An Analog-Front-End for non-invasive fetal electrocardiography monitoring

Fetal electrocardiography (fECG) based assessment techniques are likely to be more effective in ensuring fetal well-being, than currently popular methods based on ultrasound echocardiography and pulse oximetry. Employing Non-invasive procedures for measurement of fECG not only ensures minimal patient discomfort, but also makes the product suitable for wearable applications. However, non-invasive fECG extraction has proven to be difficult due to low amplitudes of fECG signals, attenuation by maternal biological tissues, overlapping spectra with other fetal and maternal bio-signals and low-frequency noise in measurement devices. This paper describes the design of a noise optimized Analog-Front-End (AFE) for a non-invasive fECG monitor. The design criterion laid down for the fECG monitor require it to be an ambulatory, low-power, inexpensive device, interfaced to a smart-phone and capable of running computationally intensive signal processing algorithms. The primary role of the AFE is to filter-out large DC offsets, movement artifacts, high-frequency noise and subsequently amplify and condition the extracted signals before digitization. This noise optimized design makes use of active-electrodes fabricated on small printed circuit boards (PCB). Each PCB is mounted on standard Ag-AgCl electrodes to buffer, filter and amplify abdominal bio-signals as soon as they are detected. The extracted signals are then sent to the fECG monitor PCB for further processing and signal conditioning, before being digitized by a 24-bit analog-to-digital converter (ADC) of an ARM Cortex M3 based system-on-chip (SoC). The monitor is designed to work with supply voltages from 3.6Vdc to 5.5Vdc, allowing it to be powered from a variety of sources; battery packs to USB connectors. Working of the AFE was tested using synthetic signals and elementary biological signals.