Electronic – Reverse biased photodiode gain resistor selection

Scanning the datasheet, it appears that fast decay or slow decay are achieved by leaving the outputs off (high impedance) or on (low impedance) but at the same logic level.

So the rise and fall times are determined by the driver output impedances (which are given in the datasheet) and the inductance of your motor's windings (which the chip manufacturer obviously doesn't know). So they cannot give a reliable answer in the datasheet.

I think you have to do the rest of the work yourself; either using the motor's datasheet or your own measurements of its parameters.

Since you are doing a lot of these, your best bet is to devote one amplifier board to testing photodiodes, and put a socket on the PD location. Now you can simply plug your diode leads into the socket and look at the output. You might do well to increase your feedback resistor to 10 pF or so just to be sure the increased input capacitance doesn't give you stability problems.

With that said, I need to ask about your op amp. You list it as an ISL2812, but I cannot find any such part number with a quick Google. What I do find is an Intersil ISL28127, which I presume is what you're using. If so, you are violating power supply specs by putting 40 volts across it - max is 36. Furthermore, you should get rid of your + input components. They are not needed for DC levels (offset currents are 10 na or less, typically 1 nA,) so your offset induced by 2.2 M is no more than 2.2 mV, and typically 100 uV. Furthermore, you are risking stability issues by using this configuration.