Datasheet MAX1450 Datasheet (MAXIM)

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General Description

The MAX1450 sensor signal conditioner is optimized for
piezoresistive sensor calibration and temperature com­pensation. It includes an adjustable current source for
sensor excitation and a 3-bit programmable-gain amplifi­er (PGA). Achieving a total typical error factor within
1% of the sensor’s inherent repeatability errors, the
MAX1450 compensates offset, full-span output (FSO), off­set tempco, FSO tempco, and FSO nonlinearity of silicon
piezoresistive sensors via external trimmable resistors,
potentiometers, or digital-to-analog converters (DACs).

The MAX1450 is capable of compensating sensors that
display close error distributions with a single tempera­ture point, making it ideal for low-cost, medium-accuracy
applications. Although optimized for use with popular
piezoresistive sensors, it may also be used with other
resistive sensor types such as strain gauges.

Customization

Maxim can customize the MAX1450 for unique require­ments including improved power specifications. With a
dedicated cell library consisting of more than 90 sen­sor-specific functional blocks, Maxim can quickly pro­vide customized MAX1450 solutions. Contact the
factory for additional information.

Applications

Piezoresistive Pressure and Acceleration

Transducers and Transmitters
Manifold Absolute Pressure (MAP) Sensors
Automotive Systems
Hydraulic Systems
Industrial Pressure Sensors

Features

♦ 1% Sensor Signal Conditioning
♦ Corrects Sensor Errors Using Coefficients Stored

in External Trimmable Resistors, Potentiometers,
or DACs

♦ Compensates Offset, Offset TC, FSO, FSO TC,

and FSO Linearity

♦ Rail-to-Rail

®

Analog Output

♦ Programmable Current Source for Sensor

Excitation

♦ Fast Signal-Path Settling Time (< 1ms)
♦ Accepts Sensor Outputs from 10mV/V to 30mV/V
♦ Fully Analog Signal Path

MAX1450

Low-Cost, 1%-Accurate Signal Conditioner

for Piezoresistive Sensors

________________________________________________________________

Maxim Integrated Products

1

PGA

A = 1

OUT

ISRC

BDRIVE

INP

INM

SOTC

SOFF

OFFTC
OFFSET

BBUF

A2

FSOTRIM

A1
A0

+

-

V

DD

V

SS

CURRENT

SOURCE

V

DD

MAX1450

Pin Configuration

19-1365; Rev 0; 5/98

PART

MAX1450CAP
MAX1450C/D
MAX1450EAP -40°C to +85°C

0°C to +70°C

0°C to +70°C

TEMP. RANGE PIN-PACKAGE

20 SSOP
Dice*
20 SSOP

*

Dice are tested at TA = +25°C, DC parameters only.

Functional Diagram

Ordering Information

Rail-to-Rail is a registered trademark of Nippon Motorola, Ltd.

查询MAX1450供应商

TOP VIEW

INP
I.C.
I.C.

SOFF

A1
A0

OFFSET

20

INM

19

V

SS

18

BDRIVE

17

ISRCSOTC

16

I.C.

15

V

DD

OUT

14

A2OFFTC

13
12

I.C.

11

FSOTRIMBBUF

1
2
3
4

MAX1450

5
6
7
8
9

10

SSOP

MAX1450

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

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(VDD= +5V, VSS= 0, TA= T

MIN

to T

MAX

, unless otherwise noted. Typical values are at TA= +25°C.)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.

