Bio-signal monitoring systems for monitoring a biological subject may involve measurement of the electrical potential differences in the tissue of the subject (biopotential). This is achieved by applying an excitation current across, in vivo, electrodes. Measurements are taken on a continuous basis. Therefore, the monitor is invariably implanted. Therefore, low power consumption whilst maintaining accuracy of the measurements is desirable.
Therefore design strategies for each building block and signal monitoring methodology focus on low power consumption. In addition, measurement accuracy has to be guaranteed with minimized power consumption. There are two things which determine monitoring accuracy of portable bio-signal monitoring system. One is accuracy of the building block and the other is accuracy of monitoring method. To achieve high accuracy measurement requires high power consumption for internal building blocks and high accurate external components which increase form factor of the system.
One specific bio-signal monitoring system is intra-thoracic fluid analysis. Such a measurement system is often required to resolve biopotential down to mΩ-range change (AC) superimposed on up to kΩ-range average (DC) impedance. In standard practice, this can be achieved by using a purely sinusoidal current source with very good harmonic distortion to minimize the error of impedance measurement as disclosed in, for example, Yan, L.; Bae, J.; Lee, S.; Kim, B.; Roh. T.; Song, K.; Yoo, H-J.;, “A 3.9 mW 25-electrode Reconfigurable Thoracic Impedance/ECG SoC with Body-Channel Transponder,” IEEE International Solid-State Circuits Conference, vol., no., pp.490-491, 7-11 Feb. 2010. However, this approach consumes many mWs of power which is not acceptable for an implantable device. There is research that uses multi-level quantized signal as a modulation and demodulation signal as disclosed in M. Min, and T. Parve, “Improvement of Lock-in Electrical Bio-Impedance Analyzer for Implantable Medical Devices,” IEEE Tran. On Instrumentation and Measurement, 2007. As a result, pW-range power consumption can be achieved by using a square-wave current combined with quadrature demodulation as disclosed, for example, in Yazicioglu, R. F.; Kim, S.; Torfs, T.; Merken, P.; Van Hoof, C.; , “A 30 μW Analog Signal Processor ASIC for biomedical signal monitoring,” IEEE International Solid-State Circuits Conference, vol., no., pp.124-125, 7-11 Feb. 2010. This technique, however, requires multi-level quantized current as well as multi-level demodulation amplifier which has a complex architecture and consumes more power. In addition, this technique requires modulation and demodulation signal which have accurately generated phase shift. This limits programmability of the generated signal as well as the measurement accuracy can be degraded due to the phase variations. Further, resulting in intolerable measurement errors as high as 23% because the quadrature demodulation will fold all odd harmonics of the square wave current into the baseband.