I'm trying to measure the bandwidth of a TIA with a gain of 1e9 on the first stage and 10x on the second stage. The first stage OpAmp is the LTC6268 with a 1G Ohm feedback resistor and the second is a LTC2050 with a 10k/100k feedback. I'm getting correct values for measured gain and my noise floor is fairly good, but the next step in testing the performance of this TIA is its bandwidth.
I know that typically, you would hook up a sinewave and sweep it to find the -3dB cutoff, but I've been told this is not entirely correct, since it doesn't compensate for the capacitance.
The set up is a function generator with a 100mVrms sine wave, sweeping from 0Hz to 100kHz, feeding into a 1G Ohm resistor to give a 100pArms current. This is connected directly to the input of the TIA.
I don't understand enough of the physics and elemental electronics to know why this setup is incomplete. Any pointers and literature would be most welcome!