PIN diode frontend might not reach 100MHz BW
In the specs it is mentioned that the BW of the TIA has to be "preferably" >100MHz. The OPA857 datasheet specifies a 105MHz BW but for a parasitic capacitance of only 1.5pF. Now, for our PCB SIwave gave me a 1pF parasitic, which can go down to 0.7pF if we add the cut-outs in the planes around the TIA input (see #18), but the PIN diode itself already seems to have a capacitance of 2pF.
Would reaching a BW somewhat below 100MHz be acceptable? Should different TIAs or a different PIN (perhaps the same-series but smaller active area FCI-InGaAs-75) be considered?
Two extra points here:
- The extra input capacitance would also raise Tom's estimates in issue #11.
- The plots on p. 9 of OPA857's datasheet show a BW that's only marginally less than 100MHz for 4.7pF input capacitance, however I don't know how much we can trust them since they're for T=25°C and RL=500R, not over the whole range.