Sensors

6

Freescale Semiconductor, Inc.

MPL115A2

Example Binary Format Definitions:

a0 Signed, Integer Bits = 12, Fractional Bits = 3 :

b1 Signed, Integer Bits = 2, Fractional Bits = 13 :

b2 Signed, Integer Bits = 1, Fractional Bits = 14 :

c12 Signed, Integer Bits = 0, Fractional Bits = 13, dec pt zero pad = 9 :

Padc Unsigned, Integer Bits = 10 :

Tadc Unsigned, Integer Bits =10 :

NOTE: Negative coefficients are coded in 2’s complement notation.

3.2

Compensation

The 10-bit compensated pressure output, Pcomp, is calculated as follows:

Eqn. 1

Where:

Padc is the 10-bit pressure ADC output of the MPL115A

Tadc is the 10-bit temperature ADC output of the MPL115A

a0 is the pressure offset coefficient

b1 is the pressure sensitivity coefficient

b2 is the temperature coefficient of offset (TCO)

c12 is the temperature coefficient of sensitivity (TCS)

Pcomp will produce a value of 0 with an input pressure of 50 kPa and will produce a full-scale value of 1023 with an input pressure

of 115 kPa.

Eqn. 2

3.3

Evaluation Sequence, Arithmetic Circuits

The following is an example of the calculation for Pcomp, the compensated pressure output. Input values are in bold.

c12x2 = c12 * Tadc

a1 = b1 + c12x2

a1x1 = a1 * Padc

y1 = a0 + a1x1

a2x2 = b2 * Tadc

Pcomp = y1 + a2x2

This can be calculated as a succession of Multiply Accumulates (MACs) operations of the form y = a + b * x:

Pcomp

a0

b1

c12 Tadc

⋅

+

() Padc b2 Tadc

⋅

+

⋅

+

=

Pressure (kPa)

P

=comp

115

50

–

1023

----------------------

50

+

⋅

a

b

x

y

+

X