General > General Technical Chat
100G -1000G Resistors
bdunham7:
--- Quote from: jwet on May 14, 2023, 09:23:49 pm ---Lab gear exists that can do all this but it has a lot of features that they don't need and costs about 4k and is not easy to source.
--- End quote ---
Just curious--what is the $4k candidate that can measure 2.5kVDC with a 1T input impedance and also measure current with 10fA resolution?
strawberry:
FET input capacitance fc = 1/(2*PI*1Tohm*10pF) = 0.0159Hz ~100s settling time
bootstrap preamp
jwet:
bduham7- The customer won't tell me what they think is the $4k solution until after the study! I suspect its some kind of semi custom Keithley solutions like a modified 6500 type meter- some of these have a 200T Ohm input Z but only a 200V input range for DC voltage (divider?). The old and still widely used solution in the lab is an electrostatic voltmeter mostly made by Sensitive Research- real old timey analog voodoo- they use a mirrored scale analog meter and come in a faux walnut box. They're actually somewhat plentiful surplus but should never leave a lab, very fragile. Input Z isn't measurable.
strawberry- that is the fundamental physics problem, you've got it- the C has be reduced by 1000x or more. Like everything in the modern world, people have been chipping away at these tough problems for a long time. Forget 1 pF, you need something that is .1 pf to start and that needs to be knocked down with circuit techniques. Passive or Active guarding (bootstrapping), nulling voltmeter techniques and other techniques are used. Look at the paper from 1965 that Tim Fox posted early in the thread- this adresses the speed problem pretty well. This is not a first order or even second of third order type of problem.
Thanks.
bdunham7:
--- Quote from: jwet on May 15, 2023, 07:23:28 pm ---bduham7- The customer won't tell me what they think is the $4k solution until after the study! I suspect its some kind of semi custom Keithley solutions like a modified 6500 type meter- some of these have a 200T Ohm input Z but only a 200V input range for DC voltage (divider?).
--- End quote ---
The only thing I haven't seen--and perhaps I missed it--is what they expect for precision. This doesn't sound like a $4K field device to me, but I suppose if it only has to have a single or even a few ranges and someone has been quite clever, it might be possible. Perhaps an entirely different sort of design would work, like an active differential system that floats up a more sensitive meter to within a few volts of the target. I'd want to see it to believe it.
jwet:
Their stated goals are 2% basic accuracy on volts full scale on volts and 20% at .1 pA current, but there are other conditions - like Mil Std 810- shock and vibration- this will allow it to be hand carried and shipped on planes in checked luggage and not require a transit case. Why not ask for the world is their philosophy- at least they're letting a paid market study contract.
I don't know if you've ever seen a nulling voltmeter- Fluke made them before sexy DMM's- they were kind of cal lab oddities. On one side of a null detector was a HV source with a 5 or 6 decade kelvin varley voltage divider and on the other was the unknown input. The null detector between the two drew very little current (zero if possible) often an electrometer was used. After you balance things out - you've measured your voltage- digitally! The null detector is also the current meter. This isn't really much different than how an opamp works really and you don't have to do it at high voltage if you scale the cancelling voltage source, etc. Long lever, short lever. The nice thing about the nulling is its like a inverting op-amp where the common mode voltage is near zero and the bias currents can be controlled with DC bootstrapping, like putting a resistor in the non inverting input equal to the feedback R only active. There are also more clever methods with multiple stages of this stuff.
I posted the resistor question because this is one issue that is key to in an house first order brute force build- can I get an off the shelf 1 T-ohm resistor that didn't suck? I suspected No and I am more sure now that the answer is no and brute force won't get it done.
I'll post something when the dust settles, knowing how these things go, it will take them 2 years to decide and after I've completely forgotten about it, They'll want to do the worst option in a month for half of what I estimated 2 years earlier....
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version