MAX1478 Datasheet, PDF (15/20 Pages) Maxim Integrated Products – 1% Accurate, Digitally Trimmed,Rail-to-Rail Sensor Signal Conditioner

1% Accurate, Digitally Trimmed,
Rail-to-Rail Sensor Signal Conditioner
Offset TC Compensation
The offset voltage at T1 was previously set to 0.5V;
therefore, any variation from this voltage at T2 is an
offset TC error. Perform the following steps:
1) Measure the offset voltage at T2.
2) Use the following equation to compute the correc-
tion required:
NewOFFTC COEF = CurrentOFFTC COEF -
( ) ( ) ^

4096
_VOFFSET
T1
-
V_OFFSET
T2




( ) ( ) ^

2.3
_VBDRIVE
T1
-
V_BDRIVE
T2




Note: CurrentOFFTC COEF is the current value
stored in the Offset TC DAC. If the Offset TC sign bit
(SOTC) is low, this number is negative.
3) Load this value into the Offset TC DAC.
4) If NewOFFTC COEF is negative, set the SOTC bit
low; otherwise, set it high.
Offset TC compensation is now complete.
Offset Calibration
At this point, the sensor should still be at temperature
T2. The final offset adjustment can be made at T2 or T1
by adjusting the Offset DAC (and optionally the offset
sign bit, SOFF) until the output (V_OUT(PMIN)) reads 0.5V
at zero input pressure. Use the following procedure:
1) Set Offset DAC to zero (Offset COEF = 0).
2) Measure the voltage at OUT.
3) If V_OUT is greater than the desired offset voltage
(0.5V in this example), set SOFF low; otherwise, set
it high.
4) Increase Offset COEF until V_OUT equals the desired
offset voltage.
Offset calibration is now complete. Table 8 and Figure 9
compare an uncompensated input to a typical compen-
sated transducer output.
Table 8. MAX1478 Calibration and Compensation
Typical Uncompensated Input (Sensor)
Typical Compensated Transducer Output
Offset ..........................................................................±80% FSO
FSO ................................................................................+15mV/V
Offset TC ......................................................................-17% FSO
Offset TC Nonlinearity ..................................................0.7% FSO
FSO TC.........................................................................-35% FSO
FSO TC Nonlinearity.....................................................0.5% FSO
Temperature Range...........................................-40°C to +125°C
V_OUT ...................................................Ratiometric to V_DD at 5.0V
Offset at +25°C ......................................................0.500V ±5mV
FSO at +25°C .........................................................4.000V ±5mV
Offset Accuracy Over Temp Range ...........±28mV (±0.7% FSO)
FSO Accuracy Over Temp Range ..............±20mV (±0.5% FSO)
UNCOMPENSATED SENSOR ERROR
30
20
10
FSO
0
OFFSET
-10
-20
-50
0
50
100
150
TEMPERATURE (°C)
COMPENSATION TRANSDUCER ERROR
0.8
0.6
0.4
FSO
0.2
0
-0.2
-0.4
OFFSET
-0.6
-0.8
-50
0
50
100
150
TEMPERATURE °(C)
Figure 9. Comparison of an Uncalibrated Sensor and a Temperature-Compensated Transducer
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