Capacitive Readout Circuit Research Articles

A high precision and low power consumption switching capacitor readout circuit

In this paper, a high precision and low power switching capacitor readout circuit for MEMS sensors is proposed. The capacitance readout circuit is realized by using the total difference separation loop structure composed of a switching capacitor charge amplifier. In order to reduce the aggravation of common-mode parasitic capacitance to the input offset error and amplifier gain error in the readout circuit, the input common-mode feedback compensation structure and oversampled successive approximation (OSA) readout technology are introduced. In addition, the circuit adopts a non-overlapping nested clock to reduce charge injection and improve output accuracy. Based on SMIC 0.18 μm CMOS technology, the circuit can recognize 0-0.2 pF capacitance signals and convert them into voltage signals, achieving good linear fitting. When the common-mode parasitic capacitance is 1-16 p, it has good resistance to common-mode parasitic capacitance, and the precision of the capacitor readout circuit is increased by 3.9%. Compared with traditional readout circuits, it has a simple structure and low power consumption.

Design of analog front-end readout circuit for capacitive silicon micro gyroscope

To design a high-performance analog front-end readout circuit for capacitive silicon micromachined gyroscope, this paper presents electrical modeling of the silicon micromachined gyroscope according to its working characteristics, proposes to construct the amplitude and frequency range of the silicon micromachined gyroscope signal in the form of a capacitor array, and designs the capacitance readout circuit of the gyroscope based on this model. Based on the SMIC 0.18 um CMOS process, the electrical model and its capacitance readout circuit are designed and simulated in this paper. The results indicate that the capacitance readout circuit can accurately detect the equivalent variable capacitance signal of the gyroscope electrical model and transform it linearly into a voltage signal. The equivalent output noise of the overall analog front-end readout circuit is 80.11 nV/√Hz@6.4kHZ. The system resolution can reach 0.29 aF. As a result, the silicon micro gyroscope analog front-end circuit has a wide range of capacitive applications, high linearity, low noise, and broad market and application prospects. Hence, this topic is of great scientific significance.

A Pseudo-Differential Capacitive MEMS Accelerometer Analog Front-End Design

Based on the current design and application of the chip and the research status at home and abroad, we proposed a silicon micro-accelerometer analog front-end integrated circuit with high resolution and wide capacitance application range by using SMIC 0.18μm CMOS technology. The circuit is composed of a capacitive readout circuit, a demodulator, and a low-pass filter. To detect the weak capacitive signal, the capacitor readout circuit of this paper adopts the continuous-time voltage mode charge amplifier structure and combines with the signal modulation technology and the optimization of the MOS transistor size to achieve low noise and high resolution. With the application of the signal modulation technology, the low-frequency capacitance sensor signal is shifted from 2-12 kHz to high-frequency 800 kHz. The signal modulation technology modulates the low-frequency capacitive sensor signal of 2~12 kHz at the high frequency of 800 kHz, and the signal is converted into voltage through the capacitive readout circuit. The simulation results show that the designed analog front-end circuit can convert the accelerometer capacitance signal with the frequency of 2~12 kHz, the dynamic capacitance peak is 0.108 pF, and the static capacitance is 10 pF into the expected voltage signal. In addition, the output noise can be as low as 556 nV/√Hz in the capacitive frequency range of the accelerometer, which meets the requirement of 2.5 aF capacitive resolution. Therefore, the silicon micro-accelerometer analog front-end circuit designed in this paper has the characteristics of a wide capacitor application range, low noise, and high resolution, and can be widely used.

Capacitive sensor readout circuit based on sample and hold method

This paper presents a capacitive sensor readout circuit using sample and hold method. The proposed readout circuit is used to convert capacitance from sensor to DC (direct current) voltage output. The basic structure of readout circuit consists of the pulse generator circuit, differentiator circuit, amplifier circuit, monostable I circuit, monostable II circuit, and sample and hold circuit. The proposed technique is based on the change of time constant from differentiator circuit corresponding to the measurement capacitance. The sample and hold circuit is used for sampling output voltage from differentiator circuit. The output voltage of the proposed readout circuit is proportional to measurement capacitance. The standard capacitors with different capacitance are used to test the proposed converter performance. Experimental results show that the proposed readout circuit can convert measurement capacitance to output voltage with satisfied values, good linearity and high sensitivity.

