Products > Test Equipment
Keithley 6485/6487 teardown
razberik:
--- Quote from: TiN on February 04, 2017, 02:21:04 am ---Can I use your info to add in my 6485 article, with link to this thread posts?
--- End quote ---
Yes, of course. No problem.
I expect to do reversing K2400 SMU in future. ;D
plesa:
--- Quote from: razberik on February 04, 2017, 07:35:58 pm ---
--- Quote from: TiN on February 04, 2017, 02:21:04 am ---Can I use your info to add in my 6485 article, with link to this thread posts?
--- End quote ---
Yes, of course. No problem.
I expect to do reversing K2400 SMU in future. ;D
--- End quote ---
If you have faulty 2400 you can check 23x service manual, there is schematic.
Two units I needs to repair has blown power mosfets only (MTP2P50 and 3N100 or in more recent meter 7N100).
razberik:
I am still not sure if I understand used relays correctly. They use EE2-3SNUH which arent special, they dont have excellent resistance.
But these relays are latching. This means that when relays not operated, coil is connected to GND. Does it mean that the coil acts as shielding/guarding ?
pelud:
Hi All,
I have some questions about how the analog circuitry of the meter works.
1. What is the purpose of the 2N3904 and 2n3906 transistors (Q4, Q19 and Q20 on kei_6487_reverse_preamp_sch2.pdf) and surrounding circuitry? It looks like some sort of compensation network, but I'm not sure what for or how it works.
2. What is the null adjustment on the AD829 for? I would have thought that the LMC6081 is the "boss", and would do whatever it can to make it's inputs the same, including compensating for any null offset on the next amp.
3. What are R185, C140, R50 and C141 (kei_6487_reverse_preamp_sch2.pdf) going into the AD829 for?
Also, some comments:
I noticed that some people mention that the triax is not necessary. However, it very much is, as you would logically expect that the exposed metal shell of the connector is at earth ground potential, while the center conductor can be at whatever voltage source you are using. Therefore, the center conductor as well as the internal circuitry can "float", and the internal feedback of the device will drive the second "guard" conductor of the cable to same as the center in order to "intercept" any current that would otherwise leak to ground.
TiN:
You can get away w/o triax if use floating BNC with external "shell" connected to GUARD. However you would still need/want low triboelectric noise cables with good insulator material. Using coax with driven shield at guard potential is very often used inside the instruments, that don't need worry about user's touching live wires (that might be at lethal potentials!). For example Keithley 7xxx series low-current switch matrix cards use coax to route signals from triax input to PTFE-floated reed COTO relays.
Caveat with this approach is obvious = your outer BNC metal shield is now under live voltage, controlled by guard driver, so if you trying to measure something with dangerous voltage levels - you need exercise great care. :bullshit:
Also this solution may provide more noise in samples, as you don't have as good shielding as with triax or quadrax cables.
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