This sounds like an interesting application, I'm afraid only a few fairly random thoughts come to mind at the moment:
- The LTC6268 does not have offset voltage adjustment pins, so there is no way to zero out the Vos contribution. You can still differentiate the relevant contributions of input bias and Vos however, by the same method of shunting the feedback resistor, measuring the resulting output (which give you the Vos if the opamp at unity gain). Then checking the difference between this and the output with the feedback resistor alone and the input open (but screened) will give you opamp input bias current contribution. Just accept the Vos contribution as a fixed (0.4uV/'C) offset. In India I suspect that the ambient temperature contribution to input bias current may be more of an issue in terms of accuracy.
- Calibration of the TIA at DC is relatively simple, as NeverDie demonstrated. This is really a confirmation though - for practical purposes, the accuracy of the TIA is determined by the accuracy of the value of the feedback resistor.
- AC performance is
heavily dependent on the physical implementation of the LTC6268 circuit. As I mentioned, stray capacitances are absolutely critical, and the 'Application Information' section of the Datasheet goes into this in significant detail. LT also have an eval board, the DC2414A, which covers three different PCB layouts....
https://www.digikey.com/catalog/en/partgroup/ltc6268-and-ltc6268-10-demonstration-board/66825 This uses a photodiode as a high frequency current source example. The DC2414A board manual contains even more detailed application information. These documents will probably give you the most specific and relevant information source.
- A 100M feedback resistor still sounds
very optimistic given your frequency goals and the significant effects of parasitic capacitance at 7uS rise time.
- Maybe some form of low capacitance C or RC discharge source would give you what you need in terms of AC current calibration source.