Electronics > Metrology
Project for standalone use of Keithley low current preamplifier.
TiN:
My own Keithley 6430 fA-SMU repair update. Thanks to TheSteve, xDevs.com also acquired own broken 6430 unit (w/o PA), with diagnosis "no output". :scared:
Received very well packed device, leaving no chance to easy damage of fragile instrument:
First smell check didn't reveal any issue, so turning on confirmed display operation, responsive buttons and only factory calibration, with count = 1.
Calibration dates as received and VFD segments brightness are good. Looks like box was never calibrated after factory. ^-^
Check with DMM at the output indeed confirmed seller's diagnosis, no output for either current or voltage. Four screws and 5 minutes later:
Analog PCB is based from Keithley 2400 SMU, with exception of 1.05A range. Good old Keithley 2400 dual-slope integrating ADC, which is in fact Keithley 2000 ADC + additional digital stuff to control onboard I-DAC and V-DACs and handle source-meter functions like sweeps, guarding, etc. Sadly reference is not LM399, but a compensated zener diode.
Photo of K2400 analog PCB shown on next image to refresh memory.
There are additional modifications for external 6430 very low-current amplifier. These include isolated 1W DC-DC "Murata/Power Convertibles HL01R05D05Z":https://xdevs.com/doc/Keithley/6430/pdf/tdc_hl01rzc-55873.pdf and extra optocouplers for SPI interface to 6430 preamplifier. DC-DC specified for 100mV[~pk-pk~] max noise in DC to 10MHz bandwidth and isolation capacitance just 25 pF.
Closed look revealed the problem :-DD:
Look closer in top right corner. See anything obvious? It can't be only that, right? It would be too easy. :-/O
But yes, after restoring connection to analog board power supply input indeed made unit fully functional again.
So much for repair in The Signal Path style :popcorn: . I almost feel violated, that we had no chance to dig into SMU analog board troubleshooting :-DD
Since we in here anyway, let's remove analog board and see on the digital/power supply boards located under.
Keithley 6430 using special multicore planar transformer assembly on 16-layer PCB to provide low-noise high-isolation power to analog board. Anybody knows what is all this magic about?
I'd guess this transformer design used to provide low leakage in power supply DC/DC that feeds analog board output stage?
Standard Keithley 2400 does not have any of this fancy stuff, but overall design is very similar.
Otherwise this is just usual Keithley SMU and it works exactly same way (minus the 1.05A current range) as 2400 when Keithley 6430 amplifier is not connected.
There are no front terminals available with K6430, so output/input provided at the rear ports only.
Quick calibration and checks for 10V and 1V source:
Noise is not great, so perhaps we should pimp this unit with LTZ1000A reference, aye? (Not that K6430 is designed to be low-noise voltage source, it's performance marvels lie in picoampere current domain with preamplifier head).
Calibraiton of the box is "simple", with help of 3458A and set of 100Meg, 1Gohm, 10Gohm and 100Gohm resistors (usually part of Keithley 5156 kit, which I do not have (yet?)).
I didn't tried it with PA yet.
antintedo:
--- Quote from: TiN on July 19, 2019, 05:04:10 am ---Keithley 6430 using special multicore planar transformer assembly on 16-layer PCB to provide low-noise high-isolation power to analog board. Anybody knows what is all this magic about?
I'd guess this transformer design used to provide low leakage in power supply DC/DC that feeds analog board output stage?
--- End quote ---
I can't tell from the photos what the layout is, but if one core is used for primary and others for secondaries, the resulting coupling capacitance can be much lower than with traditional design. "Design and Evaluation of a 10-mA DC Reference Standard" by G. Fernqvist mentions this architecture briefly.
Kleinstein:
--- Quote from: antintedo on July 19, 2019, 06:57:40 am ---
--- Quote from: TiN on July 19, 2019, 05:04:10 am ---Keithley 6430 using special multicore planar transformer assembly on 16-layer PCB to provide low-noise high-isolation power to analog board. Anybody knows what is all this magic about?
I'd guess this transformer design used to provide low leakage in power supply DC/DC that feeds analog board output stage?
--- End quote ---
I can't tell from the photos what the layout is, but if one core is used for primary and others for secondaries, the resulting coupling capacitance can be much lower than with traditional design. "Design and Evaluation of a 10-mA DC Reference Standard" by G. Fernqvist mentions this architecture briefly.
--- End quote ---
With separate cores for primary and secondary it's not a transformer anymore. There is also too little magnetic coupling.
One could sometimes use several cores instead of one larger one to get higher power and still keep the low form factor. It is possible, but would be a little odd. For very low coupling one might use several transformers in series. It is easier to get low coupling with low voltage and thus fewer, though thicker windings. So a possible choice would be to first go to a low secondary (e.g. 1 V range) than a low voltage to low voltage (e.g. only something like 2 turns - 2 turns) and than in a 3rd transformer to the final voltages.
One advantage of the planar transformer is that the coupling capacitance is predictable and reproducible. So one can find a good balance to get coupling from the right, neutral spots, effectively like having extra shields. Capacitve coupling can be low, but no real extra advanatage there over a more traditional transformer.
TiN:
I think output of this transformer should be in ballpark of 210-250V bipolar, because SMU have highest range 200V.
antintedo:
--- Quote from: Kleinstein on July 19, 2019, 07:28:36 am ---With separate cores for primary and secondary it's not a transformer anymore. There is also too little magnetic coupling.
--- End quote ---
Quote from the paper in case my explanation was confusing:
--- Quote ---The double-screened transformer uses two toroidal cores, independently wound, one each for primary and for secondary. Each is (...) coupled an external “shorted turn” winding around both cores. The resulting coupling capacitance from primary winding to screen is less than 0.1 pF and in use 10 nA of -pk AC coupling current is achieved with at least times more in most commercial units.
--- End quote ---
So yes, effectively what you described as two transformers in series. They mention a patent but it's pretty hard to find its number. There is probably something more to the physical construction than stated in the publication.
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