Author Topic: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration  (Read 27951 times)

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Offline EEVblog

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EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« on: January 01, 2013, 09:05:21 pm »


Dave.
 

Offline CarlG

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #1 on: January 01, 2013, 11:08:04 pm »
Yeah, you get a real big  :-+

I love that you show all of these "good old" instruments. You can get a lot of old instruments having features that may be hard to find in modern equipment. One of the first things I got added to the lab when I started my current work, was a Fluke 8921A true RMS voltmeter, first design late in the 70's I think (manual dated Oct -78), but I see there are newer versions (still 8921A).

The 8921A is 4 1/2 digits, has up to 20 MHz bandwidth, 700V max input, and resolution down to 0.1 1 uV on lowest range, and also selectable dB scale. Great stuff! Very useful for measuring residual noise on (switching) power supplies for instance.

//C
« Last Edit: January 02, 2013, 09:01:37 am by CarlG »
 

Offline SeanB

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #2 on: January 01, 2013, 11:48:50 pm »
I would say the power switch had dirty contacts. Looking at the insides you can see corrosion products on the back of the gold plated contacts inside the shield, so there was likely an exposure at some time to an atmosphere that was humid and contained thiols ( probably downwind of a coal power plant or industrial plant that used coal or a oil refinery that was processing heavy crude oil or near a harbour) that contaminated the boards. They would benefit from cleaning with a contact cleaner, along with the range switches, as you can see on the video they are noisy when operated ( decimal point flicker when range changing and flicker on power on) but have wiped themselves clean after a few operation cycles.
 

Offline McMonster

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #3 on: January 02, 2013, 01:34:00 am »
The strange cable mounting is there so you can nicely wind the cable around the case stands. I don't like equipment with permanent mains or any other cables sticking out of it, and I carried a lot of it at work. Really annoying unless you have a third hand to hold the cables, otherwise it is easy to trip over the cable.
 

Offline Bloch

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #4 on: January 02, 2013, 01:49:25 am »
I am looking at the schematic and there must be some thing i do overlook :-[

How is it possible to have so low burden voltage then the resistor array go from 1K to 99M ?
 

Offline robrenz

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #5 on: January 02, 2013, 02:15:20 am »
Great video Dave! As always I learned something new :-+

Offline CarlG

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #6 on: January 02, 2013, 03:13:10 am »
I am looking at the schematic and there must be some thing i do overlook :-[

How is it possible to have so low burden voltage then the resistor array go from 1K to 99M ?

Well....here's a clue: Q104 + U104 acts as a difference amplfier. So what voltage do you get at the gate of Q104-A?

//C
 

Offline SeanB

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #7 on: January 02, 2013, 03:20:19 am »
I am looking at the schematic and there must be some thing i do overlook :-[

How is it possible to have so low burden voltage then the resistor array go from 1K to 99M ?

The resistor arrays are the feedback loop of an opamp arranged to invert. The input is connected to this virtual earth point, and the opamp tries hard ( via open loop gain) to keep this voltage equal to ground ( non inverting terminal) by sinking/sourcing current via the resistors in the feedback loop. This means any input current is translated to a voltage on the opamp output  while the input is kept very close to ground. The meter then simply shows this output voltage suitably scaled and you interpret it as a current.

 

Offline bitwelder

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #8 on: January 02, 2013, 03:42:39 am »
The mysterious empty DIP connector near the big IC seems to be another diagnostic connector (check J1002 on the right size of diagram) with basically all the digital communication with the display board available on its pins.

Looking at the manual, the apparently unused upper ground clip maybe has a real function when the battery pack gets installed.
 

Offline quarks

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #9 on: January 02, 2013, 03:48:05 am »
Hello Dave,
I just watched this very nice video  :-+ and I hope you will do the video you are talking about accurate low value resistance measurement (45:40 min.), because that is what I am just evaluating myself.
 
BTW Happy new year to all.
« Last Edit: January 02, 2013, 04:08:25 am by quarks »
 

Offline Bloch

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #10 on: January 02, 2013, 03:56:21 am »
Ok I see.


But only if S107 is always closed. So my question is now that is S107 funktion?
 

Offline Bloch

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #11 on: January 02, 2013, 04:19:46 am »
But only if S107 is always closed. So my question is now that is S107 funktion?


Zero check.


Well....here's a clue: Q104 + U104 acts as a difference amplfier. So what voltage do you get at the gate of Q104-A?

I am not sure my FET knowledge is not that good.

But the Q104 was a part off the difference amplifier ... right ?

That are Q102+Q103 doing ?
 

LLR1967

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #12 on: January 02, 2013, 04:47:02 am »
Bloch: "That are Q102+Q103 doing ?"

If you're asking what the purpose of Q102/Q103 is, they are used as low-leakage diodes for input protection.
« Last Edit: January 02, 2013, 04:48:38 am by LLR1967 »
 

Offline CarlG

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #13 on: January 02, 2013, 05:00:37 am »

But the Q104 was a part off the difference amplifier ... right ?

Right. Q104-U104 act as an OP-amp. So if the gates of Q104 were the inputs to an ideal OP-amp, what is the voltage between the gates?

//C
 

Offline CarlG

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #14 on: January 02, 2013, 05:06:25 am »
Bloch: "That are Q102+Q103 doing ?"

If you're asking what the purpose of Q102/Q103 is, they are used as low-leakage diodes for input protection.

