Electronics > Metrology

Cables for mesuring high ohms resistors

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jfphp:
Hello,
I am trying to mesure directly resistors 1 Mo and above (Fluke5450 calibrator, ESI standart resistors...) on a 3458A in 4 wires configuration with the well known Fluke cables 5440-7002/4 ; I am very surprised by the noise on the readings at full resolution (not coming from the 3458A : cal and certificate of conformity one week old), no neon bulb above the cables, no vibration and no HF noise coming from a SA, synthesyzer... Any idea ? Thank you.

HighVoltage:
How big is the noise.
And the shield of your Fluke cable is connected on both ends?

I had a similar problem before and it definitely was RF noise from a neighbor.
Since then I am testing my environment each time before I do some ppm-level testing
and keep smartphones and DECT phones far away.
 

Awesome14:

--- Quote from: HighVoltage on October 18, 2017, 04:23:05 pm ---
And the shield of your Fluke cable is connected on both ends?
--- End quote ---

I always connect the shield on just one end. Otherwise you can get looping. It only needs to be grounded once.

VintageNut:
Two sources of error are AC noise and DC current leakage. Cables that shield against noise do not necessarily help reduce leakage while good cables that reduce or eliminate leakage probably will also shield against noise.

DMMs use a small constant current which is not a great way to measure a large resistance. Large resistances are best measured with a fixed voltage. Electrometers use a voltage source for a reason.

How much accuracy do you require?

My Fluke 5450A was calibrated with a Measurements Internation bridge. The low resistances use a constant current bridge while the large resistances use a constant voltage bridge.

You can create a bridge with a collection of well-known standards (Keithley 5155) and some ordinary instruments.

e61_phil:

--- Quote from: Awesome14 on October 18, 2017, 04:54:05 pm ---
--- Quote from: HighVoltage on October 18, 2017, 04:23:05 pm ---
And the shield of your Fluke cable is connected on both ends?
--- End quote ---

I always connect the shield on just one end. Otherwise you can get looping. It only needs to be grounded once.

--- End quote ---

You should connect both ends to use the full shielding inside the units. To avoid loops you have the guard switches on the 3458A and the 5450A (and all the other calibrators)

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