Prema 6048 teardown

[Rax]

After performing the DCV cal on the Prema on the .2V, 2V and 20V ranges - using my 5440A, which seems to be vetted by a KS-calibrated 3458A - I am seeing what to me like pretty reassuring agreement. For instance, between the 2V "non-DIV" input and the 2V "DIV" input, there's sub-unitary (ppm) divergence between the two hardware paths (preliminary conclusion). Of course, not very easy to say if what I'm seeing confirms the Prema, the Fluke, of both of them in "joint venture." I'd need to do more work to clear my data and experimental conditions.

But to me this provides some level of comfort with the linearity of the Prema ADC away from zero crossing, as it compares with the pretty good linearity the 5440A in known to have.

[Rax]

The other provisional conclusion I feel I can draw is that I'm seeing much more consistency from the Prema than before the upgrade.

I feel that the zeroing is now pretty reliable and and the drifts I used to see (and not know whether they are caused by the Prema or the 5440A) are no longer occurring. If anything, the zeroing of the Prema seems just about as reliable and resulting in a level of relative accuracy (between the two) as the AUTOCAL the 3458A seemed to provide (and direly need all the time). In fact, for my particular samples, the Prema needs zeroing less than the 3458A needed the AUTOCAL.

[Kleinstein]

The upgrade via better OP-amps can be really woth it.  The change OP77 to OPA140 seems to largely fix the weak point in the INL.  The OPA140 and LT1007 for the reference are major contributors to the zero point drift. Here there can still be variations in the actual parts used. Even in the original configuration the stability of the zero can vary between units. If the zero is stable there is no need to change things. If it is drifting it can make sense to try an alternative chip (could be just another LT1007 or similar or an AZ OP).

How often the zero for the 6048 or ACAL for the 3458 is needed  can also vary. Some units can be more stable than others.

For the prema meter the gain of the ADC and input is set by 1 resistor ratio. The 2 V range may be slightly more stable in this respect, using 2 x 5 K and thus equal value resistors. The other ranges still also still have only 1 pair of resistors, just exchanging one of the resistors to a different value. So there is no clear prime range with like with the 3458. The 6048 does not look that it could easily measure it's own reference directly. So a kind of ACAL would not work so easy for the Prema meter. The test in 2 ranges is more like a self test, but no way to correct drift.

The 3458 has additional resistors in the ADC gain and amplifier gain. So less stable to start with, but able to correct. On the other side it has a stable zero.