DA5541B_005.doc

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00005541766 ECN865

FUNCTION

GENERAL

The MS5541B consists of a piezoresistive sensor and a sensor interface IC. The main function of the MS5541B

is to convert the uncompensated analog output voltage from the piezoresistive pressure sensor into a 16-Bit

digital value, as well as providing a 16-Bit digital value for the temperature of the sensor.

• measured pressure (16-Bit)

“D1”

• measured temperature (16-Bit)

“D2”

As the output voltage of a pressure sensor is strongly dependent on temperature and process tolerances, it is

necessary to compensate for these effects. This compensation procedure must be performed by software using

an external microcontroller.

For both pressure and temperature measurement the same ADC is used (sigma delta converter):

• for the pressure measurement, the differential output voltage from the pressure sensor is converted

• for the temperature measurement, the sensor bridge resistor is sensed and converted

During both measurements the sensor will only be switched on for a very short time in order to reduce power

consumption. As both, the bridge bias and the reference voltage for the ADC are derived from VDD, the digital

output data is independent of the supply voltage.

FACTORY CALIBRATION

Every module is individually factory calibrated at two temperatures and two pressures. As a result, 6 coefficients

necessary to compensate for process variations and temperature variations are calculated and stored in the 64-

Bit PROM of each module. These 64-Bit (partitioned into four words of 16-Bit) must be read by the

microcontroller software and used in the program converting D1 and D2 into compensated pressure and

temperature values.

PRESSURE AND TEMPERATURE MEASUREMENT

The sequence of reading pressure and temperature as well as of performing the software compensation is

depicted in flow chart, Fig. 3 and Fig. 5.

First the WORD1 to WORD4 have to be read through the serial interface. This can be done once after reset of

the microcontroller that interfaces to the MS5541B. Next the compensation coefficients C1 to C6 are extracted

using Bit-wise logical- and shift-operations (refer to Fig. 4 for the Bit-pattern of word 1 to word 4).

For the pressure measurement the microcontroller has to read the 16 Bit values for pressure (D1) and

temperature (D2) via the serial interface in a loop (for instance every second). Then, the compensated pressure

is calculated out of D1, D2 and C1 to C6 according to the algorithm in Fig. 3 (possibly using quadratic

temperature compensation according to Fig. 5). All calculations can be performed with signed 16-Bit variables.

Results of multiplications may be up to 32-Bit long (+sign). In the flow according to Fig. 3 each multiplication is

followed by a division. This division can be performed by Bit-wise shifting (divisors are to the power of 2). It is

ensured that the results of these divisions are less than 65536 (16-Bit).

For the timing of signals to read out WORD1 to WORD4, D1, and D2 please refer to the paragraph “SERIAL

INTERFACE”.

Sensor

D1

D2

Word1..4

Calculation

in external

micro-

controller

Pressure

Temperature

re