> The "basic" HP patent is 4357600: > <http://www.google.com/patents?id=vCUxAAAAEBAJ> > I think this is the HP3458A. > There is a later HP one 4951053 which refines it with a greatly improved > switching technique<http://www.google.com/patents?id=INImAAAAEBAJ>. But > the 3458A does not use it AFAIK. Well, only the 2nd HP patent is for the 3458A, as the author Wayne Goeke definitely was one of the engineers designing its A/D. (See HP Journal 4/1989). But there should be several others, concerning cancellation of charge injection effect, as described in the journal, and realized by the ASIC they have designed. It's quite interesting, that no other instrument, wether DVM, or calibrator (i.e. Fluke 5720A) comes near the linearity of the 3458A (0.02..0.05ppm of input), after all those years. And as far as I overview the situation, the 3458A and the 5720A are today the only instruments having complete auto calibration from 2 or 3 references only. Btw: There's now the full Fluke 5700/5720A service manual including schematics hosted on ko4bb.com, saved just before Fluke has violently cut off the most interesting part of this manual. It's an interesting read about the 13V reference hybrid and the D/A. Frank

Frank Stellmach <frank.stellmach@freenet.de> writes: >> The "basic" HP patent is 4357600: >> <http://www.google.com/patents?id=vCUxAAAAEBAJ> >> I think this is the HP3458A. > >> There is a later HP one 4951053 which refines it with a greatly improved >> switching technique<http://www.google.com/patents?id=INImAAAAEBAJ>. But >> the 3458A does not use it AFAIK. > > Well, only the 2nd HP patent is for the 3458A, as the author Wayne > Goeke definitely was one of the engineers designing its A/D. > (See HP Journal 4/1989). Hi Frank, I think you are right there. The reason I said that was the article you refer to shows the switches in the arrangement from the first patent (before the input resistors). The whole point of the second patent is to put them afterwards. But looking at the circuit in the CLIP (thanks for sending it to me) shows it may be as the second patent after all. So the article does not reflect the actual circuit in this detail. > But there should be several others, concerning cancellation of charge > injection effect, as described in the journal, and realized by the > ASIC they have designed. The fluke patents claim to improve this aspect too, by fiddling with the switch sequencing. > It's quite interesting, that no other instrument, wether DVM, or > calibrator (i.e. Fluke 5720A) comes near the linearity of the 3458A > (0.02..0.05ppm of input), > after all those years. > And as far as I overview the situation, the 3458A and the 5720A are > today the only instruments having complete auto calibration from 2 or > 3 references only. Yes, and they can do this as a direct result of the amazing linearity, as I understand it. > Btw: There's now the full Fluke 5700/5720A service manual including > schematics hosted on ko4bb.com, saved just before Fluke has violently > cut off the most interesting part of this manual. > It's an interesting read about the 13V reference hybrid and the D/A. Thanks, I'll have a look. John -- John Devereux