Supply Voltage, VDDto VSS......................................-0.3V to +6V

All Other Pins ...................................(VSS- 0.3V) to (VDD+ 0.3V)

Short-Circuit Duration, OUT, BBUF, BDRIVE.............Continuous

Continuous Power Dissipation (TA= +70°C)

SSOP (derate 8.00mW/°C above +70°C) ....................640mW

Operating Temperature Range

MAX1450CAP .....................................................0°C to +70°C

MAX1450EAP ..................................................-40°C to +85°C

Storage Temperature Range.............................-65°C to +165°C

Lead Temperature (soldering, 10sec).............................+300°C

TA= +25°C (Note 1)

DC to 10Hz, gain = 39,
sensor impedance = 5kΩ

V

OUT

= (VSS+ 0.25V) to (VDD- 0.25V)

,

TA= +25°C

(Note 5)

(Note 4)

5kΩ load to VSSor V

DD,

TA= +25°C

From VSSto V

DD

63% of final value

(Notes 2, 3)

Eight selectable gains (Table 3)

CONDITIONS

µV

RMS

500Output Noise

mA

-1.0 1.0

(sink) (source)

Output Current Range

V

V

SS +

V

DD -

0.25 0.25

Output Voltage Swing

V/V36 39 44

Minimum Differential Signal
Gain

V/V39 to 221Differential Signal Range Gain

mA2.8 3.5I

DD

Supply Current

mV/V10 to 30

Input-Referred Adjustable
Full-Span Output Range

mV±100

Input-Referred Adjustable Offset
Range

dB90CMRRCommon-Mode Rejection Ratio

ms1Output Step-Response Time

MΩ1.0R

IN

Input Impedance

µV/°C±0.5

Input-Referred Offset
Temperature Coefficient

%V

DD

0.01Amplifier Gain Nonlinearity

UNITSMIN TYP MAXSYMBOLPARAMETER

No load, TA= T

MIN

to T

MAX

V

SS +

V

DD -

0.05 0.05

V4.5 5.0 5.5V

DD

Supply Voltage

At any gain ppm/°C±50

Differential Signal Path
Temperature Coefficient

V/V1.15Offset TC Gain

V/V1.15Offset Gain

∆∆V

V

OUT

OFFSET

∆∆V

V

OUT

OFFTC

GENERAL CHARACTERISTICS

ANALOG OUTPUT (PGA)

ANALOG INPUT (PGA)

SUMMING JUNCTION (Figure 1)

MAX1450

Low-Cost, 1%-Accurate Signal Conditioner

for Piezoresistive Sensors

_______________________________________________________________________________________

3

ELECTRICAL CHARACTERISTICS (continued)

(VDD= +5V, VSS= 0, TA= T

MIN

to T

MAX

, unless otherwise noted. Typical values are at TA= +25°C.)

Note 1: Contact factory for high-volume applications requiring less than 1.5mA.
Note 2: All electronics temperature errors are compensated together with the sensor errors.
Note 3: The sensor and the MAX1450 must always be at the same temperature during calibration and use.
Note 4: This is the maximum allowable sensor offset at minimum gain (39V/V).
Note 5: This is the sensor’s sensitivity normalized to its drive voltage, assuming a desired full-span output (FSO) of 4V and a bridge

voltage of 2.5V. Operating at lower bridge excitation voltages can accommodate higher sensitivities.

V

BDRIVE

= 2.5V

No load

(V

BDRIVE

- V

BBUF

) at V

BDRIVE

= 2.5V, no load

CONDITIONS

µA-100 100Current Drive

V

V

SS +

V

DD -

1.3 1.3

Voltage Swing

mV-20 20V

OFS

Offset Voltage

UNITSMIN TYP MAXSYMBOLPARAMETER

∆I

BDRIVE/

∆I

ISRC

(Figure 2)

V

V

SS +

V

DD -

1.3 1.3

V

BDRIVE

Bridge Voltage Swing

mA0.1 0.5 2.0I

BDRIVE

Bridge Current Range

µA/µA13AACurrent-Source Gain

V

V

SS +

V

DD-

1.3 1.3

V

ISRC

Current-Source Input Voltage
Range

BUFFER (BBUF)

CURRENT SOURCE

______________ Detailed Description

Analog Signal Path

The MAX1450’s signal path is fully differential and com­bines the following three stages: a 3-bit PGA with
selectable gains of 39, 65, 91, 117, 143, 169, 195, and
221; a summing junction; and a differential to single­ended output buffer (Figure 1).

