Electronics > Metrology

Short teardown of a Keithley 2500 Dual Photodiode Meter

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Echo88:
I recently aquired an electrometer, as i needed one to build a high-accuracy voltage buffer for my Fluke 720A KVD. After looking for a fitting electrometer on ebay for a few weeks and missing an ebay-listing for a cheap Keithley 617 by about 5mins, i couldnt be bothered anymore and got myself a Keithley 2500 which nobody was interested in on ebay.
Its a Dual Photodiode Meter, essentially it consists of two simultaneous measuring electrometers and two voltage outputs to bias the connectable photodiodes. It has 1 fA-resolution on the lowest 2nA range with 6.5 Digits, 20mA max range, low voltage input burden: 4mV max (since its as feedback type amperemeter), max 100V voltage-output and GPIB-connectivity.
Therefore it fits my needs to precisely measure the input bias of the planned KVD-voltage buffer, but of course isnt as good as a Keithley 617 or other top-notch-electrometers.
With the triax-input shielded by aluminiumfoil i measured about 90fApp-noise at 10NPLC over about 10h, by averaging these measurements over 20 samples i get about 20fApp-noise.

Manual can be found here:  https://cdn.testequity.com/documents/pdf/manuals/2500-2502-M.pdf

Now to the teardown (finally a device which doesnt already has a teardown-thread on the eevblog-forum  :-DD ):

I was expecting to see old clunky rectangular reed-relays and long gigaohm-resistors in a separate metal-shield, instead a LMC6001 and minimal surrounding components do the job pretty well.
The LMC6001 is protected by J2611 JFETS connected as low leakage diodes and the whole range-switching on the board is done by a COTO-relay, AQV201A OptoFETs and MAX319 Analog Switches. The feedback-resistor is a 500M 1% resistor.
Interesting detail: the slots which isolate the LMC6001 and the triax-input-pin.

The first image shows the K2500 while measuring only channel 1 with 6.5 digits, because i was logging the noise-characteristic over GPIB. Normally it shows both input-channels on the display. The front is still a bit dirty and needs to be cleaned.

TiN:
Great job, keep it coming. I didn't even see one.

It's interesting combo of K2000 platform with dual-line matrix VFD front panel.

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