No, the key part is an old small-signal mosfet.
I built a home-made picoamp meter and the Radio Shack binding posts leaked like anything. I had to mount them on a slab of polycarb.
Nah, a discrete FET diff pair. My Keithley 405 Micro-Micro-Ammeter uses a tube and has a 10 fA full-scale range. It takes a good couple of hours' warm-up time to get there.
$5 on eBay (plus probably $35 in shipping), back in the palmy days of
2009ish when you could get good boat anchors for 2 cents on the dollar, as opposed to the current 5-cents-ish.Cheers
Phil Hobbs
Teflon (PTFE) works better. It's less mechanically stable and prone to cold-flow.
A Cambridge Instruments we bought pins that came with a PTFE standoff which we could plug int a circular hole in a printed circuit board.
Putting the dynode chain for a photo-multiplier tube onto a printed circuit board would have been impractical without them. We did have one small printed circuit board made with an alumina-loaded Teflon substrate, but that was for a half-nanosecond-wide -pulse generator.
Did they "nail it" with the first small signal FETs and mosfets? Are there new ones that are any more sensitive or would subbing a modern version cause problems in old test equipment that expects olden-day characteristic?
What's the white material the components trapeze off? I was expecting the old nylon posts, not the "fancy" two-toned ones.
Ceramic terminal strips that were used in the 500-series tube Tektronix scopes. I scored a box of them at a flea market.
Google for Tektronix 545.
Not really. Tube-type vibrating reed/AC amplifier worked well, Cary electrometers used sapphire insulators to keep leakage down. Keithley 427's included some overdesigned input stages of another sort: they lowered the FET bias to a few tenths of a volt, so the input leakage is less trouble than wiring capacitive pickup.
The old solutions weren't any worse than modern technology, actually.
Maybe the Keithley FETs were enhancement oddballs (but ALD sells low-threshold MOS that would substitute nicely). Keithley also used lots of polystyrene capacitors, and those are no longer available (and wouldn't take reflow-solder temperatures). Teflon standoffs are a key ingredient, too; the sensitive current paths just barely can be routed in driven-shield cables, but printed wiring would kill the sensitivity.
Here's the 610 manual.
I was talking to a guy what worked at Western Electric. He said the capacitance of the polystrene caps were tuned or set with heating cycles as the parts circulated on giant wheels and some sort of heat source aimed at them. The other stories make it sound like WE was still ahead of places these days when it came to mass production of quality, consistent components of all types.
ADA4530 is < 1 fA input current typ. That's less than 6000 electrons per second.
To measure that, make a follower and connect to a 10 pF cap. Watch that for a few days.
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