And that's also a clue to how the circuit operates. As stated above, Q102/Q103 are for protection, not needed for the basic operation. Maybe easier to understand if you remove them and short the 1M?

//C
 

Offline Bloch

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #15 on: January 02, 2013, 06:01:54 am »
Thanks guys

I have the flu today not a excuse  :P  just saying.

In 1mA range selected

Gain = R110/R107 = 1M / 1K = 1000
Vout =  Gain * (Vin+ - Vin-) =  1000 * (0 - Vin-)

Not sure that the Vin will be. `

But Vout will be inverting voltage. That will cancel out the burden impedance..... Am i close  ;D
« Last Edit: January 02, 2013, 06:08:07 am by Bloch »
 

Offline Rick

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #16 on: January 02, 2013, 06:33:19 am »
Time to buy a "for parts or repair" type Keithley 480 and try a repair...
At this trend I shall financially ruin myself.
I simply love those repair sessions. It's crazy how many devices I threw away whereas they could have been fixed very simply...
I have acquired a dead Fluke 8800A, that I shall try to resuscitate with my "big" repair experience when it arrives. That will be my first 5 1/2 digit and bench multimeter, if it works. Fluke not Keithley of course.
« Last Edit: January 02, 2013, 06:41:31 am by Rick »
 

Offline CarlG

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #17 on: January 02, 2013, 06:43:03 am »
Thanks guys

I have the flu today not a excuse  :P  just saying.

In 1mA range selected

Gain = R110/R107 = 1M / 1K = 1000
Vout =  Gain * (Vin+ - Vin-) =  1000 * (0 - Vin-)

Not sure that the Vin will be. `

But Vout will be inverting voltage. That will cancel out the burden impedance..... Am i close  ;D

Yep, you've got it  :-+ Good job! Have a look at Fig 2-3.

Q104 and U104 will ideally sink/source current in the FB resistor in order to keep Q104-A base at the same potential as Q104-B, which happens to be at ground. I.e the instrument input will be at ground potential. The burden voltage will mainly depend on the matching of the Q104 pair and Q104 source resistors (U104 trimmed to zero readout by R110). The offset voltage due to load/temperature dependent leakage current in Q102, Q103, and Q104-A via R110 is more or less negligible (a wild guess ~10pA * 1M << the specified 200uV).

//C
 

Online sorin

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #18 on: January 02, 2013, 07:13:38 am »
what is the part number of Q102-Q103 ?
 

LLR1967

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #19 on: January 02, 2013, 07:26:28 am »
I have acquired a dead Fluke 8800A, that I shall try to resuscitate with my "big" repair experience when it arrives. That will be my first 5 1/2 digit and bench multimeter, if it works. Fluke not Keithley of course.

I have one similar to that one at work (it's been there for >30 years), and I fixed it by replacing, as one might expect, all of the electrolytic capacitors. I was told that A.C. was an option on that meter; it has the switch, but it is locked and can't be moved. Also, in my case, the meter wasn't dead. It was very unstable.

Larry
 

Offline Rick

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #20 on: January 02, 2013, 07:30:40 am »
I have acquired a dead Fluke 8800A, that I shall try to resuscitate with my "big" repair experience when it arrives. That will be my first 5 1/2 digit and bench multimeter, if it works. Fluke not Keithley of course.

I have one similar to that one at work (it's been there for >30 years), and I fixed it by replacing, as one might expect, all of the electrolytic capacitors. I was told that A.C. was an option on that meter; it has the switch, but it is locked and can't be moved. Also, in my case, the meter wasn't dead. It was very unstable.

Larry

Ok.
Mine is completely dead I think:
http://www.ebay.com/itm/380444720310?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649
Note the shipping cost is more than twice the cost of the item. So I bought it for fun really.
A dead transformer, a blown fuse or something in the power supply, or may be an empty lunch box (if it has cannibalized)... We shall see.
« Last Edit: January 02, 2013, 08:12:12 am by Rick »
 

Offline Bloch

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #21 on: January 02, 2013, 07:48:15 am »
what is the part number of Q102-Q103 ?
ITE4392

Well i am not 100% sure ! I did search for TG-77 and found out that is a Keithley Part Number in an other Keithley see here http://physics.ucsd.edu/neurophysics/Manuals/Keithley/Model%20227%20Current%20Source
« Last Edit: January 02, 2013, 07:55:13 am by Bloch »
 

Offline CarlG

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #22 on: January 02, 2013, 09:14:44 am »
what is the part number of Q102-Q103 ?
ITE4392

Well i am not 100% sure ! I did search for TG-77 and found out that is a Keithley Part Number in an other Keithley see here http://physics.ucsd.edu/neurophysics/Manuals/Keithley/Model%20227%20Current%20Source

At least it makes sense. I've seen 2N4392 = ITE4392 being used this way before.

//C
 

Offline nixxon

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #23 on: January 02, 2013, 09:41:04 am »
There is a Keithley 440 Picoammeter for sale on ebay that obviously needs som tweaking:
http://www.ebay.com/itm/KEITHLEY-440-DIGITAL-PICOAMMETER-/360550605394

How does the 440 compare to the 480?
 

Offline NickS

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Re: EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration
« Reply #24 on: January 02, 2013, 11:27:37 am »
Why can't you compensate for the burden voltage?

You can measure the burden voltage, pump it in to a formula and you can tell what the output is supposed to be.
So why don't ammeters do that and adjust the output number?
 


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