MLX90323 Datasheet, PDF (10/25 Pages) Melexis Microelectronic Systems – 4 – 20 mA Loop Sensor Interface with Signal Conditioning and EEPROM

MLX90323
4 – 20 mA Loop Sensor Interface
with Signal Conditioning and EEPROM
7.4 Parameters calculation
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.
GAIN
I_out = FG * DAC_GAIN * CSGN[1:0] * {Vin+DAC_OFFSET+CSOF} * 8.85mA/V
FG = Hardware Gain (~72V/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.
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)
3rd 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]
(0.97 − 0.48) * ^{GN[9 : 0]} + 0.48 = DAC _ GAIN
1023
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.
(1.83 − −1.57) * ^{OF[9 : 0]} −1.57 = DAC _ OFFSET ^[mV/V]
1023
3901090323
Rev 003
Page 10 of 25
Data Sheet
Feb/12