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.