Robust Capacitance-to-PWM Converter With Enhanced Zoom-in Capabilities for Single and Differential Capacitive Sensors

A novel capacitance-to-pulsewidth modulated converter (CPC) capable of efficiently measuring the change in capacitance (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta C$ </tex-math></inline-formula>) with efficient zoom-in capabilities is presented in this article. The CPC works with both single-element-type and differential-type capacitive sensors (SCS and DCS), ensuring that measurements are unaffected by leakage resistance and offset capacitance mismatches in the DCS. The CPC uses a closed-loop mechanism to overcome common challenges such as the op-amp’s offset voltage, bias current, <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small>-resistance of switches, charge injection, parasitic capacitance, drift in reference voltages, and thermal noise. It ensures high sensitivity, constant resolution, and efficient zoom-in capabilities across a wide <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta C$ </tex-math></inline-formula> range. Additionally, its ratiometric output configuration makes it suitable for both linear and nonlinear DCS applications. Existing methods for measuring <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta C$ </tex-math></inline-formula> suffer from at least one of these challenges: slow measurements (measurement time ¿ 100 ms), errors due to offset capacitance and leakage resistance, and reduced resolution over wide <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta C$ </tex-math></inline-formula> ranges. In contrast, the proposed CPC demonstrates high linearity with a worst case error of 0.29% over a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta C$ </tex-math></inline-formula> range of ±2500 pF. The CPC efficiently eliminates the influence of leakage resistance, achieving an error of 0.28% when leakage resistance is made as low as 50 k<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega $ </tex-math></inline-formula>. Extensive testing confirms the negligible influence of circuit imperfections on the CPC’s performance. The proposed CPC is an efficient converter that can be directly used in many applications of both the SCS and DCS.