MAX1452_09 Datasheet, PDF (7/25 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
sensor bridge excitation current or voltage. The PGA
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 234V/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 output the coefficient
value at index 00hex. The maximum indexing value is
AFhex, which is the highest lookup table entry. All tem-
peratures higher than approximately 184°C output the
highest lookup table index value. No indexing 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 output the coefficient value at index
00hex. The maximum indexing value is AFhex, which is
the highest lookup table entry. All temperatures higher
than approximately 184°C output the highest lookup
table index value. No indexing wrap-around errors are
produced.
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