Yep. My UT61C can go as low as 40nF. pF is way out of it’s league. Maybe it’s time for new multimeter.
40 nF is maximum in lower scale.
Yours have resolution 10 pF and accuracy 3%, +/- 50 pF. And may be capacitance 50 pF is too small for it.
In that case most probable value is 47pF like Samsung CL10C470JB8NNNC (LCSC C1671).
I’ve uploaded schematics to github:
What type of license for schematics should I use? CC-SA-BY or something else like TAPR OHL or CERN OHL?
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I try to do some simulations in tina-ti. That is response of 10 ns 100 uA rectangle current.
I think real C1 is 10 - 50 pF.
real response of tia (yellow channel) ~150 us (but may be signal not 100 ns but 1000 or more)
This design have some different photodiode connection, because in normal schematic there are some errors.