MAX1455 Datasheet, PDF (7/25 Pages) Maxim Integrated Products – Low-Cost Automotive Sensor Signal Conditioner

Low-Cost Automotive Sensor Signal
Conditioner
temperature indexed lookup table with one hundred
seventy-six 16-bit entries. The on-chip temperature sen-
sor provides a unique FSO trim from the table with an
indexing resolution approaching one 16-bit value every
1.5°C from -40°C to +125°C. The temperature indexing
boundaries are outside the specified absolute maximum
ratings. The minimum indexing value is 00hex, corre-
sponding to approximately -69°C. All temperatures below
this value output the coefficient value at index 00hex. The
maximum indexing value is AFhex, which is the highest
lookup table entry. All temperatures higher than approxi-
mately +184°C output the highest lookup table index
value. No indexing wraparound errors are produced.
Linear and Nonlinear Temperature
Compensation
Writing 16-bit calibration coefficients into the offset TC
and FSOTC registers compensates first-order tempera-
ture errors. The piezoresistive sensor is powered by a
current source resulting in a temperature-dependent
bridge voltage due to the sensor’s temperature coeffi-
cient resistance (TCR). The reference inputs of the off-
set TC DAC and FSOTC DAC are connected to the
bridge voltage. The DAC output voltages track the
bridge voltage as it varies with temperature, and by
varying the offset TC and FSOTC digital code and a
portion of the bridge voltage, which is temperature
dependent, is used to compensate the first-order tem-
perature errors.
The internal feedback resistors (R_ISRC and R_STC) for
FSO temperature compensation are set to 75kΩ.
To calculate the required offset TC and FSOTC com-
pensation coefficients, two test temperatures are need-
ed. After taking at least two measurements at each
temperature, calibration software (in a host computer)
calculates the correction coefficients and writes them to
the internal EEPROM.
With coefficients ranging from 0000hex to FFFFhex and
a +5V reference, each DAC has a resolution of 76µV.
Two of the DACs (offset TC and FSOTC) utilize the sen-
sor bridge voltage as a reference. Since the sensor
bridge voltage is approximately set to +2.5V, the FSOTC
and offset TC exhibit a step size of less than 38µV.
For high-accuracy applications (errors less than
0.25%), the first-order offset TC and FSOTC should be
compensated with the offset TC and FSOTC DACs, and
the residual higher order terms with the lookup table.
The offset and FSO compensation DACs provide
unique compensation values for approximately 1.5°C of
temperature change as the temperature indexes the
address pointer through the coefficient lookup table.
Changing the offset does not affect the FSO; however,
changing the FSO affects the offset due to the nature of
the bridge. The temperature is measured on both the
MAX1455 die and at the bridge sensor. It is recom-
mended to compensate the first-order temperature
errors using the bridge sensor temperature.
Typical Ratiometric
Operating Circuit
Ratiometric output configuration provides an output that is
proportional to the power-supply voltage. This output can
then be applied to a ratiometric ADC to produce a digital
value independent of supply voltage. Ratiometricity is an
important consideration for battery-operated instruments,
automotive, and some industrial applications.
The MAX1455 provides a high-performance ratiometric
output with a minimum number of external components
(Figure 2). These external components include the fol-
lowing:
• One supply bypass capacitor
• One optional output EMI suppression capacitor
Typical Nonratiometric
Operating Circuit
(5.5VDC < VPWR < 28VDC)
Nonratiometric output configuration enables the sensor
power to vary over a wide range. A low-dropout voltage
regulator, such as the MAX1615, is incorporated in the
circuit to provide a stable supply and reference for
MAX1455 operation. A typical example is shown in
Figure 3. Nonratiometric operation is valuable when
wide ranges of input voltage are to be expected and
the system A/D or readout device does not enable
ratiometric operation.
Internal Calibration Registers
The MAX1455 has five 16-bit internal calibration regis-
ters (ICRs) that are loaded from EEPROM, or loaded
from the serial digital interface.
Data can be loaded into the ICRs under three different
circumstances.
Normal Operation, Power-On Initialization Sequence:
• The MAX1455 has been calibrated, the Secure-
Lock byte is set (CL[7:0] = FFhex), and UNLOCK is
low.
• Power is applied to the device.
• The power-on reset (POR) functions have been
completed.
• Registers CONFIG, OTCDAC, and FSOTCDAC are
refreshed from EEPROM.
_______________________________________________________________________________________ 7