Two different ASIC approaches can be used for signal-conditioning a MEMS piezoresistive silicon pressure sensor. One is based on a digital scheme for applications requiring a low operating voltage and low power consumption. The other involves an analog method for low-cost, high-volume applications. In both cases, the ASIC is employed to calibrate and compensate the sensor with a total error of less than ±1% of full scale over two different operating-temperature ranges. The total error includes effects due to offset and sensitivity, as well as the offset and sensitivity temperature coefficients (TCs).
Researchers chose ASICs because a typical output signal for a piezoresistive pressure sensor depends on temperature. ASICs also operate in the few-millivolt range. This level is too low for control and interface with microprocessors. Moreover, ASICs offer advantages over traditional approaches. Passive laser trimming of resistor networks, for example, provides high resolution and a wide range of resistor values. But the resistors have to be serially and individually trimmed, so it's capital intensive. It requires complicated and expensive production fixtures, as well.
Researchers at the Electron Device Laboratory of Fujikura Ltd. in Tohoku and Tokyo, Japan, devised a DSP-based circuit that corrects for the sensor's offset and sensitivity and their TCs. Fujikura's circuit operates from −30°C to 80°C. Meanwhile, researchers at the Institute of Microelectronics in Singapore used a fully customized ASIC with a fusible-link array that achieves the aforementioned performance from −40°C to 125°C.
Fujikura's ASIC was made on a 0.7-µm double-polysilicon, double-metal, n-well CMOS process. It consists of a sigma-delta 16-bit analog-to-digital converter, a reference voltage with a built-in temperature sensor, the 16-bit DSP core, 101 polysilicon fuses, a step-up voltage regulator, a 10-bit digital-to-analog converter (DAC), and a 4-MHz oscillator (see the figure).
The DSP does most of the offset and TC corrections. Corrected coefficients are stored using the polysilicon fuses. The output code is then accessible with a serial interface or an analog signal provided by the 10-bit DAC. This circuit also compensates for secondary temperature characteristics. It has an I2C serial interface. A built-in charge pump lets it work in circuits rated under 3 V. A "sleep" mode reduces power consumption.
The analog-based Institute of Microelectronics ASIC is made on a 0.8-µm double-polysilicon, double-metal CMOS process. The ASIC consists of a core analog signal processor, a 64-bit fusible link array, and a serial fusible-link interface.
The ASIC's digital portion provides the interface between the analog signal processor and the controller (the computer). This controller writes data to the interface and reads data back from it by a serial-in and serial-out communications protocol. Data in the serial interface can be loaded into the fusible-link array to control various resistor networks in the analog signal processor. These resistor networks are used for various programmable functions.
All of these programmable elements make it possible to compensate for the calibration, sensitivity, temperature, and TC effects to the first order. The ASIC features an output of 0.5 to 4.5 V using a 5-V power supply. The output is ratiometric when the power supply is varied between 4.5 and 5.5 V.
Both techniques were described at the recent Sensors Expo Conference, Anaheim, Calif. For more information from Fujikura, contact Tatsuya Ito at +81 3 5606 1073 or at [email protected]. For details on the Institute of Microelectronics' ASIC, contact Zheng Zheng at +65 7705907 or at [email protected].