Seismic Floor Vibration

Seismic measurements (very low frequency) require extremely low noise A/D converters. This is due to the low frequency sensors used. To measure BBN criterion E effectively requires a noise floor of > -50 dB

When measuring the different criterion if the noise floor of the A/D is unknown then the measurements can be erroneous. Start by understanding the sensor’s sensitivity and noise floor and then determine the measurement system’s noise floor and dynamic range. An example is the PCB XYZ with a frequency range of .1 Hz to 1000 Hz, sensitivity of 1000 mV/mG and a noise floor of -45 dB. The measurement system must have a lower noise floor or it can introduce noise into the measurement. Crystal Instruments Spider-80X series and the CoCo series noise floor is -70 dB well below the sensor noise floor so seismic measurements can be made with confidence in the accuracy. The dynamic range is important since the levels may vary from .1 mG to 10 G’s. This large range can be measured by the Crystal Instruments dual 24 bit A/D design resulting in a total of 124 dB dynamic range.

The vibration environment of facilities used for research, photolithography, microelectronics manufacture, and similar activities has a significant effect on the performance of the equipment used within. As the dimensional requirements of today’s equipment and processes become more demanding, their sensitivity to external vibration increases. When designing such facilities, the stiffness of the floors and supporting structure must be such that the vibration levels are kept below that which causes problems for the equipment.

Crystal Instruments provides solutions to conduct testing according to the associated IEST, ISO and NIST-A criteria. The CoCo portable signal analyzer is equipped to take the 1/3 octave vibration measurements that are needed.  The CoCo also contains pre-configured modules to make it simple and easy to perform the tests according to the standards. For larger environments, the Spider series of hardware can be used to measure up to 512 channels.