I'm planning to use an Avalanche Photodiode (APD) to perform sequential light sensing – something around 100 readings under a minute. From my calculations this implies a frequency of around 1.67~2Hz. My APD has a capacitance of 320 pF and a dark current around 0.15 uA. The particular light signals I'm trying to measure would output ~0.8 uA (these are all estimates based on the device's datasheet)
When designing a op-amp transimpedance amplifier, what kind of bandwith would I be looking for? The datasheet suggests using an op-amp able to work at 1GHz due to the fast photodiode response (in the nanosecond range – a photon detection pulse has FWHM of 0.3 ns). However as my application doesn't require sugh high frequencies,could I use slower op-amps or I would lose some signal?
Obviously the gain must be taken into account, but even if I place a 47 KOhm resistor on the feedback loop, I'still get ~10 Hz bandwidth and would not require such high-frequency op-amp. Am I missing something relevant?
EDIT: The APD is the Hamamatsu S12572-100P model, biased with 65V.
Best Answer
Let's assume you want a reasonable response from the TIA, say 1 Volt. For 0.8 uA signal that gives you a feedback resistor of 1.25 M\$\Omega\$.
Using \$F_p=\sqrt{\frac{GBW}{2{\Pi}{R_f}{C_D}}} \$ you see you need a high GBW to be able to get a high frequency response.
With your 2 Hz requirement your GBW of the opamp need only be ~ 1600 KHz.