MLX90320_12 Datasheet, PDF (16/33 Pages) Melexis Microelectronic Systems – Automotive sensor interface

MLX90320
Automotive sensor interface
FNOff _{ana log}
=
(0.2272 ×V_DD
+
FNOff _real
× (0.318 ×V_DD
−
0.2272
×
V_DD
))
×
V_DD
5
Equation 4
Explanation of parameters used in equation 4:
FNOff_analog is the voltage at the output of the fine offset DAC.
FNOff_real is the value of the fine offset in the [0..1] range with a 10 bit resolution.
The ^VDD term of equation 3 and 4 is due to the ratio-metric behaviour of the fine and coarse offset DACs.
5
The fine offset calibration allows also a second order compensation of the temperature drift of the sensor offset.
The value of the fine offset is given by the formula:
Equation 5
FNOff_real = O₀ + O₁ × (T − T₁ ) + O₂ × (T − T₁ )²
Explanation of parameters used in equation 5:
T₁ is the output of the temperature chain corresponding to the room temperature. The ADC of the
temperature chain outputs 10 bits but 12 bits are stored (address 0 to 2 of the EEPROM). The MSB
must always be 0 and the other 11 bits are obtained from an averaging from the previous temperature
readings. This gives more accuracy to the output of the temperature chain. The T1 value used in
equation 5 is in the [0..1] range with an 11 bit resolution.
O₀ is the zero order fine offset coefficient (independent from the temperature) used to compensate
accurately the sensor offset at room temperature. 12 bits are stored (address 28 to 30 of the EEPROM)
but only the 10 first are used. The two MSB must be 0. The O₀ value used in equation 5 is in the [0..1]
range with a 10 bit resolution.
O₁ is the first order fine offset coefficient used to compensate the sensor offset drift with temperature.
12 bits are stored (address 24 to 26 of the EEPROM). The MSB is the sign bit (two’s complement): If
O1[11] = 1 then O1 is negative, if O1[11] = 0 then O1 is positive. The O1 value used in equation 5 is in
the [-2..2] range with an 11 bit resolution.
O₂ is the second order fine offset coefficient used to compensate the sensor offset drift with
temperature. 12 bits are stored (address 20 to 22 of the EEPROM). The MSB is the sign bit (two’s
complement): If O2[11] = 1 then O2 is negative, if O2[11] = 0 then O2 is positive. The O2 value used in
equation 5 is in the [-2..2] range with an 11 bit resolution.
The ALU computes the equation 5 with 12 bits but the result is truncated to 10 bits because the Offset DAC is a
10 bit DAC. When the MLX90320 is not able to compensate for the sensor offset drift with temperature, the fine
offset calibration parameters stored in EEPROM will lead to a FNOff_real value out of the [0..1] range. In this case
the MLX90320 will indicate an overflow in the digital calculations by putting the output voltage in a fault band.
When this occurs, a reset of the chip is required to go back into the normal mode of operation.
The MLX90320 also offers the possibility to set clamping levels to the output voltage. This allows creating fault
bands necessary to detect external and internal faults.
3901090320
Rev 007
Page 16 of 33
Data Sheet
Feb/12