Fully Integrated Low-Noise Readout Circuit with Automatic Offset Cancellation Loop for Capacitive Microsensors

Abstract

Capacitive sensing schemes are widely used for various microsensors; however, such microsensors suffer from severe parasitic capacitance problems. This paper presents a fully integrated low-noise readout circuit with automatic offset cancellation loop (AOCL) for capacitive microsensors. The output offsets of the capacitive sensing chain due to the parasitic capacitances and process variations are automatically removed using AOCL. The AOCL generates electrically equivalent offset capacitance and enables charge-domain fine calibration using a 10-bit R-2R digital-to-analog converter, charge-transfer switches, and a charge-storing capacitor. The AOCL cancels the unwanted offset by binary-search algorithm based on 10-bit successive approximation register (SAR) logic. The chip is implemented using 0.18 μm complementary metal-oxide-semiconductor (CMOS) process with an active area of 1.76 mm2. The power consumption is 220 μW with 3.3 V supply. The input parasitic capacitances within the range of −250 fF to 250 fF can be cancelled out automatically, and the required calibration time is lower than 10 ms.

Oversampling Successive Approximation Technique for MEMS Differential Capacitive Sensor

“…where = 2 0 / . The relationship between two steps is ∆ ( + 1) = ( + 1) − ( ) (13) By combining the equations (9), (11), (12) and (13), the output in N-th step is,…”

“…by combining the equation (23) with the equations (11) and (12), the general expression of level of the output in N-th OSSA step considering holding error in each OSSA step is,…”

A novel design methodology for the mixed-domain optimization of a MEMS accelerometer

“…The power consumption constraint, on the other hand, has been selected as 60µW. The power consumption of the read-out circuitry may span from a few hundred µWs [17] to few mWs [18]. Since the read-out circuitry used only has a single stage that works as a C/V converter, the constraint has been set to low values to enable low power designs where the circuit noise will have an impact on the system level noise as well.…”

“…The population size and the number of generations of the optimization algorithm are 200 and 300, respectively. The only difference of the constraints compared to the single-objective optimization is the power consumption, that was set to 300µW, as a practical design value [17]. Figure 9 shows an approximation to the two-dimensional Pareto Front (PF), which includes the solutions trading-off system noise and manufacturing cost.…”

Low-Noise Multimodal Reconfigurable Sensor Readout Circuit for Voltage/Current/Resistive/Capacitive Microsensors

“…Microsensors provide output signals based on changes in voltage, current, resistance, or capacitance by utilizing specific physical/chemical variations. Various analog front-end architectures that can convert and amplify the input signals from microsensors to an amplified voltage or digital output have been reported, for example, pressure, acceleration, humidity sensors, and so on [2][3][4][5][6][7]. In the recent IoT environment, which demands multi-functional sensing capability, a readout circuit for multimodal sensing is required.…”