An ultra-efficient recycling folded cascode OTA based on GAA-CNTFET technology for MEMS/NEMS capacitive readout applications

This paper demonstrates an ultra-efficient recycling folded cascode (RFC) operational transconductance amplifier (OTA) as one of the cardinal blocks in most analog microsystems. Two positive feedbacks are utilized in the proposed RFC OTA to enhance the input transconductance and the output impedance. All GAA-CNTFETs are biased in the near-threshold region (VT≈0.37 V and VDD=±0.2 V) to reduce power consumption. The simulation results indicate that with CLoad = 5pF, the proposed RFC OTA shows 102 dB open-loop gain and 17.2 MHz unit gain-bandwidth with an ultra-low power consumption of 111.5nW and a reasonable slew rate response (17.3 V/µs). A capacitive readout circuit for micro-and nano-electromechanical systems (MEMS/NEMS) applications is suggested to assess the proposed RFC OTA's performance in practical applications. By utilizing an RC sensing feedback structure, the proposed transimpedance amplifier presents 155dBΩ gain with a 400 MHz bandwidth. Compared to the other counterparts, the proposed GAA-CNTFET-based TIA presents fascinating power savings 86% total average improvement) and 1.6 times total average gain improvement. Our simulation results accentuate that the proposed GAA-CNTFET-based RFC OTA is a powerful candidate for designing ultra-efficient MEMS/NEMS capacitive accelerometers readout circuits.

Design, modelling and characterization of comb drive MEMS gap-changeable differential capacitive accelerometer

This paper presents design optimization, numerical modelling, fabrication and characterization of a single-axis comb drive MEMS gap-changeable differential capacitive accelerometer with a compact footprint of 2 mm × 2 mm. High sensitivity is achieved by optimizing the ratio of the anti-finger gap to finger gap spacing. It is found that a gap ratio of 3.44 leads to a displacement sensitivity of 0.0139 µm/g and a capacitive sensitivity of 80 fF/g. Finite element analysis is used to find the mode shape and frequency. Scanning electron microscope and capacitance-voltage measurements are employed to confirm the devices work. An enlarged static capacitance of 14.46 ± 0.42 pF is measured due to the enlarged stator anchor width. Electrical sensitivity of 35.93 mV/g is obtained by using the capacitance readout circuit at the system level, and the electrical sensitivity is practically constant over the frequency range of 100–200 Hz.

Development of a Real-Time Force and Temperature Sensing System with MEMS-LSI Integrated Tactile Sensors for Next-Generation Robots

This study proposes a sensing system that can sense force and temperature at the same time. The system consists of MEMS-LSI integrated tactile sensor devices called sensor nodes, a field-programmable gate array (FPGA) based relay node, and a host PC. For real-time temperature and force data acquisition, a time-sharing force and temperature task processing mechanism was implemented with a dedicated computer architecture in the FPGA configuration and the host program. This study firstly reports the temperature dependency analysis of a capacitive sensor readout circuit in the sensor node by circuit-level simulation. With a fabricated sensor node, sensor output data were measured and analyzed with varying temperatures and applied force. Based on the measured data, linear multiple regression equations for temperature compensation of sensed force data were developed. In the temperature range of 24.8°C–60°C, the average/maximum force errors when considering the temperature effect were −0.98%/65% without the compensation, and 0.072%/17% with the compensation, respectively. One cycle time of temperature and force sensing for one sensor node was 113 ms on average. The experimental results showed that real-time temperature and force sensing and temperature compensation for accurate force sensing could be achieved successfully. The study also demonstrated the system with hot-coffee cup and finger touch examples.

A sensitivity-enhanced capacitance readout circuit with symmetric cross-coupling structure.

This paper presents a proposed capacitance readout circuit that enables a quadrupled (x4) output strength. A symmetric cross-coupling structure is proposed to amplify the voltage difference between two adjacent channels; hence, the detected signal can be integrated twice every clock cycle. Compared with conventional schematics, the proposed readout circuit shows an increased output strength for integration times within dozens of μs. In addition, the measurements show that the integrator resistors should be less than 1 kΩ to suppress the resistance-capacitance delay effects. Although the proposed capacitance readout circuit is implemented using discrete transistors, it has a good signal integrity at an operating clock cycle of 100 µs. Therefore, the proposed readout circuit is a promising way to detect small capacitance variations with short integration times.

Toward High Throughput Core-CBCM CMOS Capacitive Sensors for Life Science Applications: A Novel Current-Mode for High Dynamic Range Circuitry.

