The application of integrated circuits in medical implants and the complexity of these implants have increased at a rapid pace in the past few years. The need, however, still exists for a highly accurate and stable telemetry system for the measurement of blood flow. Two custom-designed ICs have been realized to resolve this problem. These ICs form the heart of a totally implantable pulsed Doppler ultrasonic bidirectional blood flowmeter; one circuit performs the basic timing functions, and the second implements low-level linear signal processing. For a small implanted package (3.8×2.8×0.8 cm/SUP 3/), these ICs must meet the stringent requirements of low-voltage operation (2.2-2.8 V), low power (<40 mW), high stability (short-term timing jitter <50 ppm), and the minimum of external components. Using a quadrature direction detecting technique, the circuits sense both positive and negative flow and produce a multiplexed telemetry signal. The approach used minimizes parts count and power drain and maximizes channel-to-channel matching in the multiplexed signal.