FUNCTION
GENERAL
The MS5540B consists of a piezoresistive sensor and a sensor interface IC. The main function of the MS5540B
is to convert the uncompensated analogue output voltage from the piezoresistive pressure sensor to a 16-Bit
digital value, as well as providing a 16-Bit digital value for the temperature of the sensor.
• measured pressure (16-Bit)
• measured temperature (16-Bit)
“D1”
“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.
D1
Pressure
Sensor
Calculation
in external
micro-
D2
Word1..4
Temperature
controller
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 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 MS5540B. 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 Word1 to Word4).
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”.
DA5540B_007
September 25th, 2006
9
00005540767 ECN 865
INTERSEMA [ INTERSEMA SENSORIC SA ]
