0.5？V Cardiac Sense Amplifier Realization Using Log-Domain Filtering

A novel configuration of a cardiac sense amplifier for pacemakers, realized using the concept of Log-Domain filtering, is introduced in this paper. The analog part of the amplifier operates under a single 0.5？V power supply voltage. Compared to the corresponding already published configuration, the proposed scheme offers the benefits of reduced operating voltage and dc power dissipation. The performance of the intermediate stages, as well as of the whole system, has been evaluated through the utilization of the Analog Design Environment of the Cadence software and, also, the design kit provided by the AMS 0.35？μm CMOS process. 1. Introduction A pacemaker is a closed-loop system where the intracardiac signal events are monitored and detected by a cardiac sense amplifier. The output of the amplifier is directly connected to a microcontroller which provides the appropriate signals to the stimulator in order to establish a therapy scheme for the heart. Taking into account that the biological signals are handled in the body’s noisy environment, the cardiac sense amplifier should offer a relatively high signal-to-noise ratio. In addition, this device should be capable of operating in an ultralow voltage environment and, simultaneously, of drawing as small as possible power from the battery in order to achieve maximum life of operation for the pacemaker. A promising technique for realizing circuits with high dynamic range is the Log-Domain technique. This originated from its companding nature, where the input current is converted into a compressed voltage and then is processed by the nonlinear core. The derived output compressed voltage is converted into a linear current, in order to preserve the linear operation of the whole system. In addition, the internal compressed voltages offer the capability for operation in a low-voltage environment, because of their reduced swings in comparison with those observed in the conventional linear filters. Log-Domain filters could be realized through the utilization of bipolar transistors in the forward active region or MOS transistors in the subthreshold region. The last approach offers potential for reducing the power consumption and this is very important in biomedical applications, since such devices should have a long battery life. Taking also the low-frequency range of biological signals into account, the employment of MOS transistors in the subthreshold region offers the benefit for realizing Log-Domain filters with small capacitor area, due to the large values of the realized equivalent resistors [1–9]. The concept