Calibration of a Digital Current Transformer Measuring Bridge: Metrological Challenges and Uncertainty Contributions^†

Round 1

Reviewer 1 Report

I congratulate the authors about the clear presentation of their thorough work on combining precise timing with accurate analog signal generation.

General:

• About line 132: You could mention here that the exact synchronization is also needed between the calibrator and the DUT.
• Line 321: I think the SV protocol resolution is 32 bits. Please check.
• About lines 386-338: You could also mention that on lower current levels the resolution of the SV values increase the relative uncertainty of both amplitude and phase. When the signal is small compared with the nominal range setting most of the higher bits are zeros, and only few of the lower bits are in use.

Editorial:

• Line 113: Replace "reasonably" by "reasonable"
• Line 246: Replace "introduce" by "introduced"
• Line 337-338: Replace "uncertainties" by "uncertainty"
• Line 370: Replace "±5400'" by e.g. "±10'"

Author Response

I congratulate the authors about the clear presentation of their thorough work on combining precise timing with accurate analog signal generation.

We thank the Reviewer for the positive comment.

General:

• About line 132: You could mention here that the exact synchronization is also needed between the calibrator and the DUT.

We do agree with the Reviewer's suggestion. We modified the text in order to underline this aspect.

• Line 321: I think the SV protocol resolution is 32 bits. Please check.

The Reviewer is right. This was a typo and we fixed it in the revised version. Thank you very much for the careful reading.

• About lines 386-338: You could also mention that on lower current levels the resolution of the SV values increase the relative uncertainty of both amplitude and phase. When the signal is small compared with the nominal range setting most of the higher bits are zeros, and only few of the lower bits are in use.

We thank the Reviewer for the insightful comment. Indeed, given the fixed resolution of the SV format, lower current levels correspond to an inefficient exploitation of the measurement range and thus result in a higher relative uncertainty (for both amplitude and phase). We included this comment in the revised version.

Editorial:

• Line 113: Replace "reasonably" by "reasonable"
• Line 246: Replace "introduce" by "introduced"
• Line 337-338: Replace "uncertainties" by "uncertainty"
• Line 370: Replace "±5400'" by e.g. "±10'"

We thank the Reviewer for the careful reading. The manuscript has been thoroughly proofread and polished from typos.

Reviewer 2 Report

Very solid paper I enjoyed reading. The ENOB of 17bits needs more explanation, since this usually requires temperature stability/calibration, and also power supply (of the electronic components) stabilization techniques not described here. Maybe that will be part of the announced "Minimization of the uncertainty contributions". Overall interesting concept for transformer measuring bridges. Please also note that you might want to take a look onto GMR/TMR sensors for current measurements, usually superior to shunt measurements.

Author Response

Very solid paper I enjoyed reading.

We thank the Reviewer for the positive comment.

The ENOB of 17bits needs more explanation, since this usually requires temperature stability/calibration, and also power supply (of the electronic components) stabilization techniques not described here. Maybe that will be part of the announced "Minimization of the uncertainty contributions".

The Reviewer is right. At such levels of accuracy, temperature stability and power supply represent possible sources of uncertainty. For this reason, the measurement campaign was carried out in METAS lab with a controlled temperature of 23° and adopting a power supply at 60 Hz for both the calibrator and the amplifier to avoid beating effects or interferences. We included these details in the revised version.

Overall interesting concept for transformer measuring bridges. Please also note that you might want to take a look onto GMR/TMR sensors for current measurements, usually superior to shunt measurements.

We totally agree with the Reviewer. In our setup we use calibrated shunts as we assume that their uncertainty contribution is negligible with respect to the amplifier stage. Nevertheless, the Reviewer suggestion is totally right and we included a comment about that in the revised version.