Datasheet MAX1457CWI, MAX1457CCJ, MAX1457C-D Datasheet (Maxim)

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________________General Description

The MAX1457 is a highly integrated analog-sensor sig­nal processor optimized for piezoresistive sensor cali­bration and compensation. It includes a programmable
current source for sensor excitation, a 3-bit program­mable-gain amplifier (PGA), a 12-bit ADC, five 16-bit
DACs, and an uncommitted op amp. Achieving a total
error factor within 0.1% of the sensor’s repeatability
errors, the MAX1457 compensates offset, full-span out­put (FSO), offset TC, FSO TC, and full-span output non­linearity of silicon piezoresistive sensors.

The MAX1457 calibrates and compensates first-order
temperature errors by adjusting the offset and span of
the input signal via digital-to-analog converters (DACs),
thereby eliminating quantization noise. If needed, resid­ual higher-order errors are then compensated using lin­ear interpolation of the first-order coefficients stored in
a look-up table (in external EEPROM).

The MAX1457 integrates three traditional sensor­manufacturing operations into one automated process:

• Pretest: Data acquisition of sensor performance under
the control of a host test computer.

• Calibration and Compensation: Computation and storage
(in an external EEPROM) of calibration and compensation

coefficients determined from transducer pretest data.

• Final Test: Verification of transducer calibration and
compensation, without removal from a pretest socket.

Analog outputs are provided for both pressure and tem­perature. A general-purpose, uncommitted op amp is also
included on-chip to increase the overall circuit gain, or to
facilitate the implementation of a 2-wire, 4–20mA transmit­ter. The serial interface is compatible with MicroWire™
and SPI™, and directly connects to an external EEPROM.
Additionally, built-in testability features of the MAX1457
facilitate manufacturing and calibration of multiple sensor
modules, thus lowering manufacturing cost.

Although optimized for use with piezoresistive sensors,
the MAX1457 may also be used with other resistive
sensor types (i.e., accelerometers and strain gauges)
with the addition of a few external components.

_______________________Customization

Maxim can customize the MAX1457 for unique require­ments. With a dedicated cell library of more than 90
sensor-specific functional blocks, Maxim can quickly pro­vide customized MAX1457 solutions. Contact Maxim for
additional information.

____________________________Features

♦ High Accuracy (within ±0.1% of sensor’s

repeatable errors)

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

Temperature/Pressure Nonlinearity

♦ Rail-to-Rail

®

Analog Output for Calibrated,
Temperature-Compensated Pressure
Measurements

♦ Programmable Sensor Excitation Current
♦ SPI/MicroWire-Compatible Serial Interface
♦ Fast Signal-Path Settling Time (<1ms)
♦ Accepts Sensor Outputs from 5mV/V to 30mV/V
♦ Pin-Compatible with MCA7707

MAX1457

0.1%-Accurate Signal Conditioner

for Piezoresistive Sensor Compensation

________________________________________________________________

Maxim Integrated Products

1

MAX1457

BIAS

GENERATOR

OSCILLATOR

16-BIT DAC - OFFSET TC

16-BIT DAC - OFFSET

16-BIT DAC - FSO

16-BIT DAC - FSO TC

16-BIT DAC - FSO LINEARITY

FSOTCDAC
OTCDAC
OFSTDAC
FSODAC

LINDAC

LINOUT

FSOTCOUT

VBBUF

V

BDRIVE

A = 1

A = 1

A = 1

AMPOUT

V

SS

VOUT

NBIAS

V

DD

FADJ
FOUT

SERIAL

EEPROM

INTERFACE

AGND

PGA

MCS

V

DD

ECS

ECLK

EDI

EDO

LINDACREF

AMP+

AMP-

BDRIVE

ISRC

INM

INP

V

SS

V

DD

12-BIT ADC

V

DD

19-1342; Rev 1; 8/98

PART

MAX1457CWI
MAX1457CCJ 0°C to +70°C

0°C to +70°C

TEMP. RANGE PIN-PACKAGE

28 Wide SO
32 TQFP

EVALUATION KIT

AVAILABLE

_______________Ordering Information

Ordering Information continued at end of data sheet.

