This is an improved version of the one in the X-chapters.
The dumb summer should be smarter to optimize s/n. If we digitize V1 and V2 and do some tweaks, that should work.
BFT25 is sadly EOL. Its c-b junction is about the best diode anywhere. I'll have to find a comparable part somewhere.
Version 4 SHEET 1 1192 680 WIRE -16 -304 -64 -304 WIRE 32 -304 -16 -304 WIRE 288 -304 256 -304 WIRE 320 -304 288 -304 WIRE -64 -272 -64 -304 WIRE 144 -240 112 -240 WIRE 256 -240 256 -304 WIRE 256 -240 224 -240 WIRE 288 -240 256 -240 WIRE 384 -240 352 -240 WIRE 448 -240 384 -240 WIRE 560 -240 528 -240 WIRE 592 -240 560 -240 WIRE -64 -176 -64 -192 WIRE 384 -176 384 -240 WIRE 560 -144 512 -144 WIRE 592 -144 560 -144 WIRE 512 -96 512 -144 WIRE 112 -48 112 -240 WIRE 112 -48 -64 -48 WIRE 224 -48 112 -48 WIRE 384 -48 384 -96 WIRE 384 -48 288 -48 WIRE -64 0 -64 -48 WIRE 112 16 112 -48 WIRE 224 16 112 16 WIRE 384 16 384 -48 WIRE 384 16 304 16 WIRE 512 16 512 -16 WIRE -64 112 -64 80 WIRE 288 144 256 144 WIRE 320 144 288 144 WIRE 0 176 -64 176 WIRE 32 176 0 176 WIRE 256 192 256 144 WIRE -64 208 -64 176 WIRE 112 208 112 16 WIRE 224 208 112 208 WIRE 384 224 384 16 WIRE 384 224 288 224 WIRE 432 224 384 224 WIRE 464 224 432 224 WIRE 224 240 192 240 WIRE 192 272 192 240 WIRE -64 320 -64 288 WIRE 256 320 256 256 WIRE 304 320 256 320 WIRE 320 320 304 320 FLAG 192 272 0 FLAG -64 112 0 FLAG 432 224 V1 FLAG 304 320 V- FLAG 560 -240 V- FLAG -64 -176 0 FLAG -64 320 0 FLAG 288 144 V+ FLAG -16 -304 V+ FLAG 0 176 V- FLAG 288 -304 V2 FLAG 512 16 0 FLAG 560 -144 SUM SYMBOL Opamps\\UniversalOpamp2 256 224 R0 WINDOW 0 19 -45 Left 2 SYMATTR InstName U1 SYMATTR SpiceLine2 En=0 Enk=0 In=0 Ink=0 Rin=500Meg Ro=100 Ccm=1p Cdiff=2p SYMBOL res 320 0 R90 WINDOW 0 64 61 VBottom 2 WINDOW 3 70 62 VTop 2 SYMATTR InstName R1 SYMATTR Value 1e7 SYMBOL diode 352 -256 R90 WINDOW 0 71 46 VBottom 2 WINDOW 3 80 47 VTop 2 SYMATTR InstName D1 SYMATTR Value BFT25 SYMBOL res 240 -256 R90 WINDOW 0 70 54 VBottom 2 WINDOW 3 78 54 VTop 2 SYMATTR InstName R2 SYMATTR Value 400K SYMBOL cap 288 -64 R90 WINDOW 0 -41 31 VBottom 2 WINDOW 3 -35 29 VTop 2 SYMATTR InstName C1 SYMATTR Value 1p SYMBOL current -64 0 R0 WINDOW 0 19 -32 Left 2 WINDOW 3 -71 -74 Left 2 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName I1 SYMATTR Value PULSE(0 2u 0 1 0 10) SYMBOL res 368 -192 R0 WINDOW 0 52 37 Left 2 WINDOW 3 52 71 Left 2 SYMATTR InstName R3 SYMATTR Value 5K SYMBOL res 544 -256 R90 WINDOW 0 -39 55 VBottom 2 WINDOW 3 -31 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 5K SYMBOL voltage -64 -288 R0 WINDOW 0 55 41 Left 2 WINDOW 3 57 67 Left 2 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL voltage -64 192 R0 WINDOW 0 53 39 Left 2 WINDOW 3 52 69 Left 2 SYMATTR InstName V2 SYMATTR Value -5 SYMBOL bv 512 -112 R0 WINDOW 0 62 64 Left 2 WINDOW 3 15 103 Left 2 SYMATTR InstName B1 SYMATTR Value V=V(V1) + 25 * V(V2) TEXT 528 72 Left 2 ;Dual-segment TIA TEXT 536 184 Left 2 ;JL Oct 18 2021 TEXT 568 320 Left 2 !.tran 2 TEXT 136 48 Left 2 ;0V TEXT 488 272 Left 2 !.model BFT25 D(Is=20f) TEXT 488 120 Left 2 ;Input range 20 pA to 2 uA