[volt-nuts] HP3458 ADC integrator

[Bruce Griffiths]

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
> 
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