I've two 3458A's, and only recently have a stable high voltage source to test that range. I'm seeing one of them drift (up to a couple of hundred uV) after applying 1kV. The other, connected to the same source, has negligible drift. Both have been powered up > 8hrs. The effect is only really noticeable at applied voltages of around 500V and greater.
Has anyone else seen anything similar?
Hello,
yes, of course, but it's quite difficult, or requires special equipment (i.e. the FLUKE 752A) to properly extract this power coefficient effect, and to quantitatively measure this non-linearity. Latter is different for your both 3458A, please provide the vintage of each of your instruments, or their S/N, best photos of their individual 100:1 HV dividers, as it's of general interest.
We had a longer discussion here:
https://www.eevblog.com/forum/metrology/influence-of-switch-resistance-in-hamon-dividers/msg4221751/#msg4221751First off, the 3458A is specified for 12ppm non linearity @ 1kV, as Kleinstein already stated.
The effect arises from the self-heating of this 10MOhm divider, which increases from 100V to 1kV. In conjunction with the T.C. of its 100k and 9.9M divider resistors, but more with their thermal imbalance (1mW on 100k, but 99mW on 9.9M), its ratio varies over power dissipation.
Any HV measurement @ 1kV requires at least 2 minutes of thermal stabilization, before a proper measurement can be made.
This stabilization effect occurs inside all DMMs and inside the HV source, or calibrator, or HV dividers, like the 752A!
There are several techniques to compensate for this effect, but the 3458A has none of these implemented!
1. This non-linearity is mostly quadratic, see 3458A specification.
During calibration of the DMM (DUT) @ 100V, 500V and 1kV (ext. precision voltages) it calculates the parameters of this quadratic effect.
When measuring voltages between 100V and 1kV in the 1kV range, it can then mathematically compensate this effect.
This method is used e.g. in the 34401A
2. The 100:1 divider, which is a thin film resistor structure on a ceramic chip, has another heater element on the back side of this chip.
The DMM then heats the chip externally, so that the dissipation by the input voltage plus the heater dissipation is constant, i.e. their sum is always constant 250mW.
This keeps the divider resistors nearly on a constant temperature, so that the T.C. and thermal mismatch is mostly cancelled.
This method is used in the 34410/411, in all TrueVolt KS DMMs (e.g. 34465A)
In the Fluke calibrators 5440, 57x0A, the temperature is directly kept constant by means of a temperature sensor.
3. Reduce the T.C. of the divider resistors to near zero by pairwise +/- T.C. matching, and make a thermal match between the different resistor, by e.g. distributing the power dissipation to many individual resistors. This method is used in the 752A, and in the Datron 4902S Reference Divider. The topology of latter distributes the divider equally on about 200 low T.C. resistors, i.e. 0.5mW per resistor, so the heating effect is neglectable.
For my 3458A from 2000, I determined a non linearity of about -1.6ppm, instead of 12ppm, see link above.
The 752A was required, because only this instrument has sufficiently low specified uncertainty or non linearity of 0.5ppm @ 1kV.
We already discussed, that different vintages for the 3458A had worse or better dividers.
Frank
PS: You also can extract / distinguish this heating effect between your Datron 4000 and the 3458A.
Best and safest way is to use a HV reed relays for this purpose.
Switch on 1kV on your 4000, and let it settle for several minutes. The HV will then be stable for the rest of the experiment.
Now apply this 1kV to the 3458A in its manual 1kV range, by engaging the HV reed relays.
Let the 3458A run at a fast rate, so that you can observe the drift from its initial HV reading to the stabilized value a few minutes later.
Best would be to sample the readings into memory, so that you can afterwards analyze the voltage rise and drift on your PC.
This difference is then your power dissipation effect.
You can repeat this experiment at 100V and 500V, to see if this is really about quadratic.