MAX1450 データシートの表示（PDF） - Maxim Integrated

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MAX1450

Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors

Maxim Integrated 

MAX1450

Low-Cost, 1%-Accurate Signal Conditioner

for Piezoresistive Sensors

OFFSET Calibration

Accomplish offset calibration by applying a voltage to the

OFFSET pin (SOFF determines the polarity of VOFFSET).

This voltage is generated by a resistor-divider between

VDD and VSS (ROFFA and ROFFB in Figure 4). To cali-

brate the offset, set VOFFSET to 0 and perform a minimum

pressure input reading at room temperature. If the output

voltage (VOFFZERO) is greater than 0.5V, connect SOFF

to VSS; if VOFFZERO is less than 0.5V, connect SOFF to

VDD. Adjust VOFFSET until VOUT = 0.5V.

Note that the magnitude of VOFFSET is directly propor

ional to the gain of the PGA. Therefore, if the PGA gain

changes after performing the offset calibration, the offset

must be recalibrated.

Linearity Calibration (optional)

Correct pressure linearity by using feedback from the out-

put voltage (VOUT) to ISRC to modulate the current source.

If a bridge current is constant with applied pressure, sensor

linearity remains unaffected. If, with a constant bridge cur-

rent, the output voltage is nonlinear with applied pressure

(e.g., increasing faster than the pressure), use pressure

linearity correction to linearize the output.

Performing linearity corrections through the use of a trans-

fer function is not practical, since a number of required

system variables cannot easily be measured with a high

enough degree of accuracy. Therefore, use a simple

empirical approach. Figure 5 shows the uncompensated

pressure linearity error of a silicon PRT. The magnitude of

this error is usually well below 1% of span. Curves A, B,

C, D, E, and F in Figure 5 represent increasing amounts

of linearity error corrections, corresponding to decreasing

values in the resistance of RLIN. To correct pressure lin-

earity errors, use the following equation to determine the

appropriate range for RLIN:

R LIN

≈

2 RISRC × R STC

(RISRC + R STC) × S(p)

where S(p) is the sensitivity linearity error as % best

straight-line fit (BSLF). Ideally, this variable resistor

should be disconnected during temperature error com-

pensation. If this is not possible, set it to the maximum

available value.

First measure the magnitude of the uncorrected error

(RLIN = maximum value), then choose an arbitrary

value for RLIN (approximately 50% of maximum value).

Measuring the new linearity error establishes a linear

relationship between the amount of linearity correction

and the value of RLIN.

Note that if pressure linearity correction is to be performed,

it must occur after temperature compensation is complet-

ed. A minor readjustment to the FSO and OFFSET will be

required after linearity correction is performed. If pressure

linearity correction is not required, remove RLIN.

Ratiometric Output Configuration

Ratiometric output configuration provides an output that

is proportional to the power-supply voltage. When used

with ratiometric A/D converters, this output provides

digital pressure values independent of supply voltage.

Most automotive and some industrial applications require

ratiometric outputs.

The MAX1450 has been designed to provide a highper-

formance ratiometric output with a minimum number of

external components (Figure 4).

Sensor Calibration and

Compensation Example

Calibration and compensation requirements for a sensor

involve conversion of a sensor-specific performance into

a normalized output curve. Table 4 shows an example of

the MAX1450’s capabilities.

A repeatable piezoresistive sensor with an initial offset

of 30mV and FSO of 37.5mV was converted into a com-

pensated transducer (using the piezoresistive sensor with

the MAX1450) with an offset of 0.5V and an FSO of 4.0V.

The temperature errors, which were on the order of -17%

for the offset TC and -35% for the FSO TC, were reduced

to about ±1% FSO. The graphs of Figure 6 show the out-

puts of the uncompensated sensor and the compensated

transducer.

LINEARITY

ERROR

A

B

C

D

UNCOMPENSATED ERROR

(RLIN REMOVED)

E

F

OVERCOMPENSATED ERROR

(RLIN TOO SMALL)

PRESSURE

Figure 5. Effect of RLIN on Linearity Corrections

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