DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

18 BIOPOTENTIAL AMPLIFIERSFigure 1.16 Array of 16 single-ended biopotential amplifiers. A number of these circuits may bestacked up to form very large arrays, making them ideally suited for applications such as body potentialmapping electrocardiography.The last processing stage of each channel is an active notch filter, which can be tuned tothe power line frequency by adjusting R12. Supply voltage to this circuit must be symmetricaland within the range of 5 V (minimum) to 18 V (absolute maximum). Two9-V alkaline batteries can be used efficiently due to the circuit’s very low power consumption.Capacitors C9–C12 are used to decouple the power supply and filter noise fromthe op-amp power lines.To minimize electrical interference, the circuit should be built with a compact layout onan appropriate printed circuit board or small piece of stripboard. The construction of thecircuit is straightforward, but care must be taken to keep wiring as short and clean as possible.Leads to the bioelectrodes should be low-loss coaxial cables, whose shields are connectedto their respective shield drives at J1 (J1x-2 for left-side channels and J1y-1 forright-side channels). The circuit’s ground should be connected to the subject’s reference(patient ground) electrode. When connected to a test subject, the circuit must always bepowered from batteries or through a properly rated isolation power supply. The same isolationrequirements apply to the outputs of the amplifier channels.It is important to note that the performance of a complete system is determined primarilyby its input circuitry. Equivalent input noise is practically that of the first stage(approximately 10 µV p-p within the amplifier’s 3-dB bandwidth of 0.2 to 100 Hz).BODY POTENTIAL DRIVERSRejection of common-mode signals in the prior circuit example is limited to the singleendedperformance of the input-stage op-amp and the 50/60-Hz rejection of the notch filter.Often, however, environmental noise (e.g., power line interference) is so large that common-modepotentials eclipse the weak biopotentials that can be picked up through singleendedamplifiers. Notch filters do not necessarily remove interfering signals in a substantialmanner either. The first few harmonics of the power line constitute strong interfering signalsin the recording of biopotentials. The range of these signals, however, is by no meansconfined to 100 or 200 Hz. High-frequency interference originating from fluorescent andother high-efficiency lamps commonly occurs with a maximal spectral density of approximately1 kHz and with amplitudes of up to 50% of the 50/60-Hz harmonic.