Electronics > Metrology

Project for standalone use of Keithley low current preamplifier.

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Now with electrometer I have ability to measure tiny currents, but yet don't have versatile programmable source for such, other than K6221. Gotta fix this.  :bullshit:

To resolve this issue, I thought to reuse preamplifier from Keithley 4200/6430 system together with my existing SMUs like 2400.
So here it is, preamp in it's glory:

It takes power and input current from the SMU (100uA - 1uA ranges) and can amplify or attenuate the output current by factor of 10 to 1000000, depends on selected range.
Beauty of it is bidirectional operation possiblity, so it can be used either to amplify DUT current for measurement, or attenuate current from SMU into DUT for low-current source.

Per patent block diagrams, both modes of operation are shown as concept:



DUT force and sense terminals are of course triaxial with guard and floating bootstrapped supply.

Problem is, I don't have 6430 to figure out interfacing protocol/connections between this preamp head and the SMU, so we need to figure out that part ourself. There is no service or detailed information about interconnect on Keithley documentation either, to my despair.  :scared:

After careful teardown, it become obvious why these little boxes are so expensive (ebay listing usually over 1K$ a piece  :-X).

Everything is packed very tight, with custom alumina substrate thin-film resistor networks on long legs.
These are used to set range of preamplifier. Little chip in PLCC socket is TMS370C702CWT microcontroller. There is Xicor 24164 I2C EEPROM on the back side.
Maxim chip is MAX509AC quad DAC in TSSOP.

Bottom side is busy as well:

I counted no less than ten TI LMC662 very low current bias amplifiers, bunch of NAIS V225N5's, three NAIS TQ2SS-L-5V latching relays.

Anybody have idea what are those SOT23-5's with marking?  :-//

A00 A
D3E "05 on side"

And SOT23-3's?  :-//

6X mirrored "L"
Top "CH on side"

Any help would be greatly appreciated.

I plan to restore full schematics of the thing, so we can adapt it for use with instruments other than rare Keithley 6430.
Board is 4-layer so it will be tricky. Either way I will have to desolder ceramic resistor networks to gain access on the front side.  :scared:

According to patent drawings, overall simplified schematics should resemble something like below:

So far I determined next pins on HD15 connector to SMU:

Pins 1,2,6,9,11 are not connected and unused.
Pin 10 is signal LO
Pin 4 is VCC power for I2C EEPROM/digital logic
Pin 12 is digital ground

Pins 3,5,7,8,13,14,15 are unknown.

says MAX8877 LDO regulator in 3.3 (G) or 3.6 V (H)
or FAN2508S adjustable for AAA

And RB411D for D3E (Schottky diode)

With best regards


Who other than TiN to take one of these apart.

That PCB design tho :o. The thing is like swiss cheese, more isolation slots than actual PCB. I could imagine there board manufacturer and assembler not being too happy about such a delicate and floppy board.

What i am curious is how the actual range switching is done, the diagram doesn't seam too specific about it.

Hello TiN

I followed your link to the pictures of the preamps. What you have is the preamp for the 4200-SMU and 4210-SMU which are cards that go into a 4200-SCS and the now 4200A-SCS.

The preamp for the 6430 is different enough from the 4200-PA that they are not interchangeable. This is second hand info to me. I have not tried switching preamps between a 6430 and a 4200-SMU. I have used both systems and they both have  incredible low current performance for both measuring and sourcing.

Well, as I already found out there is little uC, which may tell host "i am 4200 preamp" or "i am 6430 preamp".
I doubt there is much hardware difference between either flavour, unless you have photos of guts of "true 6430" preamp?

Anyhow, I don't have 4200 or 6430, so it all irrelevant. Question is if we can make this useful with "normal" SMU like 2400 and some Raspberry Pi in between to talk with PA's MCU?

I removed magic resistors to gain access on front side. Resistor with 9 pins measured as "19.81K isolated element + 200K+2M+20M+200M+2G+20G network", which follows block-diagram nicely.
However second ceramic resistor does not give any reliable readings. Maybe it's dead?


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