[volt-nuts] HP3458 ADC integrator
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Fri Feb 23 15:49:21 EST 2018
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.griffiths at 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 <attila at 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
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