Programmable-Gain Amplifier

The analog signal is first fed into a programmable-gain
instrumentation amplifier with a CMRR of 90dB and a
common-mode input range from VSSto VDD. Pins A0,
A1, and A2 set the PGA gain anywhere from 39V/V to
221V/V (in steps of 26).

MAX1450

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

4 _______________________________________________________________________________________

NAME FUNCTION

1 INP Positive Sensor Input. Input impedance is typically 1MΩ. Rail-to-rail input range.

PIN

2, 3,

12, 16

I.C. Internally connected. Leave unconnected.

4 SOTC

Offset TC Sign Bit Input. A logic low inverts V

OFFTC

with respect to V

SS.

This pin is internally pulled to V

SS

via a 1MΩ (typical) resistor. Connect to VDDto add V

OFFTC

to the PGA output, or leave unconnected (or

connect to VSS) to subtract V

OFFTC

from the PGA output.

8 OFFTC

Offset TC Adjust. Analog input summed with PGA output and V

OFFSET

. Input impedance is typically 1MΩ.

Rail-to-rail input range.

7 A0

PGA Gain-Set LSB Input. Internally pulled to VSSvia a 1MΩ (typical) resistor. Connect to VDDfor a logic
high or VSSfor a logic low.

6 A1

PGA Gain-Set Input. Internally pulled to VSSvia a 1MΩ (typical) resistor. Connect to VDDfor a logic high or
VSSfor a logic low.

5 SOFF

Offset Sign Bit Input. A logic low inverts V

OFFSET

with respect to VSS. This pin is internally pulled to VSSvia

a 1MΩ (typical) resistor. Connect to VDDto add V

OFFSET

to the PGA output, or leave unconnected (or con-

nect to VSS) to subtract V

OFFSET

from the PGA output.

14 OUT PGA Output Voltage. Connect a 0.1µF capacitor from OUT to VSS.

13 A2

PGA Gain-Set MSB Input. Internally pulled to VSSvia a 11kΩ (typical) resistor. Connect to VDDfor a logic
high or VSSfor a logic low.

11 FSOTRIM Bridge Drive Current-Set Input. The voltage on this pin sets the nominal I

ISRC

. See the

Bridge Drive

section.

10 BBUF

Buffered Bridge-Voltage Output (the voltage at BDRIVE). Use with correction resistor R

STC

to correct for FSO

tempco.

9 OFFSET

Offset Adjust Input. Analog input summed with PGA output and V

OFFTC

. Input impedance is typically

1MΩ. Rail-to-rail input range.

Pin Description

15 V

DD

Positive Supply Voltage Input. Connect a 0.1µF capacitor from VDDto VSS.

20 INM Negative Sensor Input. Input impedance is typically 1MΩ. Rail-to-rail input range.

19 V

SS

Negative Power-Supply Input.

18 BDRIVE Sensor Excitation Current Output. This pin drives a nominal 0.5mA through the bridge.

17 ISRC Current-Source Reference. Connect a 50kΩ (typical) resistor from ISRC to VSS.

Figure 1. Signal-Path Functional Diagram

INP

INM

A2

A1 A0

PGA

OFFTC SOTC

±

Σ

±

OFFSET SOFF

A = 1

OUT

Summing Junction

The second stage in the analog signal path consists of
a summing junction for offset, offset temperature com­pensation, and the PGA output. The offset voltage
(V

OFFSET

) and offset temperature-compensation volt-

age (V

OFFTC

) add or subtract from the PGA output
depending on their respective sign bits, offset sign
(SOFF), and offset TC sign (SOTC). V

OFFSET

and

V

OFFTC

can range in magnitude from VSSto VDD.

Output Buffer

The final stage in the analog signal path consists
of a unity-gain buffer. This buffer is capable of swinging
to within 250mV of VSSand VDDwhile sourcing/sinking
up to 1.0mA, or within 50mV of the power supplies with
no load.