Note:

Contact the factory for customized solutions.

*

Dice are tested at TA= +25°C.

Pin Configurations appear at end of data sheet.

MAX1457C/D 0°C to +70°C Dice*

Rail-to-Rail is a registered trademark of Nippon Motorola, Ltd.
SPI is a trademark of Motorola, Inc.
MicroWire is a trademark of National Semiconductor Corp.

Functional Diagram

MAX1457

0.1%-Accurate Signal Conditioner
for Piezoresistive Sensor Compensation

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(VDD= +5V, VSS= 0V, TA= +25°C, unless otherwise noted.)

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....................................(V

SS

- 0.3V) to (VDD+ 0.3V)

Continuous Power Dissipation (T

A

= +70°C)

28-Pin Wide SO (derate 12.50mW/°C above +70°C) ..........1W

32-Pin TQFP (derate 11.1mW/°C above +70°C)...........889mW

Operating Temperature Ranges

MAX1457C_ _ ......................................................0°C to +70°C

MAX1457A_ _ .................................................-40°C to +125°C

Storage Temperature Range.............................-65°C to +150°C

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

Reference voltage = 5.000V

R

BIAS

= 400kΩ, f

CLK

= 100kHz (Note 1)

Gain = 54, DC to 10Hz, sensor impedance =
5kΩ, full-span output = 4V

V

OUT

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

(Note 5)

(Note 4)

5kΩ load to VSSor V

DD

From VSSto V

DD

f

CLK

= 100kHz, to 63% of final value

(Notes 2, 3)

CONDITIONS

µV200DAC Voltage Resolution

VVSS+ 1.3 VDD- 1.3V

ISRC

Current-Source Reference Input
Voltage Range

VVSS+ 1.3 VDD- 1.3V

BR

Bridge Voltage Swing

mA0.1 0.5 2.0I

BR

Bridge Current Range

%FSO0.0025Output Noise

mA

-1.0 1.0

(sink) (source)

Output Current Range

V

VSS+ 0.02 VDD- 0.02

Output Voltage Swing

ppm/°C±50Differential Signal Gain Tempco

V/V54 to 306Differential Signal Gain Range

mA2.0 2.6I

DD

Supply Current

V4.5 5 5.5V

DD

Supply Voltage

mV/V5 to 30

Input-Referred Adjustable
Full-Span Output Range

mV±100

Input-Referred Adjustable
Offset Range

dB90CMRRCommon-Mode Rejection Ratio

ms1Output Step-Response Time

MΩ

1

MΩ

1R

IN

Input Impedance

µV/°C±0.5Input-Referred Offset Tempco

%V

DD

0.01Amplifier Gain Nonlinearity

UNITSMIN TYP MAXSYMBOLPARAMETER

Output filter capacitor = 0.1µF, f

CLK

= 100kHz LSB2Differential Nonlinearity

Bits16DAC Resolution

VSS+ 0.25 VDD- 0.25

TA= T

MIN

to T

MAX

V/V49 54 60Minimum Differential Signal Gain

GENERAL CHARACTERISTICS

ANALOG INPUT (PGA)

ANALOG OUTPUT (PGA)

DIGITAL-TO-ANALOG CONVERTERS

CURRENT SOURCE

No load

MAX1457

0.1%-Accurate Signal Conditioner

for Piezoresistive Sensor Compensation

_______________________________________________________________________________________ 3

Note 1: Circuit of Figure 5 with current source turned off. This value is adjustable through a bias resistor and represents the IC cur-

rent consumption. This excludes the 93C66 EEPROM average current, which is approximately 13µA at a refresh rate of 3Hz
(f

CLK

= 100kHz).

