Sensor Readout Circuit Research Articles

Simple oscillators-based readout circuit for low-power biomedical implant system

This paper reports a wireless sensor readout circuit for continuous physiological parameters monitoring including a potentiostat, a data generation unit and a frequency-shift-keying (FSK) modulator unit with the low drop-out (LDO) regulator for biomedical implant system. The potentiostat can generate an output potential of 0.7 V for the data generation unit. The data generation unit is designed based on a relaxation oscillator scheme and can be used to sense a current signal from any amperometric biomedical sensor and convert the signal to a square waveform in which the frequency of the square wave signal is proportional to the sensor current. FSK modulation scheme has been selected for wireless transmission. Designed with a very simple ring oscillator, this modulator integrates the modulation functionality into the oscillator itself by using the data signal to control the oscillation frequency. The prototype circuits have been fabricated in a 0.35 μm bulk complementary metal-oxide semiconductor (CMOS) process. Working with a regulated 1.8 V supply, the potentiostat consumes only 2 μA of current while the data generation unit can generate around 15.7 kHz output frequency with an input current of 1 μA. The FSK modulator consumes a total current of around 19 μA for a carrier frequency around 1 MHz. An off-chip demodulator is constructed to demodulate the data signal from the FSK modulator and the demodulated signal has less than 1.6 % variation of frequency.

A contact lens with integrated telecommunication circuit and sensors for wireless and continuous tear glucose monitoring

We present an integrated functional contact lens, composed of a differential glucose sensor module, metal interconnects, sensor read-out circuit, antenna and telecommunication circuit, to monitor tear glucose levels wirelessly, continuously and non-invasively. The electrochemical differential sensor module is based on immobilization of activated and de-activated glucose oxidase. We characterized the sensor on a model polymer eye and determined that it showed good repeatability, molecular interference rejection and linearity in the range of 0–2 mM glucose, covering normal tear glucose concentrations (0.1–0.6 mM). We also report the temperature, ageing and protein-fouling sensitivity of the sensor. We report the design and implementation of a low-power (3 µW) sensor read-out and telecommunication circuit to deliver wireless power and transmit data for the sensor module. Using this small chip (0.36 mm2), we produced an integrated contact lens with sensors and demonstrated wireless operation of the system and glucose read-out over the distance of several centimeters.

A 2.5 mW 370 mV/pF high linearity stray-immune symmetrical readout circuit for capacitive sensors

A stray-insensitive symmetrical capacitance-to-voltage converter for capacitive sensors is presented. By introducing a reference branch, a symmetrical readout circuit is realized. The linear input range is increased, and the systematic offsets of two input op-amps are cancelled. The common-mode noise and even-order distortion are also rejected. A chopper stabilization technique is adopted to further reduce the offset and flicker noise of the op-amps, and a Verilog-A-based varactor is used to model the real variable sensing capacitor. Simulation results show that the output voltage of this proposed readout circuit responds correctly, while the under-test capacitance changes with a frequency of 1 kHz. A metal-insulator-metal capacitor array is designed on chip for measurement, and the measurement results show that this circuit achieves sensitivity of 370 mV/pF, linearity error below 1% and power consumption as low as 2.5 mW. This symmetrical readout circuit can respond to an FPGA controlled sensing capacitor array changed every 1 ms.

A touch sensor readout circuit using switched- capacitor charge pump

A touch sensor readout circuit using switched- capacitor charge pump

A Low-Power, Wide-Dynamic-Range Semi-Digital Universal Sensor Readout Circuit Using Pulsewidth Modulation

We present a low-power, low complexity, and wide-dynamic-range universal sensor readout circuit that converts the sensing capacitance or resistance changes to digital duty cycles based on pulsewidth modulation (PWM). The readout circuit utilizes a RC-controlled pulse generator and produces pulse signals whose duration width is proportional to the charging time. The circuit is comprised of complementary CMOS circuit that enables the proposed design to consume significantly less power compared with traditional designs. The proposed design is universal and can be configured according to the application requirements. The sensor interface chip is designed and fabricated in TSMC 0.13-μm CMOS technology. The proposed interface achieves 13-aF to 10.7-nF capacitance and 5-Ω to 11.5-MΩ resistance sensing ranges with 60- μW power consumption. The functionality of the full circuit, including circuit analysis, noise analysis and measurement results, has been demonstrated.