Bridge Drive

Figure 2 shows the functional diagram of the on-chip
current source. The voltage at FSOTRIM, in conjunction
with R

ISRC

, sets the nominal current, I

ISRC

which sets

the FSO (refer to Figure 3 for sensor terminology.) I

ISRC

is additionally modulated by components from the
external resistor R

STC

and the optional resistor R

LIN

.

R

STC

is used to feed back a portion of the buffered

bridge-excitation voltage (V

BBUF

), which compensates
FSO TC errors by modulating the bridge-excitation cur­rent over temperature. To correct FSO linearity errors,
feed back a portion of the output voltage to the current­source reference node via the optional R

LIN

resistor.

Applications Information

Compensation Procedure

The following compensation procedure assumes a pres­sure transducer with a +5V supply and an output voltage
that is ratiometric to the supply voltage (see

Ratiometric

Output Configuration

section). The desired offset voltage

(V

OUT

at P

MIN

) is 0.5V, and the desired FSO voltage

(V

OUT(PMAX

) - V

OUT(PMIN

)) is 4V; thus the FS output volt-

age (V

OUT

at P

MAX

) will be 4.5V. The procedure requires
a minimum of two test pressures (e.g., zero and full scale)
and two temperatures. A typical compensation procedure
is as follows:

1) Perform Coefficient Initialization

2) Perform FSO Calibration

3) Perform FSO TC Compensation

4) Perform OFFSET TC Compensation

5) Perform OFFSET Calibration

6) Perform Linearity Calibration (Optional)

Coefficient Initialization

Select the resistor values and the PGA gain to prevent
gross overload of the PGA and bridge current source.
These values depend on sensor behavior and require
some sensor characterization data. This data may be
available from the sensor manufacturer. If not, it can be
generated by performing a two-temperature, two-pres-

MAX1450

Low-Cost, 1%-Accurate Signal Conditioner

for Piezoresistive Sensors

_______________________________________________________________________________________ 5

V

DD

I

BDRIVE ≈

13 (I

ISRC

)

V

BDRIVE

I

ISRC

I

ISRC

BBUF

INP

INM

SENSOR

R

ISRC

(EXTERNAL)

BDRIVE

FSOTRIM

BBUF

OUT

A = 1

R

STC

(EXTERNAL)

R

LIN

(OPTIONAL)

(EXTERNAL)

MAX1450

Figure 2. Bridge Drive Circuit

MAX1450

sure sensor evaluation. Note that the resistor values
and PGA gain obtained from this evaluation will repre­sent a starting point. The final compensated transducer
will likely use slightly different values. The required sen­sor information is shown in Table 1, and can be used to
obtain the values for the parameters shown in Table 2.

Selecting R

ISRC

R

ISRC

programs the nominal sensor excitation current
and is placed between ISRC and VSS. Use a variable
resistor with a nominal starting value of:

where Rb(T1) is the sensor input impedance at temper­ature T1 (usually +25°C).

Selecting R

STC

R

STC

compensates the FSO TC errors and is placed
between BBUF and ISRC. Use a variable resistor with
a nominal starting value of the following:

This approximation works best for bulk, micromachined,
silicon piezoresistive sensors (PRTs). Negative values
for R

STC

indicate unexpected sensor behavior that can­not be compensated by the MAX1450 without addition­al external circuitry.

Selecting PGA Gain Setting

Calculate the ideal gain using the following formula,
and select the nearest gain setting from Table 3.

SensorFSO can be derived as follows:

where S is the sensor sensitivity at T1, V

BDRIVE

is the
sensor excitation voltage (initially 2.5V), and ∆P is the
maximum pressure differential.

