Another function of the AFE is to filter unwanted frequency ranges from the sensor, for example, to satisfy the Nyquist limit or to remove a DC offset. This embedded noise must be removed before the analog signal is converted to embedded digital. The AFE must employ low-pass filter circuitry to block out high-frequency noise and/or employ high-pass circuits to remove lower-frequency noise.
A second function of the AFE is to convert signals from one signal type to another. For example, typical sensors output a voltage, but some output a current. The MCU ADC circuits do not accept current inputs, so such currents have to be converted to voltages before going to the MCU. This current-to-voltage conversion is performed by the AFE circuit, called a transimpedance (I/V) circuit, which also amplifies the resulting voltage to levels usable by the MCU.
refer to: http://embedded-computing.com/articles/latest-afe-simplifies-interfacing-hundreds-sensors/#at_pco=cfd-1.0
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