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Acquire Signals from Charge Mode Sensors

Updated Apr 5, 2021

Reported In

Hardware

  • PXI Sound and Vibration Module
  • PXIe-4480
  • C Series Sound and Vibration Input Module

Issue Details

I have a charge mode piezoelectric sensor, and its sensitivity is specified in units of charge per engineering unit - such as in the case of a pC/g accelerometer.

Is it possible to acquire a signal from it using NI DSA hardware?

Solution

The PXIe-4480 supports the use of charge mode piezoelectric sensors without the need of any additional circuitry. For more information about the connection diagram you can reference the PXIe-4480/4481 User Manual on page 18. 
  • To confirm that your charge sensor is compatible with the PXIe-4480:
You need to determine the full charge range of the sensor: Full Charge Range = Sensitivity * Sensor's Range
The result should be in pC and within the signal ranges defined on the table Signal Range (charge) on page 3 of the PXIe-4480 Specifications manual .
  • For example:
We have a charge sensor with +/- 15 pC/µε of sensitivity and +/- 1500 µε of range. The Full Charge Range will be: 
Full Charge Range = +/- 15 pC/µε * +/- 1500 µε = +/- 22500 pC

This sensor will exceed the signal range of the PXIe-4480 (which is 20000 pC), so it will not work unless your expected measurements won't use the full range of the sensor and they are still within the module's range. 

Additional Information

  • Other NI DSA devices are designed to function only with IEPE or ICP® sensors. These sensors contain both a piezoelectric element (like charge mode sensors) and extra integrated electronics, namely an amplifier. This amplifier converts the high impedance charge signal from the piezoelectric crystal into a low impedance voltage signal that can be more easily digitized. As a result of adding active circuitry (i.e., the amplifier), the IEPE sensor will require excitation between 2-20 mA, which NI DSA cards are also capable of providing along the same coaxial cable as the measured signal.
  • Thus, in order to acquire signals from a charge mode sensor, which lacks internal amplification, you will need to provide an external charge amplifier to convert the high impedance charge signal to a low impedance voltage signal. Which is what the internal circuitry of the PXIe-4480 does. 
  • Consult the manufacturer of your sensor in case you are using an external amplifier for your application. Depending on the type of amplifier, it may require 2-20 mA excitation, in which case NI DSA cards can be used to excite and read back from the amplifier over the same coaxial connection. If the amplifier requires some other type of excitation, then you will need to use a separate external power supply to excite the amplifier. Also, consult the specification sheet for your NI DSA device to ensure that, after amplification, your amplifier's output signal still falls within the DSA device's input range and compliance voltage specifications.