Strain Gage Measurement Using An Ac Excitation

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, as shown in Figure 1. The
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.
In Figure 1, a 400 Hz signal excites the bridge. The signal
at the AD8221 Äôs input is an ac voltage. Similarly, the signal
at the input of the AD630 is ac; the signal is dc at the end
of the low-pass flter following the AD630.
The 400 Hz ac signal is rectifed and then averaged; dc
errors are converted to an ac signal and removed by the
AD630. Ultimately, a precision dc signal is obtained.
The AD8221 † is well suited¬† for¬† this application because
its 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  amplifers, CMRR  falls  off  at
about 200 Hz. In contrast, the AD8221 continues to reject
common-mode signals beyond 10 kHz.
If an ac source is not available, a commutating voltage
may be constructed using switches. The AD8221 Äôs high
CMRR over frequency rejects high frequency harmonics
from a commutating voltage source.
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Using an AC Signal to Excite the Bridge
Using an AC Signal to Excite the Bridge

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