An intelligent multiplexer/driver integrated circuit for an implantable multichannel blood flowmeter

A custom intelligent multiplexer/driver (IMD) IC was designed for use with an implantable three-channel ultrasonic pulsed Doppler blood flowmeter. It also facilitated the development of a system architecture for the three-channel flowmeter by adding only 0.8 mW to the 30 mW of power consumption in the implanted portion of the single-channel flowmeter. By sending RF command signals to the implanted electronics, the flowmeter can operate in either the single- or multichannel mode. A unique transducer multiplexing circuit was incorporated on the IMD to obtain a round-trip channel isolation of 38 dB while operating on a 2.7 V battery. This channel isolation is an order of magnitude superior to that obtained by an earlier approach. A power transistor was fabricated (on the IMD) with standard MOS processes to achieve an on-resistance of 3 Ω (at a gate-source voltage of 2.7 V) and rise and fall times of 10 ns for 6-MHz transducer excitation