Ok, it was a pretty nasty measure but I finally got it.

Since the target is to test a drift on the R measured, I needed a way to check not only his value over time but also the current generated by the 34401A.

Here my setup:

I have the 34401A measuring a precision 90R in parallel with a 34461A to check the voltage across it (related to the current generated by the 34401A DUT).

Basically the voltage measured by the 34461A divided by 90R will provide us the current sourced by the 34401A. The coax cables will minimize the noise pickup.

Now I had the 34461A USB connected to the PC, and and the other 34401A connected as well using GPIB or RS232.

I had two 34401A units (GOOD #1 and GOOD #2) fresh from calibration no problem to use as reference to compare with the, according to the calibration lab, bad one (BAD).

Of course the units were properly warmed up and the measure time was over one hour period.

Here the setting used for the R measures:

`Auto Range: On`

Auto Zero: On

Measurement: Resistance (2W)

NPLC: 10

Range(Ohm): 100

and for the current ones on the 34461A:

`Auto Null Value: FALSE`

Auto Range: On

Auto Zero: On

Input Impedance: 10 MOhm

Measurement: DC Voltage

NPLC: 10

Null State: FALSE

Null Value(Vdc): 0

Range(Vdc): 0.1

Let´s dig into the results...

**GOOD #1**2W R measured:

Statistic [Ohm]

Mean 90.07426003

Standard Error 3.79745E-06

Median 90.074236

Mode 90.074083

Standard Deviation 0.000375295

Sample Variance 1.40846E-07

Kurtosis -0.493118701

Skewness 0.283102244

Range 0.002418

Minimum 90.073072

Maximum 90.07549

Sum 879755.2978

Count 9767

Largest(1) 90.07549

Smallest(1) 90.073072

Confidence Level(95.0%) 7.44379E-06

Current (90mV=1mA)

Statistic [A*90]

Mean 0.09014677

Standard Error 2.22337E-09

Median 0.090146746

Mode 0.090146714

Standard Deviation 2.22526E-07

Sample Variance 4.95176E-14

Kurtosis -0.075682427

Skewness 0.473640809

Range 1.4409E-06

Minimum 0.090146106

Maximum 0.090147547

Sum 903.0001985

Count 10017

Largest(1) 0.090147547

Smallest(1) 0.090146106

Confidence Level(95.0%) 4.35824E-09

**GOOD #2**2W R measured:

Statistic [Ohm]

Mean 90.07989073

Standard Error 3.09024E-06

Median 90.079946

Mode 90.079946

Standard Deviation 0.000302496

Sample Variance 9.15041E-08

Kurtosis -0.177549751

Skewness -0.477571608

Range 0.0019

Minimum 90.078848

Maximum 90.080748

Sum 863145.513

Count 9582

Largest(1) 90.080748

Smallest(1) 90.078848

Confidence Level(95.0%) 6.05752E-06

Current (90mV=1mA)

Statistic [A*90]

Mean 0.088696667

Standard Error 2.62486E-09

Median 0.088696721

Mode 0.088696711

Standard Deviation 2.62263E-07

Sample Variance 6.87817E-14

Kurtosis -0.557971289

Skewness -0.449007757

Range 1.5119E-06

Minimum 0.088695812

Maximum 0.088697324

Sum 885.4588291

Count 9983

Largest(1) 0.088697324

Smallest(1) 0.088695812

Confidence Level(95.0%) 5.14525E-09

**BAD**2W R measured:

Statistic [Ohm]

Mean 90.02526044

Standard Error 5.78764E-06

Median 90.025232

Mode 90.025119

Standard Deviation 0.000564228

Sample Variance 3.18353E-07

Kurtosis -0.456379127

Skewness 0.228340502

Range 0.00326

Minimum 90.02379

Maximum 90.02705

Sum 855600.0752

Count 9504

Largest(1) 90.02705

Smallest(1) 90.02379

Confidence Level(95.0%) 1.1345E-05

Current (90mV=1mA)

Statistic [A*90]

Mean 0.089503874

Standard Error 2.02854E-09

Median 0.089503873

Mode 0.089503842

Standard Deviation 2.02641E-07

Sample Variance 4.10633E-14

Kurtosis -0.520592125

Skewness -0.068937736

Range 1.3154E-06

Minimum 0.08950316

Maximum 0.089504475

Sum 893.159158

Count 9979

Largest(1) 0.089504475

Smallest(1) 0.08950316

Confidence Level(95.0%) 3.97634E-09

**CONCLUSIONS**We need a side to side comparison.

*2W R Measurements*Here the yellow values are the only one concerning me. Still not a huge drift in my eyes. The BAD shows a negative drift decreasing the R value. The GOOD 1 also does that, but the gradient is less than the BAD one.

*Current Measurements*Pretty much here everything is acceptable, since the unit cal lab reported the drift only in the 2W R mode, I was not expecting a problem with the current:

Maybe the current generated by the BAD one is suspiciously pretty nervous and irregular in the graph.

**Next steps**0) Check/Understand the Spec. Tolerance (let´s do tomorrow. Is already 00:51 AM)

1) Dig into the schematics and isolate the part of the circuit which can cause a drift only in 2W R mode

2) Perform a 4W R 1 hour test as above to verify the drift is not there in 4W

3) Check the 100 Ohm Range current generation, check if current is constant in 1K Range

3) Open the unit and look the circuit for bad joints on the part of circuits founded in point 1 and 3

I don´t know if I will fix this unit, it seems a nasty slightly out of spec. malfunction.

**On the side: EMI CRAP?**I had the bad idea to work under the bench using my 12V impact drill during the 1 hour measurement and here what I got on a good one! (left current, right R measured)

Even when I turned on my monitors from sleep, I noticed similar jumps. I decide to measure when I was not at the bench during night time (everything off, just the PC and the DMMs).