Two stage lock in amplification for improving magnetic data acquisition systems using Quantum Well Hall Effect sensors

Abstract

Quantum Well Hall Effect (QWHE) sensors are a type of magnetic field sensor with previously demonstrated applications in NDT imaging systems. These sensors are typically biased in the 10-100kHz frequency range to avoid both 1/f noise and magnetic pickup. The signal is then further amplitude modulated by the measured magnetic field. Traditionally, this magnetic signal's amplitude is then extracted through Fast Fourier Transforms (FFT). With this traditional method however, most of the measured signal consists of unwanted pick up, and large unchanging offset values resulting from both the circuit and any measurements, with the actual measured signal making up less than 1% of the ADC dynamic range. By designing a two-stage lock in amplifier, the signal is demodulated with respect to both the biasing signal and the AC magnetic signal, leaving only the DC component corresponding to the amplitude of the magnetic signal. The offset from this can then be subtracted, and further gain can be applied, allowing the full dynamic range of the measurement ADC to be utilised, while only measuring the changes in magnetic field strength resulting from the presence of any flaws in the measurement sample. This theoretically allows for smaller signals to be measured than is possible with the FFT method, provided that the signals are not lower than the noise floor of the measurement circuitry. An example of both FFT and early prototype lock in amplifier demodulation is demonstrated in this work. This method of signal demodulation also has the added benefit of decoupling the measurement parameters from the frequencies used in the biasing and magnetic signals, allowing for arbitrarily slow sample rates to be used in measurement.