Note 2: Temperature errors for the entire range are compensated together with the sensor errors.
Note 3: The sensor and the MAX1457 must always be at the same temperature during calibration and use.
Note 4: This is the maximum allowable sensor offset at minimum gain (54V/V).
Note 5: This is the sensor’s sensitivity normalized to its drive voltage, assuming a desired full-span output of 4V and a bridge volt-

age of 2.5V. Lower sensitivities can be accommodated by using the auxiliary op amp. Higher sensitivities can be accommo­dated by operating at lower bridge voltages.

ELECTRICAL CHARACTERISTICS (continued)

(VDD= +5V, VSS= 0V, TA= +25°C, unless otherwise noted.)

VBR= 2.5V to 3.5V, f

CLK

= 100kHz

V

OUT

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

(VIN- V

OUT

) at VIN= 2.5V,

R

BIAS

= 400kΩ (no load)

5kΩ load to VSSor V

DD

R

BIAS

= 400kΩ, VIN= 2.5V (no load)

R

BIAS

= 400kΩ, VIN= 2.5V,

V

OUT

= 2.5V ±20mV

f

CLK

= 100kHz

R

BIAS

= 400kΩ

R

BIAS

= 400kΩ (no load)

CONDITIONS

mA

-1.0 1.0

(sink) (source)

Output Current Range

V

VSS+ 0.02 VDD- 0.02

Output Voltage Swing

mV-20 20

Offset Voltage (as unity-gain
follower)

dB60A

V

Open-Loop Gain

VVSS+ 1.3 VDD- 1.2CMR

Input Common-Mode
Voltage Range

LSB2ADC Differential Nonlinearity

mV-20 20V

OFS

Offset Voltage

µA-50 50Current Drive

ms160Conversion Time

Bits12ADC Resolution

VVSS+ 1.3 VDD- 1.3Voltage Swing

UNITSMIN TYP MAXSYMBOLPARAMETER

VSS+ 0.25 VDD- 0.25

ANALOG-TO-DIGITAL CONVERTER

UNCOMMITTED OP AMP

OUTPUTS (LINDAC, FSOTCDAC)

No load

MAX1457

0.1%-Accurate Signal Conditioner
for Piezoresistive Sensor Compensation

4 _______________________________________________________________________________________

______________________________________________________________Pin Description

1 28

Positive Sensor Input. Input impedance >1MΩ. Rail-to-rail input range.

2 29

Negative Sensor Input. Input impedance >1MΩ. Rail-to-rail input range.
3 30 Positive Input of General-Purpose Operational Amplifier
4 31 Negative Input of General-Purpose Operational Amplifier

—

4, 16,

22, 32

Not internally connected.

7 3

PGA Output Voltage. Connect a 0.1µF capacitor from VOUT to VSS. High impedance when

MCS is low.

6 2 Sensor Excitation Current. This pin drives a nominal 0.5mA through the sensor.

5 1 Output of General-Purpose Operational Amplifier. High impedance when MCS is low.

12 9 Reference Input to FSO Linearity DAC. Normally tied to VOUT.

11 8

Buffered FSO Linearity DAC Output. Use a resistor (R

LIN

) greater than 100kΩ, from LINOUT
to ISRC to correct second order FSO nonlinearity errors. Leave unconnected if not
correcting second order FSO nonlinearity errors.

10 7 Buffered Bridge Voltage (the voltage at BDRIVE). Leave unconnected if unused.

9 6

Buffered FSO TC DAC Output. Tie to ISRC with a resistor (R

STC

≥ 50kΩ).

8 5

Current-Source Reference. Connect a 50kΩ resistor from ISRC to VSS.

INP

INM

AMP+

AMP-

N.C.

