KH400A piezoelectric accelerometer is a low-frequency high-sensitivity piezoelectric accelerometer with built-in IEPE circuit; it converts the charge output by the charge-type piezoelectric sensor into a low-impedance voltage output through a preamplifier installed inside the sensor . IEPE type sensors are usually in the form of two-wire output, that is, a constant current source is used for power supply; the same line is used for power supply and signal. Usually the DC part is filtered out by a high-pass filter at the output of the constant current power supply. The biggest advantage of IEPE type sensor is good measurement signal quality, low noise, strong anti-interference ability and long-distance measurement, especially many new data acquisition systems have been equipped with constant current sources, therefore, IEPE sensors can be directly connected to the data acquisition system. Without any other secondary instrumentation. IEPE sensors have gradually replaced traditional charge output piezoelectric accelerometers in vibration testing. KH400A has the characteristics of high sensitivity, high stiffness, and low frequency response lower limit. It is widely used in vibration and shock measurement in aerospace, aviation, transportation, construction, bridges, industrial monitoring, scientific research and teaching, measurement and other fields; especially suitable for bridge structural testing, building Low-magnitude, very low-frequency vibration fields such as vibration monitoring, earthquake detection, ground and foundation vibration monitoring.