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
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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
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WIRE 1360 112 1088 112
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WIRE 1504 112 1360 112
WIRE 1648 112 1648 64
WIRE 1648 112 1504 112
WIRE 1360 128 1360 112
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WIRE 496 160 464 160
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WIRE -304 176 -400 176
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WIRE 96 176 96 -160
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WIRE 624 176 624 96
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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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