Hi,
I am having a few problems designing the OPA657 as a transimpedance amplifier.
Using the 3dB frequency equations outlined in the datasheet for a gain of 1G (Cf = 8.92fF (not shown) with photodiode shunt capacitance 400pF) we should achieve 25.231kHz bandwidth?
However, running a simulation in LT Spice the bandwidth achieved is much lesser than expected. This variation from the relationship shown in the datasheet seems to worsen with increasing gain.
The bandwidth required from the system is 20kHz.
The feedback loop is in a T-circuit to minimise resistor values for the very high gain and minimise johnson noise. Gain is also switchable between 10M and 1G in this configuration. At 10M the bandwidth is also lesser than expected although i have achieved the specification of 20kHz and so this is less of a problem.
Is there any reason why the achievable bandwidth does not match the applications example in the datasheet?
I hope this is clear.
Thanks in advance,
Michael