VOUT

BDRIVE

AMPOUT

LINDACREF

LINOUT

VBBUF

FSOTCOUT

ISRC

13 10 FSO Linearity DAC Output Voltage. Connect 0.1µF capacitor from LINDAC to VSS.LINDAC
14 11 Negative Power Supply InputV

SS

15 12 OFFSET TC DAC Output Voltage. Connect a 0.1µF capacitor from OTCDAC to VSS.OTCDAC
16 13 FSO DAC Output Voltage. Connect a 0.1µF capacitor from FSODAC to VSS.FSODAC
17 14 FSO TC DAC Output Voltage. Connect a 0.1µF capacitor from FSOTCDAC to VSS.FSOTCDAC
18 15 OFFSET DAC Output Voltage. Connect a 0.1µF capacitor from OFSTDAC to VSS.OFSTDAC

19 17

Serial Input (data from EEPROM), active high. CMOS logic-level input pin through which the
MAX1457’s internal registers are updated with EEPROM coefficients. Disabled when MCS is
low.

EDO

20 18

Serial Output (data to EEPROM), active high. CMOS logic-level output pin through which
the MAX1457 gives external commands to the EEPROM. Temperature-compensation data
is available through this pin. Becomes high impedance when MCS is low.

EDI

21 19 CMOS Logic-Level Clock Output for external EEPROM. High impedance when MCS is low.ECLK
22 20

Chip-Select Output for external EEPROM. CMOS logic-level output pin through which the
MAX1457 enables/disables EEPROM operation. High impedance when MCS is low.

ECS
23 21 Frequency Output. Internal oscillator output signal. Normally left open.FOUT
24 23

Frequency Adjust. Connect to VSSwith a 1.5MΩ resistor (R

OSC

) to set internal oscillator fre-

quency to 100kHz. Connect a 0.1µF bypass capacitor from FADJ to V

SS

.

FADJ

PIN

FUNCTIONNAME

25 24

Master Chip Select. The MAX1457 is selected when MCS is high. Leave unconnected for
normal operation (internally pulled up to VDDwith 1MΩ resistor). External 5kΩ pull-up may
be required in noisy environments.

MCS

26 25

Bias Setting Pin. Connect to VDDwith a 400kΩ resistor (R

BIAS

). Connect a 0.1µF bypass

capacitor from NBIAS to V

SS

.

NBIAS

27 26 Mid-Supply Reference for Analog Circuitry. Connect a 0.1µF capacitor from VSSto AGND.AGND
28 27 Positive Power-Supply Input. Connect a 0.1µF capacitor from VDDto VSS.V

DD

TQFPSO

MAX1457

0.1%-Accurate Signal Conditioner

for Piezoresistive Sensor Compensation

_______________________________________________________________________________________ 5

_______________Detailed Description

The MAX1457 provides an analog amplification path for
the sensor signal and a digital path for calibration and
temperature correction. Calibration and correction are
achieved by varying the offset and gain of a program­mable-gain amplifier (PGA) and by varying the sensor
bridge current. The PGA utilizes a switched-capacitor
CMOS technology, with an input-referred offset trim­ming range of ±100mV (20mV/V) and an approximate
3µV (input referred, at minimum gain of 54V/V) resolu­tion (16 bits). The PGA provides eight gain values from
54V/V to 306V/V. The bridge current source is program­mable from 0.1mA to 2mA, with a 15nA step size.

The MAX1457 uses five 16-bit DACs with calibration
coefficients stored in a low-cost external EEPROM. This
memory (an external 4096-bit EEPROM) contains the
following calibration coefficients as 16-bit words:

• FSO (full-span output)

• FSO TC (including nonlinearities)

• Offset

• Offset TC (including nonlinearities)

• Pressure nonlinearity

Figure 1 shows a typical pressure-sensor output and
defines the offset, full-scale, and full-span output values
as a function of voltage.

Offset Correction

Initial offset calibration is accomplished by reading a
16-bit word (coefficient) from the EEPROM and writing it
to the OFFSET DAC. The resulting voltage (OFSTDAC)
is fed into a summing junction at the PGA output for
compensating the sensor offset with a resolution of
±0.2mV (±0.005% FSO).

