amplifier (TIA) for the detection of weak optical signals. The aim is to achieve a 10MHz bandwidth with a white voltage noise floor of 10-20nV/rtHz. I am using the FGA21 photodiode and the OPA847 Op-Amp with a 10kohm feedback resistor operating in photoconductive mode.
Key specifications include:
- Gain banwidth product: GBW = 3.9GHz
- Input voltage noise: e_n = 0.85nV/rtHz
- Input current noise: i_n = 2.5pA/rtHz
- photodiode capacitance: C_d = 100pF @ 3V bias
The PCB design followed many of the suggested layout techniques (minimising track length, passing feedback components under the op-amp, isolating sensitive tracks from the ground plane etc.). Additionally, the voltage supply was heavily filtered using decoupling capacitors and the OPA820 Op-Amp was used to buffer the output.
The dashed lines represent the corresponding theoretical noise curves. Clearly the capacitor causes the noise peaking to broaden and shift in frequency, this contradicts theory which suggests that a feedback capacitor dampens the transimpedance gain and reduces high frequency noise.
To test this further a circuit was constructed without the photodiode, instead a 100pF capacitor was added to mimic the diode junction capacitance and the noise measurements were retaken:
In this circuit the addition of a feedback capacitor causes the noise to dampen similar to how the theory predicts it to, suggesting to me that the simple photodiode model of a junction capacitance and current source may not be completely accurate. However, searching through literature I have not yet found discussions of the limitations of this model, nor have I seen any examples of this behaviour.
So I am wondering if anyone else has come across this issue before or can understand how the addition of a single capacitor causes a large disparity between theory and experiment?
(Please excuse the lack of circuit diagrams, I am a new user and as of yet can only attach two links per question)
Edit 2: Note in the above circuit diagrams the photodiode is not reverse biased, in all noise spectra shown it is soldered in the correct bias