A Low-Power Portable ECG Touch Sensor with Two Dry Metal Contact Electrodes

This paper describes the development of a low-power electrocardiogram (ECG) touch sensor intended for the use with two dry metal electrodes. An equivalent ECG extraction circuit model encountered in a ground-free two-electrode configuration is investigated for an optimal sensor read-out circuit design criteria. From the equivalent circuit model, (1) maximum sensor resolution is derived based on the electrode’s background thermal noise, which originates from high electrode-skin contact impedance, together with the input referred noise of instrumentation amplifier (IA), (2) 60 ㎐ electrostatic coupling from mains and motion artifact are also considered to determine minimum requirement of common mode rejection ratio (CMRR) and input impedance of IA. A dedicated ECG read-out front end incorporating chopping scheme is introduced to provide an input referred circuit noise of 1.3 ?VSUBrms/SUB over 0.5 ㎐ ~ 200 ㎐, CMRR of IA ＞ 100 ㏈, sensor resolution of 7 bits, and dissipating only 36 ㎼. Together with 8 bits synchronous successive approximation register (SAR) ADC, the sensor IC chip is implemented in 0.18 ㎛ CMOS technology and integrated on a 5 ㎝ × 8 ㎝ PCB with two copper patterned electrodes. With the help of proposed touch sensor, ECG signal containing QRS complex and P, T waves are successfully extracted by simply touching the electrodes with two thumbs.

Area-efficient low power CMOS image sensor readout circuit with fixed pattern noise cancellation

A low cost of die area and power consumption CMOS image sensor readout circuit with fixed pattern noise (FPN) cancellation is proposed. By using only one coupling capacitor and switch in the double FPN cancelling correlative double sampling (CDS), pixel FPN is cancelled and column FPN is stored and eliminated by the sample-and-hold operation of digitally programmable gain amplifier (DPGA). The bandwidth balance technology based on operational amplifier (op-amp) sharing is also introduced to decrease the power dissipation of traditional multi-stage switched capacitor DPGA. The circuit is designed and simulated using 1P6M 0.18 μm 1.8 V/3.3 V process. Simulation results indicate that the proposed CDS scheme can achieve an FPN of less than 1 mV. The total sampling capacitor per column is 0.9 pF and no column-wise power is dissipated. The die area and FPN value are cut by 70% and 41% respectively compared with amplifier-based CDS. The op-amp sharing gain stage can achieve a 12-bit precision and also implement an 8-bit gain controlling within a gain range of 24 dB. Its power consumption is 1.4 mW, which is reduced by 57% compared with traditional schemes. The proposed readout circuit is suitable for the application of low power cost-sensitive imaging systems.

Low-Power Low-Voltage Current Readout Circuit for Inductively Powered Implant System

Low voltage and low power are two key requirements for on-chip realization of wireless power and data telemetry for applications in biomedical sensor instrumentation. Batteryless operation and wireless telemetry facilitate robust, reliable, and longer lifetime of the implant unit. As an ongoing research work, this paper demonstrates a low-power low-voltage sensor readout circuit which could be easily powered up with an inductive link. This paper presents two versions of readout circuits that have been designed and fabricated in bulk complementary metal-oxide semiconductor (CMOS) processes. Either version can detect a sensor current in the range of 0.2 μA to 2 μA and generate square-wave data signal whose frequency is proportional to the sensor current. The first version of the circuit is fabricated in a 0.35-μ m CMOS process and it can generate an amplitude-shift-keying (ASK) signal while consuming 400 μ W of power with a 1.5-V power supply. Measurement results indicate that the ASK chip generates 76 Hz to 500 Hz frequency of a square-wave data signal for the specified sensor current range. The second version of the readout circuit is fabricated in a 0.5-μ m CMOS process and produces a frequency-shift-keying (FSK) signal while consuming 1.675 mW of power with a 2.5-V power supply. The generated data frequency from the FSK chip is 1 kHz and 9 kHz for the lowest and the highest sensor currents, respectively. Measurement results confirm the functionalities of both prototype schemes. The prototype circuit has potential applications in the monitoring of blood glucose level, lactate in the bloodstream, and pH or oxygen in a physiological system/environment.

Low Parasitic Capacitance and Low-Power CMOS Capacitive Fingerprint Sensor *

In this paper, a low parasitic capacitance and low-power CMOS capacitive fingerprint sensor readout circuit is presented. The side effect of parasitic capacitance has been under control with novel layout structure in sensor cell, and minimal size switch is used to reduce non-ideal effects of MOS switch and achieve good linearity. Power dissipation is also reduced with quiescent current control in buffer amplifier of sensor cell. A prototype chip with 32 × 32 array size has been fabricated using TSMC 0.35μm CMOS process. The chip works at 3.3V power supply and operates at 4MHz clock rate. Capacitance value from 0fF to 60fF can be sensed, corresponding analog output voltage is from 3.02V to 1.57V and the digital output is 6 bits. The overall power consumption is less than 5.5mW.

