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.

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

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