For example, the capacitive micromachined ultrasound transducer converts the acoustic energy into the capacitance variation, and the capacitive accelerometer converts the acceleration into delta C. The FET-based biosensor detects the biomolecules and generates delta R while the piezoresistive force sensor transforms the mechanical force into the resistance variation. The electrode often used for biopotential recording converts the biopotential resulted from the flow of ions into the voltage variation generated by the flow of electrons. The piezoelectric strain sensor transforms the mechanical strain into delta V. Because the sensor generates the output signal to readout, it can be regarded as a signal source, but quite nonideal signal source in many cases, unfortunately. It often generates a weak output signal at low frequencies.

In many cases, the sensor also has a nonideal output impedance, meaning a high output impedance when the sensor is modeled as a voltage source, and a low output impedance when modeled as a current source.
Also, the sensor tends to show a large underlying nominal output value over which the actual sensing signal is superposed. All these nonidealities of sensors make the design and implementation of sensor readout circuits difficult.