[volt-nuts] 3458A integrator

Randy Scott scottr9 at yahoo.com
Thu Nov 10 14:16:00 UTC 2011


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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