MAX1452 Datasheet, PDF (7/24 Pages) Maxim Integrated Products – Low-Cost Precision Sensor Signal Conditioner

Low-Cost Precision Sensor
Signal Conditioner
V_DD
V_DD
IRO
DAC
MAX1452
BIAS
GENERATOR
OSCILLATOR
CLK1M
TEST
INP
∑
PGA
OUT
INM
ISRC
BDR
VDDF
DIO
UNLOCK
AMP+
AMP-
CURRENT
SOURCE
TEMP
SENSOR
8-BIT ADC
INTERNAL
EEPROM
6144 BITS
416 BITS
FOR USER VDD
ANAMUX
A=1
FSOTC
176
TEMPERATURE
LOOK UP
POINTS FOR
OFFSET AND
SPAN.
BDR _OP-AMP
AMPOUT
V_SS
Figure 1. Functional Diagram
utilizes a switched capacitor CMOS technology, with an
input referred offset trimming range of more than
±150mV with an approximate 3µV resolution (16 bits).
The PGA provides gain values from 39V/V to 240V/V in
16 steps.
The MAX1452 uses four 16-bit DACs with calibration
coefficients stored by the user in an internal 768 x 8
EEPROM (6144 bits). This memory contains the follow-
ing information, as 16-bit wide words:
• Configuration Register
• Offset Calibration Coefficient Table
• Offset Temperature Coefficient Register
• FSO (Full-Span Output) Calibration Table
• FSO Temperature Error Correction Coefficient
Register
• 52 bytes (416 bits) uncommitted for customer pro-
gramming of manufacturing data (e.g., serial num-
ber and date)
Offset Correction
Initial offset correction is accomplished at the input
stage of the signal gain amplifiers by a coarse offset
setting. Final offset correction occurs through the use of
a temperature indexed lookup table with 176 16-bit
entries. The on-chip temperature sensor provides a
unique 16-bit offset trim value from the table with an
indexing resolution of approximately 1.5°C from -40°C
to +125°C. Every millisecond, the on-chip temperature
sensor provides indexing into the offset lookup table in
EEPROM and the resulting value transferred to the off-
set DAC register. The resulting voltage is fed into a
summing junction at the PGA output, compensating the
sensor offset with a resolution of ±76µV (±0.0019%
FSO). If the offset TC DAC is set to zero then the maxi-
mum temperature error is equivalent to one degree of
temperature drift of the sensor, given the Offset DAC
has corrected the sensor at every 1.5°C. The tempera-
ture indexing boundaries are outside of the specified
Absolute Maximum Ratings. The minimum indexing
value is 00hex corresponding to approximately -69°C.
All temperatures below this value will output the coeffi-
cient value at index 00hex. The maximum indexing
value is AFhex, which is the highest lookup table entry.
All temperatures higher than approximately 184°C will
output the highest lookup table index value. No index-
ing wrap-around errors are produced.
FSO Correction
Two functional blocks control the FSO gain calibration.
First, a coarse gain is set by digitally selecting the gain
of the PGA. Second, FSO DAC sets the sensor bridge
current or voltage with the digital input obtained from a
temperature-indexed reference to the FSO lookup table
in EEPROM. FSO correction occurs through the use of
a temperature indexed lookup table with 176 16-bit
entries. The on-chip temperature sensor provides a
unique FSO trim from the table with an indexing resolu-
tion approaching one 16-bit value at every 1.5°C from
-40°C to +125°C. The temperature indexing boundaries
are outside of the specified Absolute Maximum
Ratings. The minimum indexing value is 00hex corre-
sponding to approximately -69°C. All temperatures
below this value will output the coefficient value at
index 00hex. The maximum indexing value is AFhex,
which is the highest lookup table entry. All tempera-
tures higher than approximately 184°C will output the
highest lookup table index value. No indexing wrap-
around errors are produced.
Linear and Nonlinear Temperature
Compensation
Writing 16-bit calibration coefficients into the offset TC
and FSOTC registers compensates first-order tempera-
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