Hello,
I am trying to implement a TIA using the OPA847 because of its low noise and high bandwidth. My first task was to determine the correct values of the feedback components considering the desired closed loop bandwidth (around 170MHz at -3dB) and taking into account the capacitance of the detector (33pF). It results in feedback component values of 500 omhs and 2.5pF.
Next, I performed the stability analysis of OPA847 by means of phase and gain margin criteria. Using the TINA methodology of
the following circuits were simulated (the resistance and capacitance at the output are parasitic components of the scope)
obtaining the following Bode plots
Notice that I compute loop gain using the ratio between open loop gain and closed loop gain (since I understand that the use of Noise gain is just an approximation of loop gain). The phase margin and the gain margin are higher than the requeriments of stability.
Afterwards I checked the stability by performing a PCB of the simulated circuit in TINA, but the typical oscillations from resonant phenomena appear (the red peaks are the Fourier analysis):
I can see that this has already happened to other people, and I will try another op. amp. Specifically, I will try the OPA 656 because is unity gain stable, is it a good choice? However, I would like to know how to solve this problem because in the future I will need to use larger closed loop bandwidth.
Thanks,
Noel