SensorFSO S x V x P

1.5mV/V psi x 2.5V x 10 psi

0.0375V

BDRIVE

=
=
=

∆

R

R x 500ppm/ C

TCR TCS

65k x 500ppm/ C

2600ppm/ C 2100ppm/ C

65k

STC

ISRC

≈

°

−

≈

°

− −

=

Ω

Ω

o o

R 13 x Rb(T1)

13(5k ) 65k

ISRC

≈
≈ =Ω Ω

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

6 _______________________________________________________________________________________

Table 1. Sensor Information

Table 2.Compensation Components/Values

PARAMETER

SENSOR

DESCRIPTION

TYPICAL

VALUE

Rb(T) Input/Output Impedance 5kΩ at +25°C

TCR

Input/Output Impedance
Tempco

2600ppm/°C

S(T) Sensitivity

1.5mV/V psi at
+25°C

TCS Sensitivity Tempco -2100ppm/°C

O(T) Offset

12mV/V at
+25°C

OTC Offset Tempco

-1030 ppm­FSO/°C

S(p)

Sensitivity Linearity Error as
% FSO BSLF (Best Straight­Line Fit)

0.1% FSO
BSLF

P

MIN

Minimum Input Pressure 0 PSI

P

MAX

Maximum Input Pressure 10 PSI

PARAMETER DESCRIPTION

R

ISRC

Resistor that programs the nominal sensor
excitation current

R

STC

Resistor that compensates FSO TC errors

A

PGA

Programmable-gain amplifier gain

OFFTC

Offset TC correction voltage, including its
respective sign bit

R

LIN

Resistor that corrects FSO linearity errors
(optional)

Figure 3. Typical Pressure-Sensor Output

4.5

FULL-SPAN OUTPUT (FSO)

VOLTAGE (V)

0.5
OFFSET

P

MIN

PRESSURE

P

MAX

FULL-SCALE (FS)

where OUTFSO is the desired calibrated transducer
full-span output voltage, and SensorFSO is the sensor
full-span output voltage at T1.

Determining OFFTC Initial Value

Generally, the OFFTC coefficient can be set to 0V,
since the offset TC errors will be compensated in a later
step. However, sensors with large offset TC errors may
require an initial coarse offset TC adjustment to prevent
the PGA from saturating as the temperature increases
during the compensation procedure. An initial coarse
offset TC adjustment would be required if the magni­tude of the sensor offset TC error is more than about
10% of the FSO. If a coarse offset TC adjustment is
required, use the following equation:

which can be approximated by:

where OTC is the sensor offset TC error in ppm of FSO,
∆T is the operating temperature range in °C, and OTC
Correction is the offset TC resistor-divider ratio. For

positive values of OTC correction, connect SOTC to
V

DD

; for negative values, connect SOTC to VSS.

Select the Offset TC resistor divider (R

OTCA

and

R

OTCB

, Figure 4) using the following equation:

where 500kΩ ≥ (R

OTCA

+ R

OTCB

) ≥ 100kΩ. Choose

R

OTCB

= 100kΩ and R

OTCA

= 20kΩ.

Transfer Function

The following transfer function (linearity correction not
included) is useful for data modeling or for developing
compensative algorithms:

(AA = current source gain)

FSO Calibration

Perform FSO calibration at room temperature with a full­scale sensor excitation.

1) At +25°C (or T1), set V

FSOTRIM

to 2.5V. Adjust

R

ISRC

until V

BBUF

= 2.5V.

2) Adjust V

OFFSET

until the room temperature offset

voltage is 0.5V (see

OFFSET Calibration

section).

3) Measure the full-span output (measuredV

FSO

).

4) Calculate V

BIDEAL(25°C)

using the following equation:

Note: If V

BIDEAL(25°C)

is outside the allowable bridge
voltage swing of (VSS+ 1.3V) to (VDD- 1.3V), readjust
the PGA gain setting. If V

BIDEAL(25°C)

is too low,
decrease the PGA gain setting by one step and return
to Step 1. If V

BIDEAL(25°C)

is too high, increase the PGA

gain setting by one step and return to Step 1.

V

V 1

desiredV measuredV

measuredV

BIDEAL(25 C)

FSOTRIM

FSO FSO

FSO

o

=

+

[ ]−[ ]

[ ]















V V x
V x PGA + 1.15 x

V

V

1.15 x V

where V =

V

R

V

R

1

AA x Rb(T)

1

R

OUT BDRIVE

S

OFFTC

DD

OFFSET

BDRIVE

DD

ISRC

DD

STC

STC

=











+

+

+

OTC Correction

R

R R

R

R R

OTCA

OTCA OTCB

OTCA

OTCA OTCB

.

