ture errors. The piezoresistive sensor is powered by a

current source resulting in a temperature-dependent

bridge voltage due to the sensor's temperature resis-

tance coefficient (TCR). The reference inputs of the off-

set TC DAC and FSOTC DAC are connected to the

bridge voltage. The DAC output voltages will track the

bridge voltage as it varies with temperature, and by

varying the offset TC and FSOTC digital code a portion

of the bridge voltage, which is temperature dependent,

is used to compensate the first order temperature

errors.

FSO temperature compensation are optimized to 75k

Ω

for silicon piezoresistive sensors. However, since the

required feedback resistor values are sensor depen-

dent, external resistors may also be used. The internal

resistors selection bit in the configuration register

selects between internal and external feedback resis-

tors.

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 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 effect the FSO, however

changing the FSO will affect the offset due to nature of

the bridge. The temperature is measured on both the

MAX1452 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 MAX1452 provides a high-performance ratiometric

output with a minimum number of external components

lowing:

•

One supply bypass capacitor.

•

One optional output EMI suppression capacitor.

•

Two optional resistors, RISRC and RSTC, for special

sensor bridge types.

Low-Cost Precision Sensor

Signal Conditioner

8

_______________________________________________________________________________________

Figure 2. Basic Ratiometric Output Configuration

MAX1452

OUT

GND

RSTC

RISRC

0.1

µF 0.1µF

INM

INP

7

9

2

16

1

8

3

BDR

OUT

5

6

4

FSOTC

ISRC

SENSOR