Frank Robert Riedijk

Developments in integrated smart sensors

Abstract An integrated smart sensor (ISS) can be defined as a chip that contains the functions of sensing, signal conditioning, AD conversion and a bus output. It may also have on-chip calibration and self-test. This paper concentrates on the standardization of the output interface using on-chip AD conversion, calibration and bus output. First, it is shown how indirect AD-conversion techniques yield simple circuits which can easily coexist with the sensor on one chip. Sigma-delta conversion is a good example. Second, calibration methods are described for analog and digital sensor output signals. Finally, the use of two simple busses is described, the I2C bus and the IS2 bus. The required bus interfaces are relatively simple and can be integrated with sensors in bipolar as well as in CMOS processes.

CMOS quad spinning-current Hall sensor system for compass application

A Hall sensor system for a compass application is presented. The Hall sensor, its instrumentation amplifier and ADC are realized in standard CMOS, while the timing and control signals are generated by an external microprocessor. The Hall sensor consists of four eight-direction spinning-current Hall-plates in a quad configuration, which decreases the cross-sensitivity for stress by a factor of 1200, and decreases the cross-sensitivity for temperature by a factor of 5. The system has a typical offset of 2 /spl mu/T (20 mGauss), which is a factor 25 lower than the single spinning-current Hall-plate by Munter and a factor 5 lower than the continuous-spinning Hall-plate by Steiner. No trimming or calibration has been used to reduce the offset.

A fully integrated CMOS Hall sensor with a 3.65 /spl mu/T 3/spl sigma/ offset for compass applications

A fully-integrated CMOS Hall sensor suitable for use in compass applications has a 3/spl sigma/ offset spread less than 4.5 /spl mu/T. After artificial aging it drifts less than 250 nT. Using two such sensors, a fully integrated electronically calibrated compass with a stability of 0.5/spl deg/ is realized. The sensor has an analog PWM and a digital RS232/SPI//spl mu/wire interface.

An integrated absolute temperature sensor with sigma-delta A–D conversion

Abstract A smart absolute temperature sensor is described, which provides a pulse-rate output and is highly suitable for communication with microprocessors. The temperature sensor generates an absolute temperature current as well as a reference current. Both signals are used in a sigma-delta A–D converter, which provides a clock-synchronous pulse rate. This fully synchronized output signal offers important advantages over asynchronous signals, such as frequency or duty cycle. The last suffers from digital interference, which is often unrecognized. Smart sensors with on-chip digital processing or bus communication need synchronized A–D converters to prevent this problem.

A general circuit for resistive bridge sensors with bitstream output

A low-offset integrated circuit useful for the readout of the signal coming from various types of integrated resistive bridge sensors has been developed in standard bipolar technology. The circuit, using a first order /spl Sigma/-/spl Delta/ A/D converter, provides a clock-synchronous pulse rate output which is highly suitable for the communication with a microprocessor and for a bus environment. An almost perfect offset cancellation is achieved by chopping the bridge voltage supply. The simplicity of the circuit makes it ideal for the integration with the sensors itself, while the digital interference rejection allows the integration of the digital control circuits for the bus on the same chip. The experimental results show that with a clock frequency of 200 kHz a resolution of 12 bits is obtained with a conversion time of 20 ms. The remaining offset is /spl plusmn/5 /spl mu/V. The bridge imbalance can be up to 18 mV/V. Depending on the chosen resolution the input signal bandwidth could be from about 25 Hz (12 bits) up to 400 Hz (8 bits).

Colour-sensor system with a frequency output and an ISS or I2C bus interface

Abstract An optical sensor for operation in the visible spectrum with integrated electronic readout circuits and two different interfaces to a sensor bus systems has been realized in a BIFET process. The output signal of the basic readout circuit is a pulse series with a frequency proportional to the intensity of the incident light. Furthermore, the duty cycle of these output pulses depends on the spectral distribution of the incident light, enabling measurement of colour. No internal or external capacitor is needed for the current-to-frequency conversion, since the photodiode operates in a charge-integrating mode. The electronic circuits have been designed for operating with a large dynamic range and a relatively small chip area. Interfaces to the ISS and the I 2 C bus have been designed, fabricated and tested in the same process. Photodiodes with different spectral responses have also been fabricated on a single chip in a standard CMOS process. First measurements on the these devices have shown that the same readout principle can be used in CMOS technology. These results will be published later. In CMOS technology a much smaller die area is needed if the sensor, the readout circuits and the digital interface are integrated on a single chip.

A new class of integrated thermal oscillators with duty-cycle output for application in thermal sensors

A new class of integrated thermal oscillators has been developed by measuring the heating power in the time domain. A duty-cycle output proportional to the heat-transfer coefficient has been obtained to be compatible with microprocessors. Several different structures, which belong to the same family, have been constructed. The converter family can be employed to measure flow velocity, pressure, IR radiation and true RMS with fast response and good linearity. Experimented results for measuring gas-flow velocity are presented. The structures of the oscillator are simple and only a standard bipolar technique is required.

Integrated smart two-dimensional thermal flow sensor with Seebeck-voltage-to-frequency conversion

Abstract A smart sensor has been developed for the measurements of both flow direction and velocity. The sensor uses a thermal principle, which allows complete integration together with electronic circuitry on one chip. A groove is etched to create thermal isolation between the sensor and the chip edge to obtain an essential variable (the flow temperature). On the edge a voltage-to-frequency (V/f) converter is placed as a first step towards a digital output. The V/f converter is able to convert voltages in the range of 1 μV to 1 mV. The frequency output signal is a direct measure for the wind velocity. The presence of a multiplexer enables direction measurement and calibration of the sensor.

An integrated optical position-sensitive detector with digital output and error correction

Abstract An integrated optical position-sensitive detector (PSD) with digital output is presented. A novel interpolating analog-to-digital converter which is based on a sigma-delta converter, is applied to perform three operations simultaneously. These are: calculation of the relative position cancellation of errors, both optical and electrical; generation of a microprocessor-compatible digital output. The realized smart sensor is integrated in a standard bipolar IC process, and measures 6.5 mm × 2.5 mm.

A smart balanced thermal pyranometer using a sigma-delta A-to-D converter for direct communication with microcontrollers

Abstract A conventional thermal pyranometer has been equipped with a smart interface chip, which converts the small voltage output of the applied thermocouples into a digtal pulse rate. Such a pulse rate can be used directly by a microcontroller, which also supplies the control signals for the chip. A total accuracy of 14 bits and an offset of 5 μV have been obtained by using a balanced chopper and special sigma-delta A-to-D converter. No external components are necessary. The chip, which was realized in 3 GHz bipolar process, measures 4 mm 2 . The circuits have been designed in such a way that they can be upgraded with a simple bus interface and combined with an integrated sensor on a single chip. Such an adaptability is very important with todays rapidly changing system architectures.