Does anyone have data on the modules (first from Philbrick and then from ADI) that used varactor diodes in the front end, energized through a transformer? I wonder how they compare in the 1/f range with solid-state choppers. Usually, people concentrate on the very low input current (+/- 10 pA for the Philbrick and +/- 10 fA for the later ADI) and huge input common-mode voltage (+/- 200 V). These amplifiers are very slow, with a 75 kHz unity gain frequency for the Philbrick. ADI's website has a link for the PDF datasheet from "obsolete" components, but it sends one to an announcement of brand-new parts. From an ancient ADI article, it looks like the 301 module had an input voltage noise of 70 nV/rt Hz and an input impedance of 1010 ohms at 25 C, so its voltage performance may not be as impressive as the current and common-mode performances.