50 Gigohm Resistor?

Some folks need to measure really low currents, like picoamps or even femtoamps. For instance, if you are peeking at a faraway star, you're only getting a few photons per second. Even after being enhanced by a photomultiplier tube, you're still talking picoamps. You need a few gizillion ohms as your amplifier's input impedance to get any measureable voltage from those few electrons.

If you need any parts like that and you're not working for J. Paul Getty, go to eBay and bid on any old Keithley electrometer, say the 6xx series. You'll probably snarf it up for $5.99 or so. Inside you'll find resistors from 1 ohm to 1 gigohm, all mounted on a teflon-insulated and gold-plated switch. Drat, I gave away my source for otherwise pricey components!

Regards,

A_H

Yeah, but probably like me, you have a high pile of them already. Actually, I only have one in its original condition, my technician disassembled the rest for a nice collection of high-quality parts.

--
 Thanks,
    - Win

I think I already did. Didn't I? Someone has elected to go with a higher value. I'm sure that you've already guessed why.

--
--
kensmith@rahul.net   forging knowledge

I stand corrected, and high voltage as well.

Hello Michael,

I remember those from the tube days but that's long gone (not the tubes though). Recently I could really have used an 18M resistor in a circuit but the client told me they had great difficulty sourcing even those and asked if I could get by with 15M. It was for battery monitoring and there every microamp counts.

Regards, Joerg

33M 1/4W 5% is as high as I have in my development stock. As SMT sizes have dropped, 10M is about as high as you can easily get from stock in 0603, 1% or 5%. Of course you can always use multiples in series, and often the manufactures will make higher values to order for the same price, just maybe a bit more relaxed specs, if you buy enough.

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
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I was using the same value for the same reasons for a photodiode amp, 33M. I ordered some high ohms from K&M Kobra many years ago, and still have some. I recall having to order many values at a fairly high cost. The trick about using them in some current measurments was getting them in low capacitance varieties.

greg

Hello Spehro,

They can do custom but I found that to be a hassle with production in Asia. They like everything off-the-shelf from half a dozen released vendors.

You can also take a trimmable resistor and do some aggressive meander cuts with the laser but only if laser trim is required anyway for other stuff.

Regards, Joerg

[who said]

Hi, Joerg. The thing that popped up in my mind was a memory from about a millennium ago, of me paging through the Allied catalog with almost as much lust as I used to page through the Sears Catalog, which, let's face it, was a lot! I used to drool looking at the Sears tractors with PTO options, and other farm stuff and household stuff, and, well, I don't know which part of the Allied catalog maps onto the Sears underwear section, but hopefully everyone's got the point by now) and in the order section, you select the shipping method, and one option was "Freight (rail or truck)" - yeah, back in those days Sears sold stuff so big it had to be shipped by train! ;-)

But the point of this post is, I remember looking at the ordering scheme for the Allied resistors, and there was a table of normalized values, with sort of a caption: "10 ohms to 22 megs."

Brought back memories. :-)

Cheers! Rich

--
(BTW, Joerg, could you include a little more of the "from" header in your
attributions? Thanks!)

Not necessarily seriously, when you've got 10M in a (an?) 0603 SMT, do you have to take heroic measures to deal with ordinary leakage?

Thanks, Rich

Hello Rich,

Wrong catalog. That would be Victoria's Secrets :-)

22M was popular as a resistor from gate to ground. But with the widespread demise of tubes those went away. I still have some.

Regards, Joerg

Nope, not at 10M. Well, unless you're trying for 0.1% or something like that (10G in shunt blows your error budget).

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Thanks for your service, Michael!

"Ken Smith"

** Nope.

You pompously alluded to a mysterious product.

Proves you are a liar or a wanker.

Or both.

....... Phil

We used to order 100M in 1k to 10k quantities for our instruments, at standard prices and delivery. Who knows, it's possible the distributor stocked them due to our steady, if small, orders.

--
 Thanks,
    - Win

They wanted response down to 0.1Hz?

--
 Thanks,
    - Win

Twarn't none o' that Victoria's Seecrit nonsense back then! Just good ol', practical, cotton and canvas foundation garments. ;-D Which, by the way, is _like_ VS, to a pubescent boy who was raised in the forest by a family of cows. ;-) ...

One day I'll look in my "random resistors left over from breadboard projects that I haven't bothered to sort yet" bin - I'm pretty sure I've got a couple of 1M; don't know if I have any higher, or why I would, for that matter. ;-)

Cheers! Rich

it.

Yeah, I was in the service pretty much around the same time as MAT was, but I did my Army-dodging the smart way: I joined the Air Force. ;-)

The closest I got to combat was one time in Thailand an airplane came back to the base with a .50 cal bullet hole in it. About 40 or so guys took a break from the shop and swarmed around the airplane, oohing and ahing - "Wow! They're really shooting real bullets over there!"

Then, they fixed the plane, and we all went downtown and got drunk and stoned and played with the pleasure ladies. ;-)

Before I went on that assignment, I had a few weeks or so of specialized training on the equipments that would be there, at Keesler AFB, Biloxi, MS. A coule of my buddies had already been to Thailand, so I asked them to tell me about it. They were rhapsodic. It made my eyes red just thinkin' about it. %-}

And, of course, one of them mentioned the khatoys, aka "ladyboys" - pre-op transsexuals (or maybe they have no intent of changing, but they can be as pretty as some girls - boy Thais have kinda feminine features anyway, which is probably why the straight ones take up kick-boxing and kung fu and stuff.) who offer blow jobs in the street for a couple bucks. One of the fellows quipped, "What the hell, a blow job's a blow job!"

Cheers! Rich

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A. Fire the IRS, and abolish the income tax
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D. Declare World Peace I.

I once designed a telemetry system to pick up the sound of the Saturn S1B rocket engine static tests, with pickups scattered about in the backwoods of Mississippi. We used a closed-loop-heated GR piezo-ceramic microphone and just dumped it into the gate of a jfet follower [1], no resistors at all, and got below 0.1 Hz. That was analog FM'd and fed over dedicated land-lines back to the MTF site. That's when I learned about Bessel functions and FM distortion in bandlimited systems, before I had that sort of stuff in school.

The locals liked to shoot the boxes off their poles, so there was a decent replacement market.

NASA thought we had an intermittent subsonic motorboating problem, but it turned out to be the mating call of male alligators.

Gosh, I *have* done a lot of silly things.

John

[1] a *silicon* jfet. TI was actually making some germanium jfets at the time, and they were pretty good for low-noise RF front-ends.