Moin, I am trying to understand how the HP 3458 integrator of its dual-slope ADC works, as it's a bit more complicated than a simple, single opamp integrator. As you can see from the attached picture, it consists of 3 opamps (U1-U3) for the integrator itself, and another for the slope amplification (U4). Unfortunately, I do not get what U2 and U3 are doing, much less why they are needed.
So I tried to simulate it in LTspice. But, the circuit oscillates. In both, the reset state (when the integration-C shorting switch is closed) and the integration state (though, less wildly so). I guess, this is because I replaced the LM6361 and AD848 by the LT1225 but do not understand why, or respectively, which of the differing parameters of the LT1225 causes this oscillations. Would someone be so kind and give me some hints? Thanks in advance Attila Kinali -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson
Version 4 SHEET 1 2172 680 WIRE 1936 -416 1840 -416 WIRE 208 -400 96 -400 WIRE 896 -400 208 -400 WIRE 1312 -400 896 -400 WIRE 1792 -400 1392 -400 WIRE 1792 -384 1792 -400 WIRE 1936 -384 1936 -416 WIRE 1840 -368 1840 -416 WIRE -480 -304 -480 -336 WIRE 1840 -304 1840 -320 WIRE 1888 -304 1840 -304 WIRE 1936 -304 1888 -304 WIRE 896 -256 896 -400 WIRE -480 -240 -480 -304 WIRE 352 -224 352 -272 WIRE 896 -192 848 -192 WIRE 944 -192 896 -192 WIRE 96 -160 96 -400 WIRE 304 -160 96 -160 WIRE 848 -144 848 -192 WIRE 944 -128 944 -192 WIRE -480 -112 -480 -160 WIRE -480 -112 -544 -112 WIRE 352 -80 352 -128 WIRE 496 -80 352 -80 WIRE 592 -80 496 -80 WIRE 640 -80 592 -80 WIRE -480 -64 -480 -112 WIRE 720 -48 720 -80 WIRE 640 -32 640 -80 WIRE 688 -32 640 -32 WIRE 1216 -32 1216 -80 WIRE 848 -16 848 -64 WIRE 848 -16 752 -16 WIRE 944 -16 944 -64 WIRE 944 -16 848 -16 WIRE 1008 -16 944 -16 WIRE 1184 -16 1008 -16 WIRE 688 0 624 0 WIRE 1360 0 1248 0 WIRE 1504 0 1360 0 WIRE 1648 0 1504 0 WIRE 1728 0 1648 0 WIRE 1792 0 1792 -304 WIRE 1792 0 1728 0 WIRE 1184 16 1088 16 WIRE 496 48 496 0 WIRE 496 48 464 48 WIRE 624 48 624 0 WIRE 624 48 560 48 WIRE 720 64 720 16 WIRE 1216 64 1216 32 WIRE -480 80 -480 16 WIRE 624 96 624 48 WIRE 1088 112 1088 16 WIRE 1360 112 1360 80 WIRE 1360 112 1088 112 WIRE 1504 112 1504 64 WIRE 1504 112 1360 112 WIRE 1648 112 1648 64 WIRE 1648 112 1504 112 WIRE 1360 128 1360 112 WIRE -480 144 -480 80 WIRE 528 144 528 128 WIRE 160 160 160 112 WIRE 464 160 464 48 WIRE 496 160 464 160 WIRE -400 176 -416 176 WIRE -304 176 -400 176 WIRE -144 176 -224 176 WIRE 96 176 96 -160 WIRE 96 176 -64 176 WIRE 128 176 96 176 WIRE 624 176 624 96 WIRE 624 176 560 176 WIRE -416 192 -416 176 WIRE 240 192 192 192 WIRE 288 192 240 192 WIRE 496 192 368 192 WIRE 128 208 64 208 WIRE 1360 240 1360 208 WIRE 528 256 528 208 WIRE 160 272 160 224 WIRE 368 288 368 192 WIRE 368 432 368 368 FLAG 368 496 0 FLAG 352 0 0 FLAG -544 -112 0 FLAG -480 -304 p15 FLAG 160 112 p15 FLAG 352 -272 p15 FLAG 720 -80 p15 FLAG 528 128 p15 FLAG -480 80 n15 FLAG 160 272 n15 FLAG 528 256 n15 FLAG 720 64 n15 FLAG 1008 -16 IntOut FLAG -416 272 0 FLAG 64 208 0 FLAG -400 176 in FLAG 240 192 u1out FLAG 624 96 u2out FLAG 592 -80 j1out FLAG 208 -400 cout FLAG 1888 -304 0 FLAG 1216 64 p15 FLAG 1216 -80 n15 FLAG 1360 240 0 FLAG 1728 0 out SYMBOL Opamps\\LT1001 160 128 R0 SYMATTR InstName U1 SYMBOL Opamps\\LT1225 528 112 R0 SYMATTR InstName U2 SYMBOL Opamps\\LT1225 720 -80 R0 SYMATTR InstName U3 SYMBOL res 384 176 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 33k SYMBOL res 352 272 R0 SYMATTR InstName R2 SYMATTR Value 100 SYMBOL cap 352 432 R0 SYMATTR InstName C1 SYMATTR Value 2.2ยต SYMBOL res -208 160 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 50k SYMBOL cap 560 32 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C2 SYMATTR Value 100p SYMBOL res 480 -96 R0 SYMATTR InstName R4 SYMATTR Value 1.5k SYMBOL res 336 -96 R0 SYMATTR InstName R5 SYMATTR Value 475 SYMBOL njf 304 -224 R0 SYMATTR InstName J1 SYMATTR Value U309 SYMBOL res 832 -160 R0 SYMATTR InstName R6 SYMATTR Value 243 SYMBOL cap 928 -128 R0 SYMATTR InstName C3 SYMATTR Value 100p SYMBOL cap 880 -256 R0 SYMATTR InstName C4 SYMATTR Value 330p SYMBOL voltage -480 -256 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL voltage -480 -80 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value 15 SYMBOL voltage -416 176 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V3 SYMATTR Value 0 SYMBOL sw 1792 -288 R180 SYMATTR InstName S1 SYMATTR Value MYSW SYMBOL voltage 1936 -400 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V4 SYMATTR Value PULSE(1 0 0.5u) SYMBOL Opamps\\LT1225 1216 64 M180 SYMATTR InstName U4 SYMBOL res 1344 -16 R0 SYMATTR InstName R7 SYMATTR Value 12.1k SYMBOL res 1344 112 R0 SYMATTR InstName R8 SYMATTR Value 1k SYMBOL diode 1488 0 R0 WINDOW 0 2 -22 Left 2 WINDOW 3 -24 -44 Left 2 SYMATTR InstName D1 SYMATTR Value 1N4148 SYMBOL diode 1664 64 R180 WINDOW 0 0 87 Left 2 WINDOW 3 -23 116 Left 2 SYMATTR InstName D2 SYMATTR Value 1N4148 SYMBOL ind 1296 -384 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L1 SYMATTR Value 160n SYMBOL ind -160 192 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L2 SYMATTR Value 160n TEXT -568 520 Left 2 !.tran 1u TEXT 1464 -592 Left 2 !.model MYSW SW(Ron=1 Roff=10Meg Vt=.5 Vh=-.4)
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