=

+

=

+

0 17

OTC Correction

OTC x FSO x ( T)

TCS x V x 1.15 x ( T)

1

2100 x .5V x 1.15

0.68

BDRIVE

≈

≈

− °

−

=

∆

∆

030 4

2

ppm C x V/

OTC Correction

V

V x 1.15

OUT(T)

BDRIVE(T)

=

∆

∆

A

OUTFSO

SensorFSO

4V

0.0375V

106V/V

PGA

≈

≈ =

MAX1450

Low-Cost, 1%-Accurate Signal Conditioner

for Piezoresistive Sensors

_______________________________________________________________________________________ 7

Table 3. PGA Gain Settings

PGA GAIN (V/V) PGA VALUE A2 A1 A0

39 0 0 0 0
65 1 0 0 1

91 2 0 1 0
117 3 0 1 1
143 4 1 0 0
169 5 1 0 1
195 6 1 1 0
221 7 1 1 1

MAX1450

5) Set V

FSOTRIM

= V

BIDEAL(25°C)

. Adjust R

ISRC

until

V

BBUF

= V

BIDEAL(25°C)

.

6) Readjust V

OFFSET

until the offset voltage is 0.5V (see

OFFSET Calibration

section).

FSO TC Compensation

Correct linear span TC by connecting BBUF to ISRC
through a resistor (R

STC

). The value of R

STC

depends
on the required correction coefficient, which is sensor
dependent, but typically around 100kΩ for most silicon
PRTs. The following procedure results in FSO TC cali­bration:

1) Measure the full-span output at T2.

2) Use the equation from Step 4 of the

FSO Calibration

section to determine V

BIDEAL(T2)

. While at T2, adjust

R

STC

until V

BBUF

= V

BIDEAL(T2)

.

3) Do not adjust V

OFFSET

or V

OFFTC

.

OFFSET TC Compensation

Connect OFFTC to a resistor divider between BBUF
and VSS. The divided-down V

BBUF

is then fed into
OFFTC and the appropriate polarity (designating
whether V

OFFTC

should be added or subtracted from

the PGA output) is selected with SOTC.

1) At T2, remeasure the offset at V

OUT

.

2) Use the following equation to determine the magni­tude of V

OFFTC(T2)

, and adjust R

OTCA

accordingly. If

V

OFFTC

is negative, connect SOTC to VSS. If V

OFFTC

is positive, connect SOTC to VDD. After OTC calibra­tion, the output may be saturated; correct this condi­tion during OFFSET calibration. In most cases
Current OFFTC will be 0. However, if a coarse
OFFTC adjustment was performed, the coefficient
must be inserted in the equation below.

where Current OFFTC is the voltage at pin OFFTC.
Note that the magnitude of V

OFFTC

is directly propor­tional to the gain of the PGA. Therefore, if the PGA gain
changes after performing the offset TC calibration, the
offset TC must be recalibrated.

V

V V

V V x 1.15

Current OFFTC

OFFTC

OFFSET(T1) OFFSET(T2)

BDRIVE(T1) BDRIVE(T2)

=

−

−

( )

+

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

8 _______________________________________________________________________________________

R

STC

R

FSOB

R

FSOA

V

DD

V

DD

R

ISRC

0.1µF

R

LIN

(OPTIONAL)

PGA

A = 1

CURRENT

SOURCE

OUT

OUT

ISRC

BDRIVE
INP

INM

SOTC

OFFSET

OFFTC

SOFF

BBUF

A2
A1
A0

V

DD

V

DD

FSOTRIM

V

SS

MAX1450

R

OTCA

R

OTCB

R

OFFA

R

OFFB

V

DD

V

DD

0.1µF

0.1µF

SENSOR

V

DD

Figure 4. Basic Ratiometric Output Configuration

OFFSET Calibration

Accomplish offset calibration by applying a voltage to the
OFFSET pin (SOFF determines the polarity of V

OFFSET

).
This voltage is generated by a resistor-divider between
VDDand VSS(R

OFFA

and R

OFFB

in Figure 4). To calibrate

the offset, set V

OFFSET

to 0 and perform a minimum pres­sure input reading at room temperature. If the output volt­age (V

OFFZERO

) is greater than 0.5V, connect SOFF to

VSS; if V

OFFZERO

is less than 0.5V, connect SOFF to VDD.

