MAX1460 Datasheet, PDF (15/20 Pages) Maxim Integrated Products – Low-Power, 16-Bit Smart ADC

Low-Power, 16-Bit Smart ADC
UNCOMPENSATED SENSOR ERROR
10
8
6
4
2
0
FSO
-2
-4
OFFSET
-6
-8
-10
0
10 20 30 40 50 60 70
TEMPERATURE (°C)
C_F
1µF
V_DD
R1
500k
OUT
R_D1
10k
UNFILTERED
BITSTREAM
AGND
R_D2
10k
R_F
500k
AMP-
MAX1460
OP AMP
AMP+
AMPOUT _FILTERED
ANALOG
OUTPUT
Figure 6. Sensor Characteristics Before Compensation
Figure 8. Filtering the Output DAC
COMPENSATED TRANSDUCER ERROR
0.20
0.15
0.10
0.05
FSO
0
-0.05
OFFSET
-0.10
-0.15
-0.20
0
10 20 30 40 50 60 70
TEMPERATURE (°C)
Figure 7. Compensated Sensor/MAX1460 Pair
where:
Equation (5a)
Y_AS
=
Signal_AS
D_S − D_OFF
+ Of₀ + Of₁T_A
+ Of₂ T_A ²
Equation (5b)
Y_BS
=
Signal_BS
D_S − D_OFF
+ Of₀ + Of₁T_B
+ Of₂ T_B2
Equation (5c)
Y_CS
=
Signal_CS
D_S − D_OFF
+ Of₀ + Of₁T_C
+ Of₂ T_C2
Equations 4a and 4b form a system of two linear equa-
tions and two unknowns, G₁ and G₂. Solve for G₁ and
G₂. Equation 1 can now be readily solved for the last
unknown, Gain.
Arithmetic manipulation can magnify measurement
errors and noise. Quantization of the calibration coeffi-
cients is another reason to consider adjusting the Gain
and D_OFF coefficients. To do this, load the MAX1460
registers with the calculated coefficients Gain, G₁, G₂,
Of₀, Of₁, Of₂, and D_OFF. Assuming the oven is still at
temperature C and the S sensor excitation is still
applied, measure the output D_CS. Change to the L sen-
sor excitation, and measure D_CL. Compute the new
Gain coefficient using equation 6. Remeasure D_CL, and
compute the new D_OFF coefficient, given by equation
7.
Equation (6)
GAIN_new
= Gain ^DL
D_CL
− D_S
− D_CS
Equation (7)
D_OFFnew = D_OFF + D_L − D_CL
The final calibration coefficients may now be written
into the MAX1460 EEPROM. The unit is now ready for
final test.
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