Version 4
SHEET 1 1200 1772
WIRE 272 -272 272 -288
WIRE 384 -272 384 -288
WIRE 272 -176 272 -192
WIRE 384 -176 384 -192
WIRE -224 208 -336 208
WIRE -128 208 -224 208
WIRE 16 208 -128 208
WIRE 80 208 16 208
WIRE 208 208 160 208
WIRE 352 208 208 208
WIRE -336 224 -336 208
WIRE -224 224 -224 208
WIRE -128 240 -128 208
WIRE 16 304 16 208
WIRE 80 304 16 304
WIRE 208 304 208 208
WIRE 208 304 144 304
WIRE -336 336 -336 304
WIRE -224 336 -224 304
WIRE -128 336 -128 304
WIRE 16 400 16 304
WIRE 80 400 16 400
WIRE 208 400 144 400
WIRE 112 512 112 496
WIRE 16 528 16 400
WIRE 80 528 16 528
WIRE 208 544 208 400
WIRE 208 544 144 544
WIRE 240 544 208 544
WIRE 352 544 352 208
WIRE 352 544 320 544
WIRE 400 544 352 544
WIRE 80 560 16 560
WIRE 352 576 352 544
WIRE 16 592 16 560
WIRE 112 592 112 576
WIRE 352 672 352 640
FLAG 384 -176 0
FLAG 272 -176 0
FLAG 272 -288 -V
FLAG 384 -288 +V
FLAG 400 544 out
IOPIN 400 544 Out
FLAG 112 496 +V
FLAG 112 592 -V
FLAG 16 592 0
FLAG -128 336 0
FLAG -224 336 0
FLAG -224 208 in2
FLAG -336 336 0
FLAG 352 672 0
SYMBOL voltage 384 -288 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 5
SYMBOL voltage 272 -288 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value -5
SYMBOL res 176 192 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName Rf
SYMATTR Value 50k
SYMBOL Opamps\\opamp2 112 480 R0
SYMATTR InstName U3
SYMATTR Value OPA381
SYMBOL cap 80 320 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName Cs
SYMATTR Value 0.2p
SYMBOL cap -144 240 R0
SYMATTR InstName Cin
SYMATTR Value 10p
SYMBOL res -240 208 R0
SYMATTR InstName Rd
SYMATTR Value 1G
SYMBOL current -336 224 R0
SYMATTR InstName Id
SYMATTR Value 0
SYMBOL cap 80 416 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName Cf
SYMATTR Value {CF}
SYMBOL cap 336 576 R0
SYMATTR InstName Cfilt
SYMATTR Value {CFI}
SYMBOL res 224 560 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName Rfilt
SYMATTR Value {RFI}
TEXT -368 -296 Left 2 ;.tran 0 50u 0 50n
TEXT -368 -344 Left 2 !.include  OPA381.LIB
TEXT -368 -384 Left 2 ;The OPA381 can be sued with he universal symbol "opamp2".
TEXT -368 -264 Left 2 ;.options plotwinsize=0
TEXT -368 -176 Left 2 !.noise V(out) Id dec 100 {FSTART} {FSTOP}
TEXT -368 -24 Left 2 !.param CF =table(n, 0, 1, 1, 1f, 2, 16p, 3, 22p)\n.param RFI=table(n, 0, 1m, 1, 1m, 2, 1m, 3, 100)\n.param CFI=table(n, 0, 1f, 1, 1f, 2, 1f, 3, 3.9n)
TEXT -368 -112 Left 2 !.param n=0 ; default, opamp noise only
TEXT -368 -72 Left 2 !.step param n list 1 2 3 ; sweep circuit 6a, 6b, 6c from datasheet
TEXT -368 96 Left 2 !.measure vtotnoise RMS V(onoise)*sqrt(FSTOP-FSTART)
TEXT -288 720 Left 2 ;View -> SPICE Error Log\n \nMeasurement: vtotnoise\nstep      RMS(v(onoise)*sqrt(20meg-1))    FROM     TO\n     \n1           0.000327568         1        2e+007     \n2           7.25442e-005        1        2e+007     \n3           2.32553e-005        1        2e+007
TEXT -368 -216 Left 2 !.param FSTART=100 FSTOP=20Meg