100G -1000G Resistors

[bdunham7]

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. 

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]

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 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....

[jonpaul]

get a Sensitive Research  Electrostatic Voltmeter 

Avail as surplus, $40..250, in ranges 5kV ...100 kV, zero current.


Jon

[jwet]

Need quantity 150 and long term support for field service people.  I mentioned this above as an electrostatic voltmeter- a standard solution for 1/2 the problem but also pretty fragile for a tech servcing airport equipment.

[bdunham7]

not require a transit case.

Now they're getting silly.

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. 

Yes, that is what I was thinking of, except that if they are always reading the same voltage you could simplify the design a bit.  It's not hard to design a device to put out a fixed or adjustable 2-2.5kV by using a lower-impedance feedback design, then null your measured voltage to it.  Insulation testers come to mind as a source.   I still have no idea how you could produce a tested and certified product like this in that low of a volume for well under $4k.  It's good they're paying you upfront.

[jonpaul]

There are small cased ESVM with 10..20 kV ranges for field use. They are rugged and are on ebay perhaps $40..100 ea


Jon

[jwet]

bunham7- I agree with everything you and others have said.  I even think that the customer knows its semi impossible.  They just can't buy something this expensive without a pretty good paper trail.  I think I'm a pawn probably but its ok- its interesting and has low risk- the only deliverable is a report.

The voltage nulling technique is looking pretty good and I think I get the electrometer for free- I'm supposed to show them all reasonable solutions with analysis of pros and cons.  One nice second order thing about the voltage nulling is that it reduces the effects of voltage coefficient as there is no voltage across the big input R.  I have found some very high value R's but the VCR's are such as to create huge errors (30%+!) if not cancelled with this nulling.  The best are 250 ppm/V- time 2kv gives you 50%!  This is not a brute force problem.

jonpaul- thank you, I have one in a faux wood box but I'll look on line at the ruggedized ones.  They do use things like my model now but they ship them to the techs and the techs ship them back to a cal lab on return for surveillance and checks.  Thanks

[TimFox]

Back in graduate school, we needed to apply about 10 kV to a test object and measure the (small) current through it.
We floated a battery-powered Keithley electrometer in a purpose-built acrylic enclosure, with acrylic rods to rotate the control knobs.
The case of the electrometer was connected to the power supply, so that any leakage or corona from the rectangular case did not flow through the electrometer's ammeter circuit.
To protect the input of the electrometer (a vacuum-tube input unit, with reasonable input voltage rating), we added an appropriate diode network across the input.

[exe]

some very old NOS Russian/Bulgarian beauties from early cold war, probably 70 years old.

Oh, those can have very bad tempco. Hope it doesn't matter for your circuit. Afaik, somebody evaluated them for a (pico-)ammeter application, but I can't find the thread. Also worth cleaning and drying it properly, any surface contamination ruins precision.

[Someone]

not require a transit case.

Now they're getting silly.

It ends up with the "product" they order being a branded pelican case factory fitted with the actual product inside. Several employers have done exactly this.

[jwet]

Tim Fox- That's pretty cool.  My first job was at General Atomics in San Diego in 1982.  As a new grad, I was supposed to work in three groups for 6 months each- they called it the Tech Grad Rotation program.  They were an aging company with a lot of Manhattan project type guys in the hierarhcy.  They were trying to get some young blood and were hiring a few dozen grads a years from good engineering schools.  General Atomic is still around and thriving- they did the  Predator drone- (good story).   My first rotation was in an instrumentation group that did radiation monitoring equipment for Nuclear Power Plants- our system was installed in most Westinghouse PWR's around the world- over 100 sites with 100's of monitors per system, all networked together in 1982- super state of the art-  software, digital, analog, physics and instrumentation problems.  I loved it and couldn't believe I was getting paid to go play there everyday.  I was in the Detector Physics group, working on analog front ends for all different kinds of radiation detectors- GM Tubes, Ion Chamber, Fission Chambers, PMT/Scintillator, CdTe-Solid State- the gamut.  I had some great old time mentors that had seen it all.  There were also a lot of commercial Nuclear Power guys mostly brought up through the Navy.   My next rotation was supposed to be to their Fusion Research Reactor called Doublet 3- a big Tokhamak that was hot shit for its time.  About a month before I was to transfer, I took a big tour and saw these guys operating instruments behind 1/2" lexan blast shields with3 foot lucite rods to turn scope knobs, etc like you're talking about.  I decided to stay put in my group and go out of the Tech Grad deal.  I've done a lot of ion detector and PMT work up to about 2 KV and have been on projects with 200 KV X-Ray power supplies.  That stuff gets weird.  I settled down to precision data acquisition and mixed signal stuff and after Nuclear dried up went to work for Maxim for about 25 years in their standard products group defining products like Analog Swtiches, Op-Amps, etc.  A lot of fun.  Retired a while back taking little consulting projects that come along.

[ZhuraYuk]

Tried to get similar 1T resistors to replace them in some old electromters. There are few options for cheap in stock:

https://mou.sr/4a3v7Sg   Mouser 1T Ohm 500ppm 5%

https://www.aliexpress.com/item/1005006645803409.html There is a lot of new Chinese high ohm resistors, all seems to be made of same material 200ppm.

https://twjohmresistor.aliexpress.com/store/912607391 This shop has large choice of Giga Ohm resistors to upgrade yuor old electrometers.