[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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