The April 1989 HP Journal is mostly dedicated to the 3458A and there's an
article about the dual slope integrator starting on page 8.
There's also a nice discussion about it in Art of Electronics 3rd Edition
starting on page 918

-----Original Message-----
From: volt-nuts [] On Behalf Of Attila
Sent: 23 February 2018 15:51
To: Discussion of precise voltage measurement
Subject: [volt-nuts] HP3458 ADC integrator


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

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

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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