Design trade‐offs of a capacitance‐to‐voltage converter with a zoom‐in technique for grounded capacitive sensors

Abstract

This paper presents a low-power, high-precision capacitance-to-voltage converter (CVC) for grounded capacitive sensors. To measure very small capacitance variations in the presence of a large offset capacitance, a new zoom-in structure is proposed. The major non-idealities of the CVC such as the settling error, charge injection, and parasitic capacitance of the switches are minimized through an optimized design. Accordingly, it is shown that the zoom-in technique can significantly reduce many of these errors. The effect of the parasitic capacitances around the sensor capacitance is significantly reduced by using a switched-capacitor-based active-shielding technique. The interface is designed as an integrated circuit using a standard 0.18-μm CMOS technology. Simulation results show that for a sensor capacitor with a nominal value of 10 pF, variation of only 200 fF, and parasitic capacitance of up to 20 pF, a worst-case capacitance error of 0.2 fF can be achieved by taking into account the layout mismatches and the interconnection effects. The achieved latency is 100 μs, and the CVC consumes only 80 μA from a 2-V power supply. The simulated input capacitance resolution for this latency is 123 aF, which is quite close to our calculated resolution (126 aF). This resolution corresponds to an energy efficiency of 9.82 pJ/Step. A temperature sweep simulation has been performed over the temperature range from −45°C to 125°C to demonstrate the small thermal drift of the designed circuit.

An accurate and power‐efficient period‐modulator‐based interface for grounded capacitive sensors

“…In the following section, the major nonidealities of the designed interface will be described. It will be shown that the zoom‐in CVC with autocalibration can reduce more efficiently the effect of any nonidealities affecting the performance of the interface, compared with the conventional CVC and the zoom‐in CVC in Ahmadpour Bijargah et al Because of the essential role of the CVC, first the operation principle of the CVC with zoom‐in structure is introduced in Section . The rest of the interface is elaborated in Section .…”

“…Figure A shows the zoom‐in CVC structure for a grounded capacitive sensor . A conventional CVC is also shown within the dashed rectangle.…”

“…A, Zoom‐in capacitance‐to‐voltage converter (CVC) for a grounded capacitive sensor. B, Related control signals and the output voltage [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…In Ahmadpour Bijargah et al, the zoom‐in CVC circuit is completely analyzed, and the key parameters, such as the capacitors, transconductance, and switch sizes, to achieve the required accuracy and resolution, are investigated. To simplify the entire circuit design process, these parameters are given as 10 pF ≤ C X ≤ 10.2 pF, C Z = 10 pF, C F = 2 pF, C S = 10 pF, C P1 = C P2 = 20 pF, g m = 0.75 mS, and ( W / L ) switch = 1/0.18 μm.…”

“…This makes the output signal simply a function of the maximum variation of the sensor capacitance. Another method has also been proposed in Ahmadpour Bijargah et al, which uses the zoom‐in technique with grounded targets. That work demonstrates that, in addition to relaxing the high dynamic range requirement of the following stage, the zoom‐in technique can remove the systematic errors of the interface itself.…”