MAX1454_11 Datasheet, PDF(21/25 Page) Maxim Integrated Products – Precision Sensor Signal Conditioner with Overvoltage Protection

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MAX1454

Precision Sensor Signal Conditioner

with Overvoltage Protection

Power-Up Control Sequence

The device uses a power-up state machine to determine

whether the device should switch to the fixed analog

mode, or enable the digital programming mode (Figure 4).

At power-up, the device loads the PWRUPCFG register to

establish a wait time (Table 11), and the number of control

words (Table 12) required to enter the digital program-

ming mode. If the wait time expires, the device automati-

cally switches to the fixed analog mode. However, if the

interface receives the correct number of control words

within the established wait times, the device enters the

digital programming mode. A serial command enables the

device to switch into the fixed analog mode after the part

has been programmed.

Note: Setting CTRLREP[2:0] to 111 in the PWRUPCFG

flash memory location forces the part into the digital pro-

gramming mode without the need for control words (an

initialization byte is still required). By default, parts shipped

from the factory are programmed to start in the digital

programming mode.

Sensor Compensation Overview

The device compensates for sensor offset, FSO, and

temperature errors by loading the internal calibra-

tion registers with the compensation values. These

compensation values can be loaded to registers directly

through the serial digital interface during calibration, or

loaded automatically from flash memory at power-on.

During the calibration process, the device is configured,

tested, and compensation values are calculated and

stored in the internal flash memory. Once programmed,

after each power-up, the device autoloads the registers

from flash memory and is ready for use without further

configuration.

Compensation requires an examination of the sensor per-

formance over the operating pressure and temperature

range. A minimum of two test temperatures and two test

pressures (zero and full scale) are required to correct the

linear component of temperature error to achieve pres-

sure calibration. For higher temperature accuracy, more

test temperatures must be used. A typical compensation

procedure can be summarized in the following sections.

Initialize the Device

Initialize the device registers with known values (e.g.,

compensation coefficients of a similar device) or deter-

mine values for IRO, PGA gain, FSO DAC, and offset

DAC based on sensor parameters (offset, sensitivity,

bridge resistance, etc). Select a current-mirror ratio value

corresponding to the sensor in use. Initialization is an

important step to ensure that the device output remains

in range over the full operating conditions. When the

device is initialized successfully, the excitation voltage is

within the normal range, and the output voltage is around

the desired offset value (when zero pressure is applied).

Characterize the Sensor at Test Temperatures

1) Set the temperature to the first test temperature point

and allow the system to reach equilibrium.

2) By changing the FSO DAC through an iterative pro-

cess, set the bridge voltage to a value that produces

the desired output span. Change the offset DAC as

necessary.

3) Once the desired output span is achieved, change

the offset DAC to produce the final offset.

4) Record the values of TEMPINDEX, FSODAC, and

ODAC. The device flash memory can be used to store

the information.

5) Change the temperature to the next value and repeat

this procedure to determine a unique value for the

TEMPINDEX, FSODAC, and ODAC at every test tem-

perature.

Calculate Compensation Coefficients

1) FSO Lookup Table: Using a fitting function, fit the

FSODAC and TEMPINDEX values obtained during the

characterization step and generate an array of 176

elements (FSODAC vs. TEMPINDEX array, where 0 â¤

TEMPINDEX â¤ 175).

2) Offset Lookup Table: Using a fitting function, fit the

ODAC and TEMPINDEX values obtained during the

characterization step and generate an array of 176

elements (ODAC vs. TEMPINDEX array).

Program Flash Memory and Final Test

1) Program the device by writing to the ODAC

and FSODAC lookup tables, and the OTCDAC,

FSOTCDAC, CONFIG1, CONFIG2, PWRUPCFG, and

user data locations in flash memory.

2) While the sensor is still at the last test temperature

point, perform a final test to verify the compensation

accuracy.

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