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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.

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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.

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  • \$\begingroup\$ But with a APD response in the nanosecond range, is 2Hz what I'm looking for (as in, would I loose something with low GBW amplifiers?). Also, would that expression still hold with a compensating feedback capacitor? \$\endgroup\$ – joaocandre May 27 '14 at 8:49
  • \$\begingroup\$ I didn't include the compensation capacitor because at these low BW's it's value is infinitesimal, of course if you crank up the bandwidth then you might need it. But it won't affect the corner frequency, just the stability. Will you miss some signal? You don't state your design goal with this circuit, it's not clear why you'd need an APD with sub ns response time. \$\endgroup\$ – placeholder May 27 '14 at 13:36

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