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

A custom CMOS integrated circuit that extends the current implantable single-channel ultrasonic pulse Doppler blood flowmeter to three channels is discussed. By operating on a single 2.7-V battery with three existing custom bipolar ICs, the integrated circuit serves as an RF command receiver, controls the channel-multiplexing sequence, and multiplexes the piezoelectric transducers. A round-trip channel isolation of 38 dB is achieved with a novel transducer-multiplexing circuit that employs three on-chip power transistors for 6-MHz transducer excitation. Each power transistor is capable of switching a current of 200 mA with an on-resistance of 3 Omega and transition times of 10 ns. The power consumption in the implanted portion of the three-channel flowmeter is only 0.8 mW higher than the 30 mW in the single-channel flowmeter.