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MEMS electrothermal displacement sensors can be operated in constant current (CC) or constant voltage (CV) excitation modes. The CV mode is more commonly used. However, there have been reports that the CC excitation mode may lead to a larger measured signal, and thus, it may be a better choice than the CV mode. In this letter, we present an analytic comparison of the two methods, and show that from a signal-to-noise-ratio point of view, benefits of operating a sensor in CC mode are only marginal. The analytical investigation is supported by experiments performed on sensors integrated in a SOI-MEMS nanopositioner with low noise read out circuits, which leads to 0.04-nm/√Hz displacement resolution for both excitation modes.