Input current is typically 0.1 fA at room temp. Noise and offset specs are impressive for a fA opamp.
0.1 fA is 625 electrons per second.-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Cute part--thanks.
Cheers
Phil Hobbs
0.1fA is what the graph shows. Spec is +/- 20Fa. What am I missing?
-- Boris
Typ vs guaranteed, I guess.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Well, that's getting down there. A fifty-year old Cary 31 electrometer has about 1E-18 A input current, so there's only another decade or two before solid state catches up. To vacuum tubes. Gold-plated elecrodes, cast iron case, sapphire insulators... they say a lot of the cost is in the packaging.
The chopper for that old amp operated with a 440 Hz (that's A above middle C, to a musician) tuning fork oscillator.
-- Wowie Zowie... Show your work. John Fields
ebay has at least one, but the shipping looks expensive.
I looked for a manual for the Cary but didn't find one. I think maybe a vibrating reed acted like a variable capacitor and modulated the input voltage into an AC amplifier. Is that how it worked? If so, a vibrating reed with a solid-state amplifier would be at least as good as the tube version.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
A changing capacitance in an electric field generates an AC current.
Analog Devices had a solid state competitor to the vibrating reed with a back-biased varactor bridge which worked much the same way.
I once thought about building one of my own, but figured that I'd have needed very well matched diodes in the bridge and gave up at that point.
The Cary would have needed a very low input capacitance amplifier, which would have been easier to realise with a valve/tube back then.
-- Bill Sloman, Sydney
Not as much as you might think: the smallest triodes (like the "acorn" style
57, 0.3pF G-H, 1.2pF G-P) and "electrometer" types are comparable to what you'll get from smaller semiconductors, especially with bootstraps.Likewise the follower performance of a triode or pentode isn't very good, due to the lower gain.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
I think there were tubes, used for tape head and mic inputs, that got close to 1 nV/rtHz noise levels.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Really? The only one of those I remember is the Philbrick P2.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
-- Oh, my... I think there were some that were twice that good, but I just can't seem to remember their part numbers. John Fields
TI briefly made an integrated opamp that had a varactor-modulator front end.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Yep, that's the principle. The cast-iron probe head, and selected tubes for low microphonics (and there were Nuvistor upgrades recommended) were necessary elements. I found one for the old lab, around thirty years ago, but the manual was a faded mimeograph.
See here:
AD310 (inverting) and AD311 (noninverting) were the part numbers. Those, Keithley 427 current amplifiers, and the Cary electrometer were all useful for accurate ion-chamber measurements of X-rays, gammas, C14 dating. The predecessors were gold-leaf electroscopes, used with stopwatches...
Cool, thanks.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
..are you suggesting that one can BUILD one using a tuning fork for the vibrating reed?
Seems that you are spot-on!
Cary did. The Cary 401 Vibrating Reed Electrometer is firmly embedded in the scientific literature.
-- Bill Sloman, Sydney
How to Convert Amps to Electrons Per Second (1:11)
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
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