Electronics > Metrology

3458A's Current shunts/resistance current source tempco study

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Since I got my hands on damaged 3458A's A1 PCB decision was made to test critical resistors used on it for current/ohm functions to evaluate their TCR and eventually long-term stability performance.
I'll be updating this thread as the measurements proceed. Unless mentioned otherwise, all measurements to be taken with 3458A, @NPLC100, OCOMP ON with 5 sec delay.
One of the reasons why I want to go into this topic is to study possibility of pimping up 3458A's ohms and current performance and long term stability, for which I need to get baselines.
Let's get started.

Reference 40K resistor

This resistor is used as stable standard for ACAL calibration and perhaps as transfer point in calibration purposes. Hence stability of this resistor is crucial for 3458A's ohm/current specs.
Older A1 PCB have non-hermetic 40K resistor (more information wanted), while newer versions are equipped with custom-ordered Vishay PG #301031 VHP hermetic resistor. It's labeled 40.000K 0.1%.

TEST 1 (Ambient-Ambient)

To get initial TCR test going, I soldered resistor to 4-wire banana plug adapter and connected to DMM.
In this test both resistor and DMM are in free ambient, and temperature delta created by room AC.

RAW CSV-data.

Graph (LIVE version on click):

Temp-Resistance deviation:

Box TCR = +2.74 ?/K

DMM was run thru ACAL on HOT/COLD points. I expect a caveat though, as there is high probability that 3458A's own 40K drifts similar magnitude, thus obscuring test result.

TEST 2 (Ambient-Thermostat)

Test 2 using same DMM, but this time with resistor located in TEC thermostat, controlled by Keithley 2510 within 0.01 °C setpoint.
DUT resistor located in copper box filled with oil and Keithley 2001 used to measure RTD temperature, which located in same chamber.

Test methodology details placeholder

Temp chamber setup placeholder

Results summary and analysis placeholder.

References to other related publications

Dr. Frank:
Hi Illya,

I assume this A1 board is from 1989, as the 2nd line on the resistor should be the date code?

Then the 40k resistor is the original one, i.e. a hermetical sealed type, specified T.C. of 1.3ppm/K (see CLIP).
The actual Vishay technology of that time were C- or K- foil types, and had T.C.s of +/- 2 or  +/- 1 ppm/K.

You have to run the 3458A in a stable environment (+/- 0.1°C), so you may well measure the DUTs T.C, independently.
Does your 3458A already contain the VHP101 resistor?

Anyhow, I'm very curious about your results.



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