This paper proposes a novel charge-based Complementary Metal Oxide Semiconductor (CMOS) capacitive sensor for life science applications. Charge-based capacitance measurement (CBCM) has significantly attracted the attention of researchers for the design and implementation of high-precision CMOS capacitive biosensors. A conventional core-CBCM capacitive sensor consists of a capacitance-to-voltage converter (CVC), followed by a voltage-to-digital converter. In spite of their high accuracy and low complexity, their input dynamic range (IDR) limits the advantages of core-CBCM capacitive sensors for most biological applications, including cellular monitoring. In this paper, after a brief review of core-CBCM capacitive sensors, we address this challenge by proposing a new current-mode core-CBCM design. In this design, we combine CBCM and current-controlled oscillator (CCO) structures to improve the IDR of the capacitive readout circuit. Using a 0.18 μm CMOS process, we demonstrate and discuss the Cadence simulation results to demonstrate the high performance of the proposed circuitry. Based on these results, the proposed circuit offers an IDR ranging from 873 aF to 70 fF with a resolution of about 10 aF. This CMOS capacitive sensor with such a wide IDR can be employed for monitoring cellular and molecular activities that are suitable for biological research and clinical purposes.

Thermomechanical Noise Characterization in Fully Monolithic CMOS-MEMS Resonators.

We analyzed experimentally the noise characteristics of fully integrated CMOS-MEMS resonators to determine the overall thermomechanical noise and its impact on the limit of detection at the system level. Measurements from four MEMS resonator geometries designed for ultrasensitive detection operating between 2-MHz and 8-MHz monolithically integrated with a low-noise CMOS capacitive readout circuit were analyzed and used to determine the resolution achieved in terms of displacement and capacitance variation. The CMOS-MEMS system provides unprecedented detection resolution of 11 yF·Hz−1/2 equivalent to a minimum detectable displacement (MDD) of 13 fm·Hz−1/2, enabling noise characterization that is experimentally demonstrated by thermomechanical noise detection and compared to theoretical model values.

Implementation of 144 × 64 Pixel Array Bezel-Less Cmos Fingerprint Sensor

Abstract This paper proposes CMOS integrated 144 × 64 pixel array fingerprint sensor without a bezel electrode. In this paper, the architecture of CMOS capacitive fingerprint sensor readout circuit is presented for general type of a switched capacitive integrator scheme. The pipelined scan driver is included in the fingerprint sensor for fast image capture. It is implemented on 0.35 μm standard CMOS process technology. The operation is validated by SPECTRE for one-pixel and RTL simulation including logic synthesis for a full chip design on condition of 0.35 µm typical CMOS process and 3.3 V power. The layout is performed by full custom flow for sensor cell array and auto P&R for a full chip. The area of a full chip is 4943 μm × 3943 μm and the gate count is 542,000. The area of one-pixel is 48 × 48 µm2. The pitch is 50 µm and image resolution is 508 dpi and power consumption is less than 3 mA.

A Robust Capacitive Digital Read-Out Circuit for a Scalable Tactile Skin

This paper presents a robust, capacitive digital read-out circuit (ROC) for sensitive skin applications in humanoid robots. The ROC can be calibrated to null the parasitic effects of transducer variation due to physical assembly. A prototype is fabricated in a 130 nm RFCMOS process, with an active area of $221\times 79~\mu \text{m}^{2}$ and 1.84 $\mu W$ power consumption at $V_{\mathrm {DD}}=1.2\text{V}$ and 1ms read-out rate. The ROC output is robust to $V_{\mathrm {DD}}$ and temperature variations in a range ${|\Delta V_{\mathrm {DD}}|}\leq 20\%$ and (25–53) °C. Furthermore, it can provide up to 200m $V_{\mathrm {pp}}$ power supply sine wave rejection in the range 50Hz–5MHz at $V_{\mathrm {DD}}=1.1\text{V}$ , for an output standard deviation lower than one LSB. Owing to its features and its digital modularity, the ROC was co-designed with a scalable and modular Multi-Walled Carbon NanoTube (MW-CNT) Nano composite transducer, to achieve tunable output sensitivity by adjusting the sensor nominal capacitance and the reference capacitance. The maximum sensitivity of 5.23 fF per LSB was reached when both match. The ROC was then validated with the MW-CNT nanocomposite sensor which exhibits a piecewise behaviour. 5.3 and 7.1 ENOB were extrapolated in the low-load and medium-load regions, respectively. Besides the major advantage of tunable sensitivity, the presented ROC features the lowest acquisition time and one of the most compact sizes among the state-of-the-art ROCs. Moreover, process voltage and temperature (PVT) robust output and ultra-low power consumption make this solution very attractive to replicate human physiology at robotic-level.

