EEVblog #1017 - Enter The World Of Atto Amps

[EEVblog]

Dave tears down the Keithley 617 Electrometer, capable of measuring sub-femtoamp (attoamps!) resolution.




Video on the relay matrix:

[amspire]

Here is the service manual with schematics:

http://www.download-service-manuals.com/download.php?file=Keithley-5513.pdf&SID=9pl9n45mrf1eenarb06m74k6e0

[Cerebus]

There's an interesting design feature that got missed.

That input op amp and JFET have just a +/- 5V supply, labelled +/- 5V(B), but the feedback loop is driven by that +/-100V voltage source. That means that the op amp input can see +/- 100V which is enough to release the magic smoke from any run-of-the-mill op amp.

So how's the magic smoke kept in? The +/- 5V(B) supply has its '0V' centre tap actually set by the output so it's not +/- 5V from ground, but +/- 5V from wherever the input is at, or to put it another way, the input amplifier supply is bootstrapped off the output. There are two reasons for doing this, firstly you couldn't find suitable JFETs that combined both low leakage and high voltage ratings, secondly it boosts the input impedance of the amplifier substantially.

Also, if you look carefully, that 250G resistor Dave mentioned as connected across the input isn't across the input, it's connected (via R337 at 2.2k) to a common rail labelled 'B', which is the bootstrap centre rail (or if you prefer, the composite amplifier output - see the 'B' connection near K301). That 250G is in parallel with the feedback loop, not across the input. That's the perils of a circuit diagram that defies convention and slaps the 250G feedback resistor down exactly where you'd conventionally draw a resistor that bridges the input.

Here's a, deliberately didactic, question for anyone who's interested in how this particular circuit works. What are R340 and R341 doing? They are respectively, 4.02k from the +5V(B) supply to the drains of the input JFET pair, and 402R from that junction to 'B'.

[Kryten 2X4B]

For anyone interested in making the type measurements of the type that the Keithley 617 was designed for you should get a copy of the Low Level Measurements Handbook

http://www.tek.com/sites/tek.com/files/media/document/resources/LowLevelHandbook_7Ed.pdf

I have no affiliation with Tek/Keithley, just a fan boy.

[LapTop006]

I picked up a 614 a few months back (and then had to repair it as the battery pack got loose in shipping), interesting how much more complex the 617 is.

Digikey have the Pomona 2-lug triax cable available 4725PO-ND, although it's not cheap. Sadly the 2-lug to 3-lug triax adapters seem to longer be readily available.

[gardner]

get a copy of the Low Level Measurements Handbook

Thanks for this.  It's interesting reading.

the electrometer can measure the voltage on a 500pF capacitor without significantly discharging the device

Dave?  I think you owe us some demos on this sort of stuff.

[grouchobyte]

For anyone interested in making the type measurements of the type that the Keithley 617 was designed for you should get a copy of the Low Level Measurements Handbook

http://www.tek.com/sites/tek.com/files/media/document/resources/LowLevelHandbook_7Ed.pdf

I have no affiliation with Tek/Keithley, just a fan boy.

This is a very well written document....a keeper. Required reading for all EEs.
Thanks for posting the link! Reminds me of the good old days at TEK when they regularly published gems like this.

[Kryten 2X4B]

I have a 619 Electrometer/Multimeter and two 220 current sources, like 617 they have the older 2-lug triax. Some time ago I converted these to the 3-lug type as cables are readily available and you can't plug in a standard BNC. The most cost effective replacement connector is the Pomona 5219 (Digikey 501-1312-ND or Mouser 565-5219). I've also used the 5219 for calibration fixtures for these instruments and this part has also been mentioned on other threads in the forum.

[EEVblog]

Also, if you look carefully, that 250G resistor Dave mentioned as connected across the input isn't across the input, it's connected (via R337 at 2.2k) to a common rail labelled 'B', which is the bootstrap centre rail (or if you prefer, the composite amplifier output - see the 'B' connection near K301). That 250G is in parallel with the feedback loop, not across the input. That's the perils of a circuit diagram that defies convention and slaps the 250G feedback resistor down exactly where you'd conventionally draw a resistor that bridges the input.

Dang, missed that!

[EEVblog]

This is a very well written document....a keeper. Required reading for all EEs.
Thanks for posting the link! Reminds me of the good old days at TEK when they regularly published gems like this.

