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Low-frequency vibration signal monitoring

Question

We are monitoring an extremely low-frequency vibration signal within our semiconductor photolithography equipment, in order to ensure accurate print reproduction. The Endevco® model 86 piezoelectric accelerometer was selected because of its low-noise, high-sensitivity and near-DC response capabilities. While we believe this to be the best technology for our application, we also see that the model 86 is only capable of vibration measurements to mg (10-3 g level) and our requirements are down to μg (10-6 g level). Can you help?

Answer

The standard version of the Endevco model 86 offers a sensitivity of 10 V/g, which is equivalent to 10 mV/mg or 10 μV/μg. When measuring down to μg level, the accelerometer output signal is indicated in μV range, which is impractical for this application, due to the presence of ambient noise. To obtain a manageable higher output signal, we need to amplify the output by at least 50 times without significant increase in ambient noise levels. The model 86 has extremely low noise, which makes such signal amplification possible. The schematic shown in figure 1 below is a low-noise amplifier with a gain of 50 which also provides the required 2 mA constant current power source for operation of the model 86. This circuit was expressly designed for this type of application environment and has the field-proven pedigree of many successful installations. Use of batteries is also recommended with the power supply to ensure that no additional line noise is introduced. With a gain of 50, the resultant sensitivity is 0.5 mV/μg, raising it to manageable levels. The noise introduced by the amplifier is OP amp noise of approximately 90 nV/P-P.

The gain can be individually tailored to the application if desired, and may be calculated by the ratio C1/C2. The -3 dB low-frequency corner for this amplifier is 0.00156 Hz, when calculated from the formula below. The high-frequency -3 dB corner is in the 100 KHz region, which falls well outside of the level of materiality of this low-frequency application. Thus, specifications of the Endevco model 86 piezoelectric accelerometer are more than suitable for conducting measurements within this semiconductor photolithography application.