Hint: start by looking at the summing junction voltage of a single opamp 
integrator with R = 50k, C = 330pF.

NB the LT1225 isnt unity gain stable so its use for any of U2, U3 and U4 is 
questionable.

They will all likely oscillate on their own. 

Bruce

> 
>     On 24 February 2018 at 08:45 Bruce Griffiths <bruce.griffi...@xtra.co.nz> 
> wrote:
> 
>     Moin attila
> 
>     I'd start with simulating and understanding the integrator in the 34401A, 
> its a little simpler but still has a coupe of opamps in the integrator. Then 
> analysing the GBW requirements of the various opamps used in the HP3458A 
> shoul become somewhat clearer.
> 
>     Bruce
> 
>         > > 
> >         On 24 February 2018 at 04:51 Attila Kinali <att...@kinali.ch> wrote:
> > 
> >         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
> > 
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