Reducing Measurement Error in Capacitive Readout Circuits Based on Weakly Coupled Resonators

A recently proposed method for measuring minute changes in capacitance based on weakly coupled resonators has been examined for its sensitivity with respect to unwanted resonant frequency changes. This sensitivity has been compared with the conventional capacitance measurement method based on the capacitance to frequency conversion. It has been analytically shown that both sensitivities are nearly equal but with opposite signs. This has led to a new hybrid capacitance measurement method to reduce the error by at least an order of magnitude. The analytical results have been validated by various simulations and experimental measurements.

Wide Input Range Supply Voltage Tolerant Capacitive Sensor Readout Using On-Chip Solar Cell

In this paper, a wide input range supply voltage tolerant capacitive sensor readout circuit using on-chip solar cell is presented. Based on capacitance controlled oscillators (CCOs) for ultra-low voltage/power consumption, the sensor readout circuit is directly powered by the on-chip solar cell to improve the overall system energy efficiency. An extended sensing range with high sensing accuracy is achieved using a two-step successive approximation register (SAR) and delta-sigma ([Formula: see text]) analog-to-digital (A/D) conversion (ADC) scheme. Digital controls are generated on-chip using a customized sub-threshold digital standard cell library. Systematic error analysis and optimization including the finite switch on-resistance, buffer input-dependent delay, and SAR quantization nonlinearity are also outlined. High power supply rejection ratio (PSRR) is ensured by using a pseudo-differential topology with ratiometric readout. The complete sensing system is implemented using a standard 0.18[Formula: see text][Formula: see text]m complementary metal-oxide-semiconductor (CMOS) process. Simulation results show that the readout circuit achieves a wide input range from 1.5[Formula: see text]pF to 6.5[Formula: see text]pF with a worst case PSRR of 0.5% from 0.3[Formula: see text]V to 0.42[Formula: see text]V (0.67% from 0.3[Formula: see text]V to 0.6[Formula: see text]V). With a 3.5[Formula: see text]pF input capacitance and a 0.3[Formula: see text]V supply, the [Formula: see text] stage achieves a resolution of 7.1-bit (corresponding to a capacitance of 2.2[Formula: see text]fF/LSB) with a conversion frequency of 371[Formula: see text]Hz. With an average power consumption of 40[Formula: see text]nW and a sampling frequency of 47.5[Formula: see text]kHz, a figure-of-merit (FoM) of 0.78[Formula: see text]pJ/conv-step is achieved.

On-Chip Touch Sensor Readout Circuit Using Passive Sigma-Delta Modulator Capacitance-to-Digital Converter

This paper describes a capacitive type touch sensor readout circuit using passive sigma-delta modulator-based capacitance-to-digital converter. With the proposed approach, the panel condition (touched or untouched) can be effectively converted into the digital level using the panel capacitance as the summing element of the first-order passive sigma-delta modulator. In addition, the proposed touch sensor readout circuit does not require an analog-to-digital converter, since the digital format corresponding to the touch panel condition can be simply obtained by counting the number of modulator output pulses within the detection period. This along with the passive architecture leads to a compact and low-power on-chip touch sensor readout circuitry. Furthermore, due to the noise shaping property of the sigma-delta modulator, a comparable SNR and detection time with respect to other touch sensor readout circuits can be achieved. The proposed touch sensor readout circuit is implemented using CMOS 0.35-μm technology with core area of 0.1 mm 2 . The operation of the sensor readout circuit is verified with a 10.4 projective capacitance type touch panel, where the measured SNR is 31.4 dB with power consumption of 65 μW.

SU-8 as a Torsional Spring: Viscoelastic Creeping and Closed Loop Control

The torsional viscoelastic creeping and recovery of an SU-8 spring is reported by studying the deflection angle variation of a vertical comb-drive actuator suspended by an SU-8 spring at three different stress levels. A maximum creep of 0.43° (corresponding to a relative change of strain of 5.7%) is observed under a constant stress of 8.3 MPa in 1 h ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V =22$ </tex-math></inline-formula> .7 V). A capacitive readout circuit is designed to sense the torsional deflection of the comb-drive actuator. The circuit is capable of actuating and sensing the capacitance change by the same comb-drives with a resolution of 0.03° (corresponding to a capacitance resolution of 0.05 pF). The readout capacitance is then used as the input variable for a proportional-integral (PI) controller in order to eliminate the time and load dependent drift by creeping of the SU-8 spring. The PI controller is tuned by the Ziegler–Nichols method and has a settling time of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 2.3$ </tex-math></inline-formula> s for the output. Compared with the open loop deflection of 0.43°, the closed-loop deflection is reduced by a factor of 7 down to 0.06°. This controller error is prominent at high actuation voltages, and is attributed to the actuator’s lateral and vertical cross-sensitivity. [2014-0124]