Adjust V

OFFSET

until V

OUT

= 0.5V.

Note that the magnitude of V

OFFSET

is directly proportion­al 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
output voltage (V

OUT

) 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 current, 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
transfer 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 R

LIN

. To correct
pressure linearity errors, use the following equation to
determine the appropriate range for R

LIN

:

where S(p) is the sensitivity linearity error as % best
straight-line fit (BSLF). Ideally, this variable resistor
should be disconnected during temperature error com­pensation. If this is not possible, set it to the maximum
available value.

First measure the magnitude of the uncorrected error
(R

LIN

= maximum value), then choose an arbitrary

value for R

LIN

(approximately 50% of maximum value).
Measuring the new linearity error establishes a linear
relationship between the amount of linearity correction
and the value of R

LIN

.

Note that if pressure linearity correction is to be per­formed, it must occur after temperature compensation
is completed. A minor readjustment to the FSO and
OFFSET will be required after linearity correction is per­formed. If pressure linearity correction is not required,
remove R

LIN

.

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 high­performance 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 com­pensated 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 outputs of the uncompensated sensor and the
compensated transducer.

R

R x R

R R x S p

LIN

ISRC STC

ISRC STC

≈

+

( )

( )

2

MAX1450

Low-Cost, 1%-Accurate Signal Conditioner for

Piezoresistive Sensors

_______________________________________________________________________________________ 9

B

C

D

E

F

PRESSURE

LINEARITY
ERROR

UNCOMPENSATED ERROR
(R

LIN

REMOVED)

OVERCOMPENSATED ERROR
(R

LIN

TOO SMALL)

A

Figure 5. Effect of R

LIN

on Linearity Corrections

MAX1450

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

10 ______________________________________________________________________________________

-20

-10

10

0

20

30

-50 0 50 100 150

UNCOMPENSATED SENSOR ERROR

TEMPERATURE (°C)

ERROR (% SPAN)

OFFSET

FSO

-0.8

-0.6

0.4

-0.2

-0.4

0

0.2

0.6

0.8

-50 0 50 100 150

COMPENSATED TRANSDUCER ERROR

TEMPERATURE °(C)

ERROR (% SPAN)

OFFSET

FSO

Figure 6. Comparison of an Uncalibrated Sensor and a Temperature-Compensated Transducer

TRANSISTOR COUNT: 1364
SUBSTRATE CONNECTED TO V

SS

Chip Information

Typical Uncompensated Input (Sensor) Typical Compensated Transducer Output

Offset ..........................................................................±80% FSO

FSO..................................................................................15mV/V

Offset TC......................................................................-17% FSO

Offset TC Nonlinearity.....................................................1% FSO

FSO TC.........................................................................-35% FSO

FSO TC Nonlinearity........................................................1% FSO

Temperature Range...........................................-40°C to +125°C

V

OUT

...................................................Ratiometric to VDDat 5.0V

Offset at +25°C ......................................................0.500V ±5mV

FSO at +25°C .........................................................4.000V ±5mV

Offset Accuracy Over Temp. Range.............±60mV (1.5% FSO)

FSO Accuracy Over Temp. Range ...............±60mV (1.5% FSO)

Table 4. MAX1450 Calibration and Compensation

MAX1450

Low-Cost, 1%-Accurate Signal Conditioner for

Piezoresistive Sensors

______________________________________________________________________________________ 11

Package Information

SSOP.EPS

MAX1450

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

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

12

____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

© 1998 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

NOTES

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