VOLTAGE

PRESSURE

FULL-SCALE (FS)

FULL-SPAN OUTPUT (FSO)

OFFSET

Figure 1. Typical Pressure-Sensor Output

V

BR

T

V

BR

I

BR

R

STC

V

DD

EDIECS

ECLK

TO/FROM

EXTERNAL EEPROM

EDO

EEPROM

INTERFACE

ADC

BDRIVE

12 16

OUTPUT

PGA

TEMPERATURE-

DEPENDENT VOLTAGE

OFFSET TC

DAC REFERENCE VOLTAGE

FSO TC

DAC

Σ

16

A = 1

Figure 2. Simplified Diagram of Temperature Error Correction

MAX1457

FSO Calibration

Two adjustments are required for FSO calibration. First
set the coarse gain by digitally selecting the PGA gain.
Then calibrate the bridge current by writing a 16-bit
calibration coefficient word to the FSO DAC. These two
adjustments result in a calibration resolution of ±0.2mV
(±0.005% FSO).

Linear Temperature Compensation

Temperature errors are compensated by writing 16-bit
calibration coefficients into the OFFSET TC DAC and
the FSO TC DAC (changing the current-source value
through resistive feedback from the FSOTCDAC pin to
the ISRC pin). The piezoresistive sensor is powered by
a current source resulting in a temperature-dependent
bridge voltage. The reference inputs of the OFFSET TC
DAC and FSO TC DAC are connected to the bridge
voltage. For a fixed digital word, the DAC output volt­ages track the bridge voltage as it varies with tempera­ture (quasi-linearly).

Multislope Temperature Compensation

The MAX1457 utilizes multislope temperature compen­sation, allowing for compensation of arbitrary error
curves restricted only by the available adjustment
range and the shape of the temperature signal.

The MAX1457 offers a maximum of 120 calibration
points (each consisting of one OFFSET TC coefficient
and one FSO TC coefficient) over the operating temper­ature range. Each 16-bit calibration coefficient provides
compensation of the output (either offset or FSO) with
±0.2mV (0.005% FSO) resolution. A 12-bit ADC mea­sures the temperature-dependent bridge voltage
(BDRIVE) and selects (by addressing the EEPROM) the
corresponding offset and FSO calibration data within a
specific narrow temperature span (e.g., ≅ 1°C). The
120-segment compensation enables the MAX1457 to
compensate temperature errors for a broad range of
sensors (Figure 2).

Calculate the correction coefficients by curve-fitting to
sensor-error test data. More test points allow for better
curve-fit accuracy but result in increased test over­head. The remaining error is further affected by the
slope of the temperature errors. For example, correct­ing a 6% nonlinearity over temperature with 60 seg­ments (half of the available calibration points) with
perfect curve fitting yields an error on the order of 0.1%
(6%/60). Figure 3 illustrates this compensation.

0.1%-Accurate Signal Conditioner
for Piezoresistive Sensor Compensation

6 _______________________________________________________________________________________

PRESSURE

PRESSURE

PRESSURE

TEMPERATURE

a) UNCOMPENSATED SENSOR ERROR

SMALL

NONLINEARITY ERROR

TEMPERATURE

b) RESULTANT ERROR AFTER LINEAR COMPENSATION

SMALL

NONLINEARITY ERROR

TEMPERATURE

c) RESULTANT ERROR AFTER MULTISLOPE COMPENSATION

Figure 3. Multislope Temperature Compensation

Pressure Nonlinearity Correction

The MAX1457 corrects pressure nonlinearity in an ana­log fashion by providing a resistive feedback path
(resistor R

LIN

in Figure 4) from a buffered main output
(LINOUT pin) to the current source (ISRC pin). The
feedback coefficient is then set by writing a 16-bit word
to the FSO LIN DAC.

For many silicon sensors, this type of nonlinearity cor­rection may reduce sensor nonlinearity by an order of
magnitude.

_____________Applications Information

Ratiometric Output Configuration

Ratiometric output configuration provides an output that
is proportional to the power-supply voltage. When used
with ratiometric ADCs, this output provides digital pres­sure values independent of supply voltage.

The MAX1457 has been designed to provide a high­performance ratiometric output with a minimum number
of external components (Figure 5). These external com­ponents typically include an external EEPROM (93C66),
decoupling capacitors, and resistors.

2-Wire, 4–20mA Configuration

In this configuration, a 4mA current is used to power a
transducer, and an incremental current of 0 to 16mA
proportional to the measured pressure is transmitted
over the same pair of wires. Current output enables
long-distance transmission without a loss of accuracy
due to cable resistance.

Only a few components (Figure 6) are required to build
a 4–20mA output configuration. A low-quiescent-cur­rent voltage regulator with a built-in bandgap reference
(such as the REF02) should be used. Since the
MAX1457 performs temperature and gain compensa­tion of the circuit, the temperature stability and calibra­tion accuracy of the reference voltage is of secondary
importance.

The external transistor forms the controllable current
loop. The MAX1457 controls the voltage across resistor
R

A

. With RA= 50Ω, a 0.2V to 1.0V range would be
required during the calibration procedure. If needed,
the PGA output can be divided using resistors RBand
RC.

For overvoltage protection, place a Zener diode across
V

IN-

and V

IN+

(Figure 6). A feedthrough capacitor

across the inputs reduces EMI/RFI.

Test System Configuration

The MAX1457 is designed to support an automated
production pressure-temperature test system with inte­grated calibration and temperature compensation.
Figure 7 shows the implementation concept for a low­cost test system capable of testing up to five transduc­er modules connected in parallel. Three-state outputs
on the MAX1457 allow for parallel connection of trans­ducers.

The test system shown in Figure 7 includes a dedicated
test bus consisting of six wires (the capacitive loading
of each transducer module should not exceed the
EEPROM fan-out specifications):

• Two power-supply lines

• One analog output voltage line from the transducers

to a system digital voltmeter

• Three MicroWire/SPI interface lines: EDI (data-in),

EDO (data-out), and ECLK (clock)

For simultaneous testing of more than five transducer
modules, use buffers to prevent overloading the data bus.

A digital multiplexer controls the two chip-select signals
for each transducer:

• Module Select (MCS) places the selected module

into an active state, enabling operation and compen­sation

• EEPROM Select (ECS) enables writing to the trans-

ducer’s EEPROM

MAX1457

0.1%-Accurate Signal Conditioner

for Piezoresistive Sensor Compensation

_______________________________________________________________________________________ 7

V

BR

V

OUT

I

BR

R

LIN

V

DD

PGA

FSO
LIN

DAC

111...1
16 BIT

Figure 4. Pressure Nonlinearity Correction

MAX1457

Sensor Compensation Overview

Compensation requires an examination of the sensor
performance over the operating pressure and tempera­ture range. Use two test pressures (e.g., zero and full­span) and two temperatures. More test pressures and
temperatures will result in greater accuracy. A simple
compensation procedure can be summarized as follows:

Set reference temperature (e.g., +25°C):

1) Initialize each transducer by loading its EEPROM with
default coefficients (e.g., based on mean values of
offset, FSO, and bridge resistance) to prevent gross
overload of the MAX1457.

2) Set the initial bridge voltage (with the FSO DAC) to
half the supply voltage. The bridge voltage can be

measured by the MAX1457 and returned to the test
computer via the serial interface or by using the sys­tem digital voltmeter to measure the voltage on either
BDRIVE or VBBUF.

3) Calibrate the transducer’s output offset and FSO
using the OFFSET and FSO DACs, respectively.

4) Store calibration data in the test computer.

Set next test