This is not a Tek document, it was written by Keithley before they got aquired, it's the "bible" of low level measurements.

[EEVblog]

the electrometer can measure the voltage on a 500pF capacitor without significantly discharging the device

Dave?  I think you owe us some demos on this sort of stuff.

Yeah, it needs proper setup and cables though. Die cast boxes etc

[Kryten 2X4B]

Even just for "simple" high value resistance measurement you may need a guarded shielded resistor, so you have a resistor in a die cast box connected to a guard potential and that box is inside another die cast box which is at the shield potential. The two boxes are insulated for each other and you have a triax connector on the outside, not to mention the cable to the instrument. There is some time, effort, and cost to set this up. I'm guessing that Dave doesn't have the time to go down that rabbit hole.

[LapTop006]

the electrometer can measure the voltage on a 500pF capacitor without significantly discharging the device

Dave?  I think you owe us some demos on this sort of stuff.

Yeah, it needs proper setup and cables though. Die cast boxes etc

I actually have the proper Keithley test enclosure for that stuff, just sitting at work over in Pyrmont.

[EEVblog]

Even just for "simple" high value resistance measurement you may need a guarded shielded resistor, so you have a resistor in a die cast box connected to a guard potential and that box is inside another die cast box which is at the shield potential. The two boxes are insulated for each other and you have a triax connector on the outside, not to mention the cable to the instrument. There is some time, effort, and cost to set this up. I'm guessing that Dave doesn't have the time to go down that rabbit hole.

Not for this video.

[Smith]

I know the video is relating to amps, but the device is also used a lot for voltages and high resistances. It's an electrometer. Perfectly for measuring voltages without loading your circuit on high resistance aplications. Also good for measuring resistances upto hundreds of TeraOhms. Third big plus are the low burden voltages on all ranges.

About the meter itself, don't forget the low leakage caps over the feedback resistors. Probably some polystyrene caps. Remember, their resistance should be way bigger than the feedback resistors and stable ofcourse.

Btw making a resistor enclosure isn't that big of a deal unless you go upto hundreds of TeraOhms. Mine even uses BNC because of costs. Only hard thing is making some easy to use internal connection to the resistor. I used some scrap teflon on plastic standoffs and some push to insert connector.

[eV1Te]

You can't use any triax cable, you need a low noise cable, preferably an ultra low noise cable (there is a difference and maybe a video in it self) 
Make sure it's the 2 lug version for your instrument as well.

Also the burden voltage should be close to zero (less than 1 mV on all ranges, on newer Keithley it's less than +-10 uV on the lowest range if I remember correctly), could be a nice comparison to your micro current box. Too bad you will need another electrometer to measure the burden voltage... unless you build your own high impedance amplifier with a lmc6001 or similar.

There is a very good reason why you need such low burden voltage, because otherwise the burden voltage would cause a leakage current in the triax cable and especially the connectors between guard and input terminals. If guard is always at same potential as input then you won't have a current flowing.

[HighVoltage]

Very nice video !
I have the Keithley 614 and have been looking for a good 617 for a while.
Like usual when Dave makes a video about some test equipment, now the prices will probably double on ebay.

[kridri]

According to a post from Free_Electron the matched jfet is a 2n59xx with presumable a 2n5904. These can be bought at Linear System as a LS830.

They use specially selected matched jfets in TO71 6 pin metal body. The type number is  2n59xx something. I have them. i think it's 5904

There is only one company that makes em and they are only made on demand. I bought the only 4 they had in stock a few years ago. 50$ a pop...
i since used 2 of them to fix broken electrometers.

edit: Source of type number http://www.linearsystems.com/lsdata/files/product_selection_guide.pdf

[alm]

From the text at 1:58: Fempto. Is that related to the Jiga prefix? Looks to me that one of the four contacts for the center pin is damaged in the input connector (see at 4:40). Possibly by some idiot trying to force a BNC connector into it. But you might want to replace it with a three-lug version anyway so you can use the more commonly available cables and interface with modern equipment.

Dave, how did it take a low-current nut like you so many years to acquire an electrometer? They are available on eBay for affordable prices quite regularly, and are not particularly heavy to ship.

