MAX1454_11 Datasheet, PDF(9/25 Page) Maxim Integrated Products – Precision Sensor Signal Conditioner with Overvoltage Protection

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MAX1454

Precision Sensor Signal Conditioner

with Overvoltage Protection

technology, with an input-referred offset-trimming range

of more than Q150mV. The PGA provides gain values

from 6V/V to 2048V/V in 32 steps.

The device includes an internal 2K x 8-bit flash memory

to store calibration coefficients and user data. The inter-

nal memory contains the following information as 16-bit-

wide words:

U Configuration Register 1 (CONFIG1)

U Configuration Register 2 (CONFIG2)

U Offset calibration coefficient (ODAC) table

U Offset Temperature Coefficient register (OTCDAC)

U Full-span output calibration coefficient (FSODAC)

table

U FSO Temperature Coefficient register (FSOTCDAC)

U Power-Up Configuration register (PWRUPCFG)

U 256 bytes (2048 bits) uncommitted for customer pro-

gramming of manufacturing data (e.g., serial number

and date)

Offset Correction

Initial offset correction is accomplished at the input

stage of the signal-gain amplifiers by a coarse offset set-

ting. 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 resolu-

tion of approximately 1.5NC, from -40NC to +125NC. Every

4ms (programmable through the CONFIG2 register), the

on-chip temperature sensor provides indexing into the

offset lookup table in flash memory, with the resulting

value transferred to the offset DAC register. The result-

ing voltage is fed into a summing junction at the PGA

output, compensating the sensor offset with a resolution

of Q76FV (Q0.0019% FSO). If the offset TC DAC is set

to zero, then the maximum temperature error is typically

one degree of temperature drift of the sensor, given the

offset DAC has corrected the sensor at every 1.5NC.

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 (and FSO TC DAC in

current excitation mode) sets the sensor bridge current

or voltage with the digital input obtained from the flash

memory. 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 one 16-bit value at every 1.5NC,

from -40NC to +125NC.

Linear and Nonlinear

Temperature Compensation

In most applications, the device and the sensor are at

the same temperature, and coefficients in the offset and

FSO lookup table correct both linear and nonlinear tem-

perature errors to an accuracy approaching the sensorâs

repeatability error. In these applications, the offset TC

DAC and FSO TC DACs should be set to nominal values.

In applications where the sensor and the device are at

different temperatures, the FSO and offset DAC lookup

tables cannot be used. Writing 16-bit calibration coef-

ficients into the offset TC and FSO TC registers compen-

sates 1st-order temperature errors. The piezoresistive

sensor is powered by a current source, resulting in a

temperature-dependent bridge voltage due to the sen-

sorâs temperature coefficient of resistance (TCR). The ref-

erence inputs of the offset TC DAC and FSO TC DAC are

connected to the bridge voltage, causing their outputs

to change as a function of temperature. When properly

programmed, they provide 1st-order temperature com-

pensation of the input signal. Only two test temperatures

are required for linear temperature compensation.

The device uses a 10kI internal feedback resistor

(RISRC) for FSO temperature compensation. Since the

required feedback resistor value is sensor dependent, the

device offers the ability to adjust the current-mirror ratio

(CMRATIO) of the bridge driver. By selecting one of four

CMRATIO settings in the CONFIG1 register, the bridge

driverâs feedback loop can be optimized for silicon piezo-

resistive sensors typically ranging from 2kI to 10kI.

Internal Temperature Sensor/ADC

The signal conditioner uses an internal temperature sen-

sor to generate an 8-bit temperature index. An ADC con-

verts the integrated temperature-sensor output to an 8-bit

value every 4ms (programmable through the CONFIG2

temperature index register.

The typical transfer function for the temperature index is

as follows:

TEMPINDEX = 0.6561 x temperature (NC) + 53.6

where TEMPINDEX is truncated to an 8-bit integer value.

Typical values for the temperature index register are

given in Table 13.

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