The receiving part of the circuit i

The receiving part of the circuit is composed of an amplifying circuit, a band-pass filter circuit and a signal conversion circuit. After the receiving transducer chip is subjected to the vertical action of the ultrasonic wave, the mechanical vibration gradually increases due to resonance. Due to the piezoelectric effect, the same number of AC charges with different signs appear on both sides of the wafer. The amount of charge is very small and can only provide a small AC voltage signal, but not a current signal. Therefore, a pre-amplifier circuit is needed to fully amplify small AC voltage signals and consider possible interference signals. When amplifying useful signals, add a filter circuit to drive the output potential of the following comparator to jump to determine the receiving time. The function of the pre-amplification circuit unit is to amplify the useful signal, suppress other noise and interference, so as to achieve the maximum signal-to-noise ratio, which is beneficial to the subsequent circuit design. Because the ultrasonic signal attenuates greatly when it propagates in the air, the reflected ultrasonic signal is very weak and cannot be directly sent to the later circuit for processing. The signal must be amplified to a sufficient amplitude so that the later circuit can process it correctly. The preamplifier is a bootstrap non-inverting AC amplifier with high input impedance composed of an integrated operational amplifier. C5, C6, and C7 are DC isolation capacitors, and R5, R6, and R7 are bias resistors, which are used to set the static operating point of the amplifier. The band-pass filter uses a second-order RC active filter to eliminate the influence of interference signals in the process of ultrasonic propagation. The ultrasonic receiving circuit is shown in Figure 3-2.