Real-world measurement requires the extraction of weak signals from noisy sources. High common-mode voltages are often present even in differential measurements. The usual approach to this problem is to use an op amp or an instrumentation amplifier and then perform some type of low-pass-filtering to reduce the background noise level. The problems with this traditional approach are that a discrete op-amp circuit has poor common-mode rejection, and its input voltage range is always lower than the power-supply voltage.
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