Bruel & Kjaer 2625
When measuring vibration by means of an accelerometer, it is necessary to incorporate a preamplifier between the accelerometer and the measuring amplifier. The preamplifier is introduced into the measuring circuit for two reasons: 1) to amplify the relatively weak output signal from the accelerometer and 2) to transform the high output impedance of the accelerometer into a lower value. The signal from the piezoelectric accelerometer appears as a voltage across a capacitive impedance. This charge is proportional to the acceleration to which the accelerometer is exposed. As the capacitive output impedance of the accelerometer is very high, the associated amplifier must be of a special design with a high input impedance. This is necessary in order to avoid loading the accelerometer and thereby obtain decreased sensitivity and limitation at the low end of the frequency range. The B & K preamplifiers are designed according to two different amplifier principles: 1) the voltage preamplifier and 2) the charge amplifier. The output signal of the voltage preamplifier is directly proportional to the input voltage, whereas the output signal of the charge amplifier is proportional to the input charge. Accelerometer preamplifier
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Bruel & Kjaer 2625
