MLX90308CCCLW datasheet(11/26 Pages) MICRO-ELECTRONICS

11 / 26 page

MLX902xx Name of Sensor

Rev Y.X

22/Aug/98

Page 11

MLX90308CCC

Programmable Sensor Interface

MLX90308CCC Programmable Sensor Interface

Page 11

Rev 2.2

23/Oct/01

Different Modes

Analog Mode

The parameters OF and GN represent, respectively,

offset correction and span control, while OFTCi and

GNTCi

represent

their

temperature

coefficients

(thermal zero shift and thermal span shift). After

reset, the firmware continuously calculates the

offset and gain DAC settings as follows: The

EEPROM holds parameters GN, OF, OFTCi and

GNTCi, where “i” is the gap number and can be 1 < i

< 4. The transfer function is described below.

Vout = FG * DAC_GAIN * CSGN[2:0] *

{Vin+DAC_OFFSET+CSOF}

Iout =

FG * DAC_GAIN * CSGN[1:0] *

{Vin+DAC_OFFSET+CSOF} * 8.85mA/V

FG = Hardware Gain (~20V/V). Part of the hardware

design, and not changeable.

CSGN = Course Gain, part of byte 2 in EEPROM.

CSOF = Coarse Offset, part of byte 2 in

EEPROM.

GAIN

DAC_GAIN (new value) ~ GN[9:0] + [GNTCi * dT]

GN[9:0] = Fixed Gain, bytes 3 and 17 in EEPROM.

GNTCi = Gain TC for a given temperature

segment I. GNTCiL and GNTCiH in

EEPROM table.

dT = Temp. change within the appropriate gap.

How to calculate gain in the first temp. gap?:

DAC_GAIN = GN[9:0] - GNTC1 * (T1 – Temp_f1)

How to calculate gain in the other temp. gaps?:

2nd gap:DAC_GAIN = GN[9:0] + GNTC2 *

(Temp_f2 – T1)

3th gap: DAC_GAIN = DAC_GAIN2 + GNTC3 *

(Temp_f3 – T2)

4th gap: DAC_GAIN = DAC_GAIN3 + GNTC4 *

(Temp_f4 – T3)

Where:

Temp_f = Filtered temp. (previously described).

If GNTC1 > 2047

=> DAC_GAIN

↑

If GNTC2,3,4 > 2047

=> DAC_GAIN

↓

[V/V]

OFFSET

DAC_OFFSET (new value) ~ OF[9:0]+[OFTCi* dT]

OF[9:0] = Fixed Gain, bytes 4 and 17 in EEPROM.

OFTCi = Offset for a given temperature

segment I. OFTCiL and OFTCiH in

EEPROM table.

dT = Temp. change within the appropriate gap.

Calculation of the offset for a given temperature

segment is performed the same way as for the gain.

[mV/V]

Digital Mode

The MLX90308 firmware provides the capability of

digitally processing the sensor signal in addition to

the analog processing. This capability allows for

signal correction.

Signal Correction

While in digital mode the firmware can perform

signal correction. This is an adjustment to the

output level based on the input signal level.

Adjustment coefficients can be set for five different

signal ranges. The output is obtained by the

following formula:

Output = (Signal – Pi) * Pci + Poff where

Signal = input signal measurement;

Poff = Pressure ordinate

Pi = Pressure signal point (I = 2,3,4,5)

Pci = programmed coefficient.

The PCi coefficients are coded on 12 bits: one bit for

the sign, one for the unity, and the rest for the

decimals. The Pi are coded on 10 bits (0-3FFh) in

high-low order.

PNB_TNB: contains the number of signal points,

coded on the four MSB’s. The four LSB’s are

reserved for the number of temperature points. See

Table 4 and Table 5.

Compensation Trade-Offs

A compromise must be made between temperature

compensation

and

pressure

correction.

The

EEPROM space where the signal coefficients are

stored is shared with the temperature coefficients,

with the result that an EEPROM byte can be used

either for a temperature coefficient or for a signal

coefficient, but not both. Table 6 presents the

possibilities

among

the

maximum

number

temperature gaps and the maximum number of

signal gaps.

GAIN

DAC

1023

]

[

)

(

−

OFFSET

DAC

1023

]

[

)

(

−

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