Hi Jonathan,
some of the questions have been answered in the previous post.
regarding the origin the the offset of the TIA, it is simple. The TIA is an op amp and all the real op amp have an offset.
this offset appears in the calculation when you want to extract the current value from the vout of the TIA.
I_sensor=(Vout-V_offeset-Internal_zero)/Rtia.
If you use a differential ADC (between C1 and C2), the offset and the Internal zero disappear and you have
I_sensor = -(Vc2-Vc1)/Rtia
Vc2=Vout=V_offset+Internal zero -Rtia*Isensor
Vc1= Internal zero+V_offset
regards,
Domenico