Analog current-mode interfaces for differential capacitance sensing

In this work, we present two analog current-mode interfaces performing differential sensor capacitance measurements through a voltage reading. The AC peak-to-peak output voltage is proportional to the measurand x that is directly related to the differential capacitance variation. The first interface is able to read the sensor change through a current-to-voltage conversion, while the second one employs an input voltage source. Both of the solutions utilize commercial second generation current conveyors (CCII) as active blocks, in particular AD844. The proposed electronic circuits have been designed and simulated through PSPICE software. Theoretical analysis, simulated and experimental results have been analyzed: the developed read-out circuits have shown a good accuracy, being the maximum percentage error between simulation and theory lower than 0.09% for the first solution, while in the second interface the maximum relative percentage error, with respect to theory, is lower than 0.2% in the simulated case and than 3.2% for the measurements, respectively. Considering a distance between electrodes variation measurand sensitivity (S) and resolution (res) values are also satisfactory for each operating point: for the first solution we obtained S=8.54 V/μm and res=674 pm (capacitance resolution value is 89.85 fF, that is -79 dB), while for the second one S=8.13 V/μm and res=814 pm (capacitance resolution value is 108.5 fF, that is -77 dB). The signal peak-to-peak voltage value detection has been implemented by using a NI DAQ device and a Virtual Instrumentation Suite (NI ELVIS).