Ultralow-Frequency Noise in High-Bias Precision DC-Coupled Voltage Measurements

In research on magnetic sensors and magnetometers (and not only in this field), there is often a need for precise measurement of small voltage variations under high bias or common-mode conditions. These variations are typically at low or ultralow frequencies (ULF, 1 mHz–1 Hz). As the noise of analog-to-digital (AD) and digital-to-analog converters (DA) are scaled with higher values of direct-current bias, we investigate the noise limits of direct measurement and also the ULF noise of voltage references and DA converters suitable for subtracting the large DC bias. We characterized several low-noise references and two D/A converters, with the help of a Weston cell as a DC bias source with ultimately low ultralow-frequency noise. We conclude that a 12-bit DAC with a recently introduced voltage reference is finally able to achieve noise below 0.1 ppm/√Hz at 0.1 Hz.