Figure 9-10 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 9-10 shows a representative schematic for a non-intrusive ultrasonic flow meter using the OPAx863 devices and 12-V transducer excitation. The OPAx863 devices 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, also using the OPAx863 devices, first provides an AC-gain followed by a DC-level shift to lead to the PGA, ADC and processing within the MSP430 microcontroller.
Node 2 and Node 1 use similar transmit and receive circuits (discussed above) for the reverse path. The OPAx863 devices wide GBW of 50 MHz introduces minimal phase-delay and low-noise for superior flow rate measurement. The amplifier stays in power-down mode for a majority of the time in battery powered systems, resulting in very small average system-level power consumption and prolonged battery lifetime with its 1.4 µA (maximum) power-down mode quiescent current. Since the transmit and receive signal chains are connected to the same point at the respective node transducers, the OPAx863's 12.6-V supply voltage capability enables 12-V transducer excitation without any damage to the front-end or need for external switches which makes a compact solution. This makes the OPAx863 devices suitable for flow measurements in large diameter pipes and non-intrusive flow meters.