This is a very well written document....a keeper. Required reading for all EEs.
Thanks for posting the link! Reminds me of the good old days at TEK when they regularly published gems like this.

This is not a Tek document, it was written by Keithley before they got aquired, it's the "bible" of low level measurements.

I am pretty sure grouchobyte was referring to publications like the Tektronix Concepts series that discussed their technologies in more detail than any other publication. For example, try to find a discussion of scope probes with more technical information than their Oscilloscope Probe Circuits book.

[4CX35000]

I have a Keithley 617 which I bought damaged with MANY internal problems. It seemed to have been opened by someone with little or no knowledge of electronics who decided to cause as much damage as possible.

The damage done.

1. The main transformer was bent forwards as in the video and one of the brackets had broken off, this required reattaching to the transformer casing. The transformer is mounted on the main circuit board.
2. The Triaxial connector on the rear was the wrong type for the leads I already had and was damaged. Keithley supplied a Two lug connector which is similar to a BNC connector, but modern instruments use a three lug connector which is intended to stop someone forcing a BNC connector on and damaging the internals of the connector.
3. The triaxial connector coax wire which connects the triaxial connector to the measurement board inside the unit had also been badly damaged requiring replacement. I did this by botching the replacement using some dual core coax as I was unable to source the original or replacement.
4. The measurement board on top with the shielding on top had been removed in a violent manner damaging the plastic supports.
5. The front display/control panel is mounted on a circuit board with a flat wire DIL connector which connects the control panel to the main circuit board had also been damaged.
6. The power switch needed replacing. Damaged by violently removing the front panel control panel.
7. Two transistors on the main circuit board and used for the 100volt DC current source had been broken off and another transistor nearby replaced.
8. Replaced some of the larger capacitors on the main board near the front. Damaged.

In all I have managed to get the instrument working, but it does need a little more TLC to finish off the repairs.

[Dr. Frank]

Also, if you look carefully, that 250G resistor Dave mentioned as connected across the input isn't across the input, it's connected (via R337 at 2.2k) to a common rail labelled 'B', which is the bootstrap centre rail (or if you prefer, the composite amplifier output - see the 'B' connection near K301). That 250G is in parallel with the feedback loop, not across the input. That's the perils of a circuit diagram that defies convention and slaps the 250G feedback resistor down exactly where you'd conventionally draw a resistor that bridges the input.

Well, I directly thought, that this 250GOhm resistor would be used for the usual compensation of input bias current of the JFET. It has nothing to do with the feedback loop, as it's neither inside, nor parallel to the feedback loop.
Its purpose can be derived from the calibration procedure:

The +/- 5V from the potentiometer R348 is divided by 1000 to +/- 5mV (by double divider R339/R354 and R338/R337. This voltage drives an additional current into the inverting input node by R332 = 250GOhm, i.e. +/- 20fA. This bias compensation calibration limits the total input bias current to +/-1.5fA.

These 20fA are also the maximum allowable leakage current for the JFET pair Q308.

Very interesting circuit and video.

Frank

[(*steve*)]

Sadly the 2-lug to 3-lug triax adapters seem to longer be readily available.

I've been after a 2 lug male to 3 lug female for a while.

Beware of sites that claim to have availability but don't.  *Cough* Element-14 *Cough*

[pmcouto]

Keithley 6172 is a 2 slot male to 3 lug female Triax adapter.
But it’s not cheap…

http://pt.farnell.com/keithley/6172/triax-adapter-2slot-male-3lug/dp/2672724

I bought one of these adapters from Farnell a few months ago.
According to the information on Farnell's website today, they have 14 units in stock.

Regards,
Pedro Couto

[(*steve*)]

According to the information on Farnell's website today, they have 14 units in stock.

I asked to be informed when they were in stock.

I received an email from them saying they had them in stock in Australia.

Their web site said they had 16 in stock.

I ordered one.

I am informed there is a 5 week delay whilst they source them from Tektronix.

Their web site still said they had 16 in stock.

[mk_]

Sadly the 2-lug to 3-lug triax adapters seem to longer be readily available.

I've been after a 2 lug male to 3 lug female for a while.

Beware of sites that claim to have availability but don't.  *Cough* Element-14 *Cough*

something like this?
http://www.helmut-singer.de/stock/-148524350.html

They do have a lot of triax-stuff, don`t know if they fit.