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Measure strain gage with AC Excitation

Posted: 02 Jan 2009     Print Version  Bookmark and Share

Keywords:Strain gage  AD8221  signal 

Strain gage measurements are often plagued by offset drift, 1/f noise, and line noise. One solution is to use an ac signal to excite the bridge. Analog Devices AD8221 gains the signal and an AD630AR synchronously demodulates the waveform. What results is a dc output proportional to the strain on the bridge. The output signal is devoid of all dc errors associated with the in-amp and the detector including offset and offset drift.

Analog Devices says that AD8221's high CMRR over frequency ensures that the signal of interest, which appears as a small difference voltage riding on a large sinusoidal common-mode voltage, is gained and the common-mode signal is rejected. In typical instrumentation amplifiers, CMRR falls off at about 200 Hz. In contrast, the AD8221 continues to reject common-mode signals beyond 10kHz.

View the PDF document for more information.





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