[Product Report]
Integrated Analog Signal-Conditioning IC Handles Harsh Environments
With 6 kbits of EEPROM on-chip, nearly 176 temperature points can be compensated over a range of –69°C to 184°C.
Amplifying and conditioning distorted sensor signals in noisy and harsh environments has never been easy. This is especially so in automotive systems where semiconductor chips and associated components are subjected to extreme temperature conditions. In severe winters, the temperature under an auto's hood can dip below −40ºC. During hot summers, it can skyrocket to over 125ºC. Interconnects linking these devices are long enough to introduce all kinds of unwanted interferences to corrupt and deform the desired signals. Consequently, finding a way for ICs to sustain performance under such hostile conditions remains a challenging task for designers.
A variety of monolithic semiconductor IC solutions have evolved over the years to address a myriad of sensor problems. Yet there's still plenty of room for improvement. Efforts continue to drive the performance of these signal-conditioning chips that must link the sensor's output to a microcontroller for processing. The trend is to pack more functions and peripherals on-chip to make it as complete as possible without draining the battery or pinching the pocket.
Minimizing the external component count to enable an easy-to-use solution is another goal. In addition, with large memory blocks on board, clever calibration and correction methods are being implemented to compensate for the nonlinearities of the sensor. This steadily raises the bar on sensor-signal conditioning techniques.
To address some of these grievances, Maxim Integrated Products has developed the MAX1455, a new-generation analog signal-conditioning (ASC) chip. This IC is optimized for automotive pressure sensors that use piezo-resistive bridge transducers. With an unprecedented level of integration, the MAX1455 provides amplification, calibration, and temperature compensation. These features enable an overall performance approaching the inherent linearity of the sensor itself.
"This highly integrated ASC chip requires a few bypass capacitors to complete the solution," asserts Maxim's director of IC development Ali Rastegar. As Rastegar states, the analog signal path of the MAX1455 is well architected to introduce no quantization noise in the output signal. At the same time, it contains sensor-repeatable errors within ±0.02% over the specified temperature range.
"The temperature compensation of this ASIC is so good that there are no additional errors contributed," notes Harold Joseph, Maxim's director for smart signal conditioning products. "As a result, the overall accuracy is limited by the accuracy of the transducer itself," he says.
One advantage of this unit is that it compensates for the sensor offset and the span of the input signal. It also provides a novel compensation strategy for correcting the offset and full-span output (FSO) over a wide temperature range. This is accomplished by varying the offset and gain of the 4-bit programmable-gain amplifier (PGA), as well as the sensor bridge excitation current. The offset trimming range for the PGA is ±150 mV with a resolution of 3 µV. Its gain values can be set from 36 to 208 V/V in 16 steps with a frequency response of 3.2 kHz.
Four on-chip 16-bit digital-to-analog converters (DACs) enable digitally controlled trimming, with calibration coefficients stored by the user in the built-in 6-kbit EEPROM (Fig. 1). A bandgap temperature sensor is linked to an 8-bit analog-to-digital converter (ADC) to generate the lookup address for the table. To keep current consumption low, the data converters use a single-bit delta-sigma (Δ−Σ) architecture. Therefore, the ADC can offer 16 bits of resolution. But for this application, it's configured to deliver 8-bit values.
Initially, the coarse offset correction is applied at the input of the PGA. The final correction occurs through the use of the temperature-indexed lookup table. In reality, over a range of −69ºC to 184ºC, up to 176 points of 16-bit temperature-correction coefficients are stored in the internal 6-kbit EEPROM. Since the EEPROM is configured as a byte-wide array, each 16-bit value is stored as two 8-bit quantities. This information is stored as a temperature-indexed lookup table.
Every millisecond, the on-chip temperature sensor provides indexing into the offset lookup table in the EEPROM. The resulting value is then transferred to the offset DAC register. Subsequently, the output voltage from this output DAC is fed into a summing junction at the PGA output. In turn, the sensor offset is compensated with a resolution of ±76 µV or ±0.002% of FSO.
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