Electronic Design

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.

Similarly, for the FSO calibration, a coarse gain is first set by selecting the gain of the PGA. Next, the FSO DAC sets the sensor bridge current with the digital input obtained from a temperature-indexed lookup table in EEPROM. Using this lookup table again, the FSO correction is implemented.

Maxim maintains that the offset and FSO compensation DACs provide compensation values in 1.5ºC temperature increments over the specified temperature range. In short, the uncompensated temperature errors can be on the order of 20% to 30% of FSO. But with compensation, nonlinear sensor offset and FSO temperature errors can be reduced to 0.02% of FSO (Fig. 2).

Based on the requirements, a user can select any number of test points between 2 and 176. In some special cases, however, a select number of calibration points can be combined with preset values to define the temperature curve. The preset values can also be loaded into the ASIC without testing. As an example, Table 1 shows seven different compensation points for offset and FSO from the lookup table, with an index corresponding to −40ºC to 125ºC. Table 2 represents an actual characterization of the sensor's offset and FSO using the compensation points given in Table 1.

A robust communications protocol permits the MAX1455 to use a single pin for programming the chip. Calibration and compensation programming is accomplished via the serial digital I/O (DIO) pin. Asynchronous serial data communications between the conditioning chip and the host calibration test system or computer are made possible by the serial interface. The data rate can vary between 4.8 and 38.4 kHz. For zero-pin programming, the DIO can be tied to the ASC chip's VOUT pin.

The MAX1455 has special features that allow transducers to be multiplexed to the chip. In fact, according to Maxim, a patent is pending for this programming technique. After the sensor has been calibrated, the MAX1455 provides a secure lock feature. This lets the user prevent any modifications of the calibration and compensation coefficients stored in the EEPROM. The lock is not permanent, though. A logic high on the unlock pin overrides this secure lock to enable factory rework and recalibration.

The MAX1455 offers a ratiometric voltage output with a minimum number of external components. It also features a bidirectional output diagnostic clip function. Four user-selectable clip levels are available as well. These include 0.10 to 4.90 V, 0.15 to 4.85 V, 0.20 to 4.80 V, and 0.25 to 4.75 V. Using an external voltage regulator, the device can be configured for a nonratiometric output (Fig. 3).

The input range for the MAX1455 is 1 to 40 mV/V. For application-specific circuit needs, the IC also furnishes an auxiliary op amp. The op amp can drive a 1-µF load without oscillation. And, it can source/sink 2 mA all the way to the rails. Plus, it can be tailored for low-pass filtering, amplifying the output signal, or generating sufficient drive for strain-gage-type bridges.

Although optimized for use with piezo-resistive pressure sensors, the MAX1455 can be extended to other resistive types like accelerometers, strain gages, and giant magnetoresistive devices. With resistive-type sensors such as these, additional external components are needed.

To ensure product reliability in the field, the MAX1455 is supported by an extensive test and qualification program. According to Maxim, the company has developed an automated-pilot production test system to support engineering prototyping and limited production.

Price & Availability
Implemented in a 0.5-µm CMOS process, the MAX1455 comes in a 16-pin SSOP. Maximum supply current is 6 mA at a 5-V supply. Sampling now, it's slated for production in the fourth quarter. In 1000-piece quantities, the MAX1455 is priced at $3.50 each. An evaluation kit also is available to facilitate manual programming. It includes an evaluation board with a silicon pressure sensor, design/applications man-ual, MAX1455 communication software, and an interface adapter and cable.

Maxim Integrated Products, 120 San Gabriel Dr., Sunnyvale, CA 94086; (408) 737-7600; www.maxim-ic.com.

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