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MAX1455C Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX1455C Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 25 page temperature indexed lookup table with one hundred seventy-six 16-bit entries. The on-chip temperature sen- sor provides a unique FSO trim from the table with an indexing resolution approaching one 16-bit value every 1.5°C from -40°C to +125°C. The temperature indexing boundaries are outside the specified absolute maximum ratings. The minimum indexing value is 00hex, corre- sponding to approximately -69°C. All temperatures below this value output the coefficient value at index 00hex. The maximum indexing value is AFhex, which is the highest lookup table entry. All temperatures higher than approxi- mately +184°C output the highest lookup table index value. No indexing wraparound errors are produced. Linear and Nonlinear Temperature Compensation Writing 16-bit calibration coefficients into the offset TC and FSOTC registers compensates first-order tempera- ture errors. The piezoresistive sensor is powered by a current source resulting in a temperature-dependent bridge voltage due to the sensor’s temperature coeffi- cient resistance (TCR). The reference inputs of the off- set TC DAC and FSOTC DAC are connected to the bridge voltage. The DAC output voltages track the bridge voltage as it varies with temperature, and by varying the offset TC and FSOTC digital code and a portion of the bridge voltage, which is temperature dependent, is used to compensate the first-order tem- perature errors. The internal feedback resistors (RISRC and RSTC) for FSO temperature compensation are set to 75k Ω. To calculate the required offset TC and FSOTC com- pensation coefficients, two test temperatures are need- ed. After taking at least two measurements at each temperature, calibration software (in a host computer) calculates the correction coefficients and writes them to the internal EEPROM. With coefficients ranging from 0000hex to FFFFhex and a +5V reference, each DAC has a resolution of 76µV. Two of the DACs (offset TC and FSOTC) utilize the sen- sor bridge voltage as a reference. Since the sensor bridge voltage is approximately set to +2.5V, the FSOTC and offset TC exhibit a step size of less than 38µV. For high-accuracy applications (errors less than 0.25%), the first-order offset TC and FSOTC should be compensated with the offset TC and FSOTC DACs, and the residual higher order terms with the lookup table. The offset and FSO compensation DACs provide unique compensation values for approximately 1.5°C of temperature change as the temperature indexes the address pointer through the coefficient lookup table. Changing the offset does not affect the FSO; however, changing the FSO affects the offset due to the nature of the bridge. The temperature is measured on both the MAX1455 die and at the bridge sensor. It is recom- mended to compensate the first-order temperature errors using the bridge sensor temperature. Typical Ratiometric Operating Circuit Ratiometric output configuration provides an output that is proportional to the power-supply voltage. This output can then be applied to a ratiometric ADC to produce a digital value independent of supply voltage. Ratiometricity is an important consideration for battery-operated instruments, automotive, and some industrial applications. The MAX1455 provides a high-performance ratiometric output with a minimum number of external components (Figure 2). These external components include the fol- lowing: • One supply bypass capacitor • One optional output EMI suppression capacitor Typical Nonratiometric Operating Circuit (5.5VDC < VPWR < 28VDC) Nonratiometric output configuration enables the sensor power to vary over a wide range. A low-dropout voltage regulator, such as the MAX1615, is incorporated in the circuit to provide a stable supply and reference for MAX1455 operation. A typical example is shown in Figure 3. Nonratiometric operation is valuable when wide ranges of input voltage are to be expected and the system A/D or readout device does not enable ratiometric operation. Internal Calibration Registers The MAX1455 has five 16-bit internal calibration regis- ters (ICRs) that are loaded from EEPROM, or loaded from the serial digital interface. Data can be loaded into the ICRs under three different circumstances. Normal Operation, Power-On Initialization Sequence: • The MAX1455 has been calibrated, the Secure- Lock byte is set (CL[7:0] = FFhex), and UNLOCK is low. • Power is applied to the device. • The power-on reset (POR) functions have been completed. • Registers CONFIG, OTCDAC, and FSOTCDAC are refreshed from EEPROM. Low-Cost Automotive Sensor Signal Conditioner _______________________________________________________________________________________ 7 |
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