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I am using Hi and Lo alligators from the preamp for short circuit current measurement with Ammeter mode. The current level should be 1fA < current < 200fA and my device is inside home build Faraday cage. Currently, I connected one end of my device to Hi alligator and the other end to Lo alligator which is also connected to grounded Faraday cage. The guard alligator is not connected to any thing yet. My device is made of PDMS (polymer) chamber and the test fixture is plastic. So I am unsure where to connect the guard cable to. Would you suggest where to connect?
Second question :
I believe the proper ground should help this low current measurement experiment. I currently connected the Lo alligator on preamp to grounded Faraday cage but not sure if this is sufficient. Should I also ground Input/output Lo on the mainframe using banana jack? If I should, can I connect Input/output Lo to chassis directly? Or connect to chassis ground screw?
- Keithley Applications
- Posts: 2849
- Joined: June 10th, 2010, 6:22 am
- Country: United States
1. Inner conductor of the IN/OUT HIGH triax connector on the preamp is the IN/OUT HIGH terminal.
2. Inner shield of the IN/OUT HIGH connector on the preamp is GUARD.
3. Outer shield of the IN/OUT HIGH connector on the preamp is IN/OUT LOW terminal.
4. Chassis or earth ground is the screw on the back panel.
Please note that a measurement should be made between HI and LO. This means between the inner shield and outer shield of the IN/OUT HIGH triax connector on the preamp. It is not between the inner conductor and inner shield like in most other Keithley electrometers.
1. Since you are measuring in the fA region the guard is very important. It should surround the entire circuit, if possible.
The triax cable will carry guard through to the end of the cable. This is good.
Now we need it to surround the device to be tested as well. Since your test fixture is plastic, this will not help.
We need to coat the test fixture with a conductive coating and connect GUARD to it. Using aluminum foil should help. So cover the entire test fixture with aluminum foil and connect to GUARD(inner shield of the triax, black alligator clip).
Guard is a low impedance signal of the same magnitude of the HIGH terminal. What is the voltage level you are testing at?
If it is above 30VDC or 42V peak then this is a saftey hazzard. Then the entire test fixture should be surrounded by another shield that does not touch the guard shield and connect it to earth ground.
2. Ground is for saftey not signal integrity. Guard is the important factor for signal integrity. If you feel the need to ground the LOW that is fine but it probably would not help with this low current measurement.
Hope this is clear.
I see your point. I am not source any voltage. I am using it as ammeter. So there should not be any voltage difference between Hi and Lo in principle. However, I understand there always exist a little bit of voltage difference.
So when I placed the aluminum foil under my device and connected it to guard but does not seem to make it more stable. So can I connect the guard to the Faraday cage? It surrounds the entire device, so I believe it can be a good guard shield. Then, Lo alligator should be floated (Originally Lo was grounded through the Faraday cage)
Or, what about this? My device is made of metal patterned silicon chips with PDMS block placed on glass plate. What if I connect the guard to third metal electrode adjacent to the real electrode on the same device? This third electrode is also through the same insulating layers (PDMS block and glass plate).
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