Measurement of position deviation and eccentricity for μ-disc-type inductive micro-motor

An innovative capacitive gap-sensing readout circuit to replace traditional gap sensor is designed to measure the motion of the levitated micro-disc embedded in an inductive micro-motor. Twelve equivalent capacitor pairs are constructed to detect the position deviation of the disc. As the position deviation of disc occurs, the capacitances of the corresponding capacitor pairs are altered. In addition, by applying the effects of inertial force and centrifugal force, an innovative non-contact measurement method to quantify the unbalance degree of the micro-disc, i.e., eccentricity, is also proposed. By commercial computer simulations and realistic experiments undertaken, the performance of the proposed capacitive gap-sensing readout circuit has been successfully verified. The mean of output voltage of gap-sensing readout circuit is about 327mV under the position deviation of the disc being 8μm. Moreover, the unbalance degree of the disc is approximately proportional to the square of the position deviation of the disc.

A Power-Efficient Capacitive Read-Out Circuit With Parasitic-Cancellation for MEMS Cochlea Sensors.

This paper proposes a solution for signal read-out in the MEMS cochlea sensors that have very small sensing capacitance and do not have differential sensing structures. The key challenge in such sensors is the significant signal degradation caused by the parasitic capacitance at the MEMS-CMOS interface. Therefore, a novel capacitive read-out circuit with parasitic-cancellation mechanism is developed; the equivalent input capacitance of the circuit is negative and can be adjusted to cancel the parasitic capacitance. Chip results prove that the use of parasitic-cancellation is able to increase the sensor sensitivity by 35 dB without consuming any extra power. In general, the circuit follows a low-degradation low-amplification approach which is more power-efficient than the traditional high-degradation high-amplification approach; it employs parasitic-cancellation to reduce the signal degradation and therefore a lower gain is required in the amplification stage. Besides, the chopper-stabilization technique is employed to effectively reduce the low-frequency circuit noise and DC offsets. As a result of these design considerations, the prototype chip demonstrates the capability of converting a 7.5 fF capacitance change of a 1-Volt-biased 0.5 pF capacitive sensor pair into a 0.745 V signal-conditioned output at the cost of only 165.2 μW power consumption.

Air-Gap Detection Circuit Using Equivalent Capacitive Changes for Inductive Micromotor

In this paper, we present an innovative capacitive gap-sensing readout circuit to replace the traditional gap sensor embedded in an inductive micromotor (IMM). Twelve equivalent capacitor pairs are constructed to detect the position deviation of the eccentric disc, which is rotating. As the position deviation of disc occurs, the capacitances of the corresponding capacitor pairs are altered due to air gap changes. The major functions of the proposed capacitive gap-sensing readout circuit are: 1) to amplify a micrometer-scale signal to be of millimeter scale; 2) to suppress the low-frequency noises caused by exerted three-phase ac power; 3) to rectify the induced ac gap-sensing signal into a dc output voltage; and 4) to eliminate the undesired dc bias caused by parasitic capacitances. In addition, by applying the effects of inertial force and centrifugal force, an innovative noncontact measurement method to quantify the unbalance degree of the microdisc, i.e., eccentricity, due to imperfect fabrication of IMM, is also proposed. By intensive computer simulations and intensive realistic experiments undertaken, the performance of proposed capacitive gap-sensing readout circuit and the unbalance degree of the disc have been successfully evaluated, respectively. From the simulation results, the minimum resolution of 1.22 mV/pF is obtained for the proposed capacitive gap-sensing readout circuit. The maximum sensitivity and bandwidth are ~118 mV/μm and 51.604 kHz, respectively.

Weakly-Coupled Resonators in Capacitive Readout Circuits

Low energy consumption and wide operating temperature range of capacitors made them common in sensor designs, e.g., MEMS accelerometers, and hence increased the popularity of capacitive readout circuits. Their main challenges, in either discrete or integrated implementations, are sensitivity, noise, energy consumption, and parasitic components at the analog front end. Compared to conventional “frequency-shift monitoring” which is one of the most accurate and common methods for capacitance measurements, weakly-coupled resonators (well-known in mechanical systems) can offer up to three orders of magnitude increase in sensitivity. Therefore, this concept has been recently applied to the design of micromechanical sensors, e.g., for sensitive mass sensing. This paper applies, for the first time in the electrical domain, the concept of monitoring the eigenstates variations in weakly-coupled resonators as a generic readout circuit technique for measuring very small capacitance changes. The outstanding sensitivity of this method is verified analytically and demonstrated using both extensive circuit simulations and experimental measurements.