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Why Does my Load Cell Signal Decay?

Hardware: CompactDAQ>>Analog Input Modules>>NI 9233

Problem:
Why is my load cell returning readings that delay exponentially, even though I am applying a constant force?

Solution:
Many load cells and external power supply contain AC Coupling circuitry.  This circuitry is intended to reject DC noise components from the external power supply.  In some cases, this can result in your signal decaying exponentially, according to the sensor's discharge time constant.

This decay looks similar to the chart shown below.  Notice both the exponential decay and the sinusoidal noise.  These are both effects of the AC coupling.



In order to overcome these effects, there are several options:
  • If you need external excitation, choose a power supply that does no contain AC Coupling circuitry.
  • Use the internal excitation provided by the NI 9233 and NI 9234.
  • Use sensors that do not contain AC Coupling circuitry.


Related Links:
Developer Zone Tutorial: Coupling
Wikipedia: AC Coupling
Product Page: NI 9233

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Report Date: 12/02/2008
Last Updated: 12/02/2008
Document ID: 4S1AJSZL

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