EEVblog #406 - Keithley 480 Picoammeter Teardown & Calibration

[ivan747]

Looks like an interesting machine. just found one on ebay for 69$ including the gpib board... on its way now ... 

This guy just wants $500 http://www.ebay.com/itm/Keithley-480-Picoammeter-/221161535990
How great are the chances the seller watched the eevblog video and thought it is time to make a killing?

After making the video on frequency standards, Dave says rubidium frequency standards are now 50% more expensive than before 

[CarlG]

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.

CarlG,  the manual I looked at showed 180µV as the lower functional limit independent of resolution. Is that what you find in practice. If you use it below 180µV how low does it go and what did you use to verify its accuracy below the 180µV. I am not disputing your info, I just may be interested in one if it works well in the single digit µV.

I have started a new topic and moved my previously posted answer to that one. Sorry for the confusion.

//C

[robrenz]

Thanks for all your help CarlG.  I am looking for something that goes below my Tek 7A22 differential amplifier plugin at 10µV/div. When you mentioned the 8921A I thought it might be the ticket for accurate very low power supply noise measurement. That is why I was pressing for the performance at single digit µV levels.  Thanks for all your help and I hope I didn't come across as argumentative.

[CarlG]

Thanks for all your help CarlG.  I am looking for something that goes below my Tek 7A22 differential amplifier plugin at 10µV/div. When you mentioned the 8921A I thought it might be the ticket for accurate very low power supply noise measurement. That is why I was pressing for the performance at single digit µV levels. 

No problem, I guessed that you were looking for something sharper.

Thanks for all your help and I hope I didn't come across as argumentative.

Oh no, not at all

[free_electron]

Mine came in yesterday.

Here's the guts WITH the GPIb installed :

so essentially the trap the BCD datastream wtween the 7103 and the 4511 , serialize it using some standard 4000 series cmos stuff ( counter + shifter ) , send it over two optocouplers (so the gpib is earth referenced but the machine input remains floating) where a motorola 6821 traps it. they use the same 6821 to do GPIb . i think this design predates the availability of real GPIb interface chips like the TMS9914 or uPD7210 or even motorola's 68488.

It uses an AMI made 6802 processor and a 2716 EPROM made by INTERSIL (i never knew those guys made eprom's ... )
Probably handcrafted assembly...

[EEVblog]

Mine came in yesterday.

Here's the guts WITH the GPIb installed :

so essentially the trap the BCD datastream wtween the 7103 and the 4511 , serialize it using some standard 4000 series cmos stuff ( counter + shifter ) , send it over two optocouplers (so the gpib is earth referenced but the machine input remains floating) where a motorola 6821 traps it. they use the same 6821 to do GPIb . i think this design predates the availability of real GPIb interface chips like the TMS9914 or uPD7210 or even motorola's 68488.

It uses an AMI made 6802 processor and a 2716 EPROM made by INTERSIL (i never knew those guys made eprom's ... )
Probably handcrafted assembly...

Wow, totally old-skool!

Dave.

[free_electron]

jep. totally old-skool. When real programmers wrote 1's and 0's straight into rom.

Not that fuddy duddy ladida erasable flash stuff programmed by 50$ wingpangpong programmers bought from ebay 

It's kinda funny. they use an intersil 71c03 duals slope controller with an 8052 analog front-end. not a microprocessor in sight in the whole meter section.
Want GPIB ? vooom. 6802, eprom , 6821 and still a pile of 4000 series to trap that data stream ... no wonder that gpib option was so expensive !

[SeanB]

Intersil started out with memories, I think I still have some around in a tube somewhere.

[JoannaK]

6502 chip + GPIB sounds a lot like those old Commodore Pet Floppy drives, except that those had two 02 chips.. One did bus protocol and another did disk-IO routines.

[ttp]

After making the video on frequency standards, Dave says rubidium frequency standards are now 50% more expensive than before 

I can certainly confirm that. Bought one days after Dave's video for about $AU36, after about 4 weeks emailed the seller that the item hasn't arrived. Next day I had my money back in Paypal account without any dispute (every other time I've complained on Ebay the sellers ask you to wait a bit in case od postal delays). Found another one soon but this time was just under $AU50, couldn't find it any cheaper.

[free_electron]

6502 chip + GPIB sounds a lot like those old Commodore Pet Floppy drives, except that those had two 02 chips.. One did bus protocol and another did disk-IO routines.

Not 6502 (mos) but 6802 ( Motorola ) completely different beast. 6821 is an io co troller

[SeanB]

Like the isolation transformer on that GPIB board as well, driven from the existing mains transformer secondary.

[free_electron]

nope. incoming power cable has a molex that plugs into main board. to install gpib : unplug molex from mainboard. install board plug molex from gpib on main board , plug power cable molex on gpib board.

so that little transofrmer on the gpib board is actually 110 volt to low voltage.