[volt-nuts] HP3458 ADC integrator

[Attila Kinali]

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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