Figure 9-6 shows how ultrasonic flow meters measure the rate of flow of a liquid using transit-time difference (t₁₂–t₂₁), which depends on the flow rate. Figure 9-6 shows a representative schematic for a non-intrusive ultrasonic flow meter using the OPAx863 and 12-V transducer excitation. The OPAx863 are used for the forward path as a unity-gain buffer for 12-V pulsed transducer excitation at NODE 1. At the same time, the receiver circuit at NODE 2 (which also uses the OPAx863) first provides an ac-gain followed by a dc-level shift to lead to the PGA, ADC, and processor within the MSP430™ microcontroller.

NODE 2 and NODE 1 use similar transmit and receive circuits (discussed previously) for the reverse path. The OPAx863 wide GBW of 50 MHz introduces minimal phase-delay and low-noise for excellent flow rate measurement. The amplifier stays in power-down mode for a majority of the time in battery-powered systems. This configuration results in very small average system-level power consumption and prolonged battery lifetime with the 1.5‑µA (maximum) power-down mode quiescent current with a 3-V supply. The transmit and receive signal chains are connected to the same point at the respective node transducers. Therefore, the OPAx863 12.6-V supply voltage capability enables 12-V transducer excitation without any damage to the front-end, or a need for external switches, thus enabling a more compact solution. These specifications make the OPAx863 an excellent choice for flow measurements in large diameter pipes and non-intrusive flow meters. The TIDM-02003 reference design discusses an ultrasonic gas flow sensing subsystem which uses high-speed amplifiers for front-end amplification.

Figure 9-6 Non-Intrusive Ultrasonic Flow Meter