| Totally implantable multichannel telemetry systems are increasingly valuable instruments to the physiological researcher. In these applications, however, conventional passive sensors are not ideal because their small analog output voltages are particularly susceptible to electromagnetic interference and can be degraded by shunt leakage currents across the cable. For these reasons, sensors that emit digital signals are becoming increasingly popular. The Center for Integrated Electronics in Medicine at Stanford University, for example, has developed the capacitive pressure transducer (CPT3) in which a pressure-sensitive capacitor modulates the period of a relaxation oscillator. Because most physiologists wish to measure a number of parameters simultaneously, however, a means of multiplexing a variety of digital signals through a single telemetry package is required. The research reported here describes a custom integrated circuit which is the key to incorporating multiple digital sensors into a single implantable package.; The asynchronous digital multiplexer (ADM) integrated circuit commutates the digital data signals from four CPT3 sensors. A unique two-tiered self-clocked multiplexing architecture enables compatibility with any type of pulse-modulated data signals (except PAM) and a programmable sampling rate set by the sensors. Designed in bipolar technology, the ADM combines both digital and analog circuitry. The digital portion of the ADM circuitry is a mixture of the EFL (for speed) and I{dollar}sp2{dollar}L (for density) logic families; careful attention focuses on the speed-power trade-offs required for low-power operation of the digital logic. Several new circuits have been developed for the ADM and include low-power EFL-compatible input-stage and output-buffer circuits, an I{dollar}sp2{dollar}L-to-EFL logic-level translator circuit that is insensitive to process and operating-point variations, and a triggerable astable time-out oscillator circuit based on a unique method for temperature-independent operation.; Fabricated at the Center for Integrated Systems at Stanford University, the ADM IC measures 3.4 x 4.6 mm and dissipates only 1 mW. Results from functional testing confirm the successful operation of this integrated circuit. |
