Measurement science at Metrosol.
A specialist team with a narrow focus: measuring electrical quantities more accurately than the instruments you can buy, expertise that underpins our precision instrumentation and Johnson noise thermometry.
It all starts with an analogue-to-digital converter
Our first product was a resistance bridge based on a switched DC technique, achieving performance that had previously required an AC bridge. The limitation was the analogue-to-digital converter: nothing commercially available could digitise the signal accurately enough. The best integrating converter available at the time offered around 3 ppm linearity, limited by dielectric absorption in its integrating capacitor. Rather than correct those errors in software, we developed a converter with NPL and Metron Designs.
The intervening step was 2009, when taking the microK into primary standards work meant 0.1 ppm. That needed timing jitter below a picosecond, which is the time an electrical signal takes to travel about a third of a millimetre, and it needed the input noise to come down far enough for the improved linearity to be worth having. No existing approach reached it, and the answer turned out to be a parallel analogue processing technique that is believed to be ours alone.
Proving a measurement is a separate problem
Discriminating linearity errors on a 0.1 ppm instrument needs a laboratory capable of 0.01 ppm. Telling whether the 2009 work had succeeded meant building that capability first, and building it was a project in its own right.
Manufacturing the resistance bridge calibrator later asked for more of the same: resistance at the nano-ohm level, leakage at the tera-ohm level, and standards held to within a few millikelvin over many hours, which we do using Isotech’s resistor maintenance baths. That capability is now the quiet part of what the company is, and it is why we can state figures on this site as measurements instead of as specifications.
In this section
- The A-D converterThe A-D converter and substitution topologyWhy nothing on the market was good enough in 2004, the multi-bit sigma-delta design that answered it, and the measurement topology that makes the result drift-free by construction.
- Who does whatCollaboration and skillsNPL, Metron Designs, the Measurement Standards Laboratory of New Zealand and the ThermoSI consortium.
The instruments this work went into are on the instrument design pages, and the thermometer it led to is in Johnson noise thermometry.