LPG Gas-Sensing System With $\hbox{SnO}_{2}$ Thin-Film Transducer and 0.7-$\mu\hbox{m}$ CMOS Signal Conditioning ASIC

The design and implementation of a complete gas sensor system for liquified petroleum gas (LPG) gas sensing are presented. The system consists of a SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> transducer, a lowcost heater, an application specific integrated circuit (ASIC) with front-end interface circuitry, and a microcontroller interface for data logging. The ASIC includes a relaxation-oscillator-based heater driver circuit that is capable of controlling the sensor operating temperature from 100degC to 425degC. The sensor readout circuit in the ASIC, which is based on the resistance to time conversion technique, has been designed to measure the gas sensor response over three orders of resistance change during its interaction with gases.

X-Ray Detection Using a Two-Transistor Active Pixel Sensor Array Coupled to an a-Se X-Ray Photoconductor

A direct-conversion X-ray sensor array using amorphous silicon (a-Si) thin-film transistor (TFT) based active pixel sensor (APS) readout circuit coupled with a stabilized amorphous selenium (a-Se) photoconductor for large-area digital imaging applications is presented. The pixel readout circuit employs a novel two-transistor (2T) active/passive pixel architecture that enables a compact pixel circuit for high-resolution sensor arrays with high large-area fabrication reliability. The X-ray detector consists of an in-house fabricated 150 mum pixel pitch 2-TFT pixel coated with an 80 mum thick a-Se photoconductor. A detector dark current of 110 pA/cm2 at 10 V/mum electric field, and a controllable pixel conversion gain up to 8.4 nA/mR with a quantum efficiency of 60% was measured. Capabilities such as voltage programmable gain and dynamic range control, as well as nondestructive readout during X-ray exposure are demonstrated. The detector in this work represents a highly promising technology for high-resolution X-ray digital imaging, adaptable to a wide range of applications owing to its gain and dynamic range programmability.

Configurable 3D‐integrated focal‐plane cellular sensor–processor array architecture

AbstractMixed‐signal cellular visual microprocessor architecture with digital processors is described. An Application Specific Integrated Circuit (ASIC) implementation is also demonstrated. The architecture is composed of a regular sensor readout circuit array, prepared for 3D face‐to‐face‐type integration, and one or several cascaded array of mainly identical (single instruction multiple data, SIMD) processing elements. The individual array elements were derived from the same general Hardware Description Language (HDL) description and could be of different sizes, aspect ratio, and computing resources. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Analysis of Random Telegraph Signal Noise in Dual and Single Oxide Device and Its Application to Complementary Metal Oxide Semiconductor Image Sensor Readout Circuit

The behavior of random telegraph signal (RTS) noise in metal oxide semiconductor field effect transistor (MOSFETs) with different oxidation processes and its effects on the readout circuit in complementary MOS (CMOS) image sensor were experimentally studied. Statistically, RTS noise in large number of devices have been measured both in the device with single oxide and dual oxide and we analyzed its amplitude and power spectrum density. Through the analysis, we found that there are significantly different behaviors of RTS noise between single oxide devices and dual oxide devices due to different characteristic of oxide traps. We not only provided information of traps in both single and dual oxide devices but also applied different RTS noise behavior of both two cases to readout circuits in CMOS image sensor.

A low power sensor signal processing circuit for implantable biosensor applications

A low power sensor read-out circuit has been implemented in0.35 µm CMOS technology thatconsumes only 400 µW of powerand occupies an area of 0.66 mm2. The circuit is capable of converting the current signal from any generic biosensor into anamplitude shift keying (ASK) signal. The on-chip potentiostat biases the chemical sensorelectrodes to create the sensor current which is then integrated and buffered to generate asquare wave with a frequency proportional to the sensor current level. A programmablefrequency divider is incorporated to fix the ASK envelope frequency to be inbetween20 Hz and 20 kHz, which is within the audible range of human hearing. The entiretransmitter block operates with a supply voltage as low as 1.5 V, and it can beeasily powered up by an external RF source. Test results emulate the simulationresults with good agreement and corroborate the efficacy of the designed system.

