High-quality biopotential acquisition without a reference electrode: power-line interference reduction by adaptive impedance balancing in a mixed analog-digital design.

Power-line-interference (PLI) is one of the major disturbing factors in almost all ground-free biopotential acquisition applications. The body is a volume conductor and collects PLI currents. Some of these currents pass through the sensing electrodes, then the electrode cables, and finally via the amplifier input impedances they reach the signal ground. The electrode impedances and the amplifier input impedances form an impedance bridge. Due to electrode impedance instability over time, the bridge tends to be imbalanced and produces differential PLI which is amplified together with the useful signal. This paper describes a powerful mixed analog-digital solution for automatic impedance bridge balance using software PLL for line synchronization. The approach is implemented and validated through recorded real ECG signals. The PLI is canceled by adding part of the common-mode voltage, with automatically adjusted amplitude and phase, to the useful differential biosignal. The described approach produces high-quality biosignals without the need for a common-mode reference electrode. It is applicable to all biosignals taken with surface electrodes like ECG, EEG, EMG, EOG, etc., and can benefit all diagnostic and therapeutic medical devices where these signals are in use.

Pseudo-real-time low-pass filter in ECG, self-adjustable to the frequency spectra of the waves.

The electrocardiogram (ECG) acquisition is often accompanied by high-frequency electromyographic (EMG) noise. The noise is difficult to be filtered, due to considerable overlapping of its frequency spectrum to the frequency spectrum of the ECG. Today, filters must conform to the new guidelines (2007) for low-pass filtering in ECG with cutoffs of 150 Hz for adolescents and adults, and to 250 Hz for children. We are suggesting a pseudo-real-time low-pass filter, self-adjustable to the frequency spectra of the ECG waves. The filter is based on the approximation procedure of Savitzky-Golay with dynamic change in the cutoff frequency. The filter is implemented pseudo-real-time (real-time with a certain delay). An additional option is the automatic on/off triggering, depending on the presence/absence of EMG noise. The analysis of the proposed filter shows that the low-frequency components of the ECG (low-power P- and T-waves, PQ-, ST- and TP-segments) are filtered with a cutoff of 14 Hz, the high-power P- and T-waves are filtered with a cutoff frequency in the range of 20-30 Hz, and the high-frequency QRS complexes are filtered with cutoff frequency of higher than 100 Hz. The suggested dynamic filter satisfies the conflicting requirements for a strong suppression of EMG noise and at the same time a maximal preservation of the ECG high-frequency components.

Digital lock-in techniques for adaptive power-line interference extraction.

This paper presents a simple digital approach for adaptive power-line (PL) or other periodic interference extraction. By means of two digital square (or sine) wave mixers, the real and imaginary parts of the interference are found, and the interference waveform is synthesized and finally subtracted. The described technique can be implemented in an open-loop architecture where the interference is synthesized as a complex sinusoid or in a closed-loop architecture for automatic phase and gain control. The same approach can be used for removal of the fundamental frequency of the PL interference as well as its higher harmonics. It is suitable for real-time operation with popular low-cost microcontrollers.

Bootstrapped two-electrode biosignal amplifier.

Portable biomedical instrumentation has become an important part of diagnostic and treatment instrumentation. Low-voltage and low-power tendencies prevail. A two-electrode biopotential amplifier, designed for low-supply voltage (2.7-5.5 V), is presented. This biomedical amplifier design has high differential and sufficiently low common mode input impedances achieved by means of positive feedback, implemented with an original interface stage. The presented circuit makes use of passive components of popular values and tolerances. The amplifier is intended for use in various two-electrode applications, such as Holter monitors, external defibrillators, ECG monitors and other heart beat sensing biomedical devices.