[volt-nuts] 3458A integrator

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Nov 10 17:50:41 UTC 2011

Randy Scott wrote:
> Since it seems that more people now have access to the 3458A CLIP, I have a question about the integrator amplifier being used.  In the HP Journal article on the 3458A, they list a set of specifications for the integrator and then go on to say that it required a custom amplifier design.  I had always taken this to mean that they had constructed a custom op amp IC for the task.  I was then surprised to see that the schematic used a set of off-the-shelf op amps in the integrator loop: one each of LT1001, LM6361, and AD848, plus a 2N4392 JFET and various passives.
> It bears some similarity to the integrator in the 34401A that uses an OP-27 and an AD711.  In the notes on the 34401A, they mention that the OP-27 is used to improve the input offset characteristics of the AD711 (a JFET amp with large Vos).  I suspect that the LT1001 in the 3458A serves a similar purpose.  Both the OP-27 and LT1001 have low Vos, but are too slow to be useful as the integrators in either circuit.
> The problem is that I am failing to even begin to understand how either of these two designs work.  Can anyone shed some light on this or maybe point me to a reference that I could use to help figure it out for myself?
> Thanks.
> Randy.
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The OP27 in the 34401A integrator reduces the effective input offset of 
the AD711 (including the finite (non negligible) input required to 
produce the desired integrator output slew rate).
Since the OP27 output swing is small only its small signal 
charateristics and not its large signal characteristics (slew rate etc) 
are important. The resistive divider network at the OP27 output adjusts 
the effective gain bandwidth of the OP27 so that the integrator loop 
remains stable.
Higher slew rate integrators with low input offset using this technique 
are feasible with the faster JFET opamps that are now available.
LTSpice simulations will quickly show how such integrators behave and 
the necessity for the OP27 output divider.

The more complex integrator of the 3458A is an earlier attempt to 
achieve a high slew rate integrator with a low effective input offset.


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