Low noise signal-to-noise ratio enhancing readout circuit for current-mediated active pixel sensors

Diagnostic digital fluoroscopic applications continuously expose patients to low doses of x-ray radiation, posing a challenge to both the digital imaging pixel and readout electronics when amplifying small signal x-ray inputs. Traditional switch-based amorphous silicon imaging solutions, for instance, have produced poor signal-to-noise ratios (SNRs) at low exposure levels owing to noise sources from the pixel readout circuitry. Current-mediated amorphous silicon pixels are an improvement over conventional pixel amplifiers with an enhanced SNR across the same low-exposure range, but whose output also becomes nonlinear with increasing dosage. A low-noise SNR enhancing readout circuit has been developed that enhances the charge gain of the current-mediated active pixel sensor (C-APS). The solution takes advantage of the current-mediated approach, primarily integrating the signal input at the desired frequency necessary for large-area imaging, while adding minimal noise to the signal readout. Experimental data indicates that the readout circuit can detect pixel outputs over a large bandwidth suitable for real-time digital diagnostic x-ray fluoroscopy. Results from hardware testing indicate that the minimum achievable C-APS output current that can be discerned at the digital fluoroscopic output from the enhanced SNR readout circuit is 0.341nA. The results serve to highlight the applicability of amorphous silicon current-mediated pixel amplifiers for large-area flat panel x-ray imagers.

Evaluation of mixed-signal noise effects in photon-counting X-ray image sensor readout circuits

In readout electronics for photon-counting pixel detectors, the tight integration between analog and digital blocks causes the readout electronics to be sensitive to on-chip noise coupling. This noise coupling can result in faulty luminance values in grayscale X-ray images, or as color distortions in a color X-ray imaging system. An exploration of simulating noise coupling in readout circuits is presented which enables the discovery of sensitive blocks at as early a stage as possible, in order to avoid costly design iterations. The photon-counting readout system has been simulated for noise coupling in order to highlight the existing problems of noise coupling in X-ray imaging systems. The simulation results suggest that on-chip noise coupling should be considered and simulated in future readout electronics systems for X-ray detectors.

Novel analog readout electronics for microacoustic thickness shear-mode sensors

In this paper, a novel purely analog electronic readout circuit for sensors based on thickness shear-mode (TSM) resonators is introduced. This system can, e.g., be used for viscosity sensing of liquids, even if the liquid shows a comparatively high viscosity. In contrast to conventionally utilized oscillator circuits, the present circuit eliminates the disturbing influence of the static capacitance of the sensor by means of a dedicated synchronous detector circuit. The sensor is externally driven by a voltage-controlled oscillator (VCO), which is tuned to the resonator's fundamental frequency by means of a control circuit utilizing a frequency modulation of the VCO signal. When the control loop has settled, either the obtained resonance frequency or the damping of the sensor, which is indicated by means of a DC output voltage, can be used as the output signal providing a measure for the viscosity.

An interface circuit for measuring capacitance changes based upon capacitance-to-duty cycle (CDC) converter

We present a direct-to-digital capacitive sensor readout circuit that converts capacitance changes of a sensor element to changes of the duty cycle of a square-wave oscillator, which, in turn, is converted to a digital output by a counter. The readout circuit resembles a single-slope analog-to-digital converter structure. There are several advantages of this readout scheme. First, due to its simplicity and low number of components, the power consumption of the circuit is expected to be significantly smaller than in similar digital readout designs. Furthermore, linearization of the output may be achieved using an EEPROM lookup table. Another advantage is the possibility of performing adaptive measurements where the sensor resolution and bandwidth may be changed via the readout circuit software. Finally, we present a theory of the adaptive measurement and an analysis of the design tradeoffs. The capacitance-to-duty cycle readout circuit may achieve large bandwidth and high resolution in a modern low-voltage, low-power CMOS implementation. The performance of a prototype readout circuit built from discrete components is 13-bit effective resolution with a 1-kHz bandwidth.

Noise Considerations of Integrators for Current Readout Circuits

Abstract. In this paper the noise analysis of a current integrator is carried out and measures to reduce the overall noise are presented. The effects of various noise sources are investigated and their dependence on the input capacitance and on the gate area of the input transistors of the OTA used for the readout is shown. Both, input capacitance and gate area, should be kept as small as possible. Moreover, the linearity of the integrator is examined. In addition to that, the available application of such sensor readout circuit, which is a CMOS photodetector readout, is introduced. It uses an automatic gain switching, so that the dynamic range is extended.

A differential charge-transfer readout circuit for multiple output capacitive sensors

This paper presents a true differential, low-noise readout scheme for multiple output capacitive sensors. It overcomes the restrictions with multiple sense capacitors connected to a common node that is common in micromachined gyroscopes and multiple axis accelerometers. The architecture is easily integrated into a CMOS process, only requiring single ended operational amplifiers, transmission gates and capacitors. It also supports cancellation of amplifier 1/ f-noise and offset voltage. Theory predicting the resolution of the circuit is presented and compared to noise-simulations. The ultimate noise performance is limited by the fundamental k B T/ C-noise from the sense capacitors. Measurements on a fabricated ASIC verify the feasibility of the concept. The ASIC used as a readout circuit for a triple axis accelerometer shows a resolution of 5.7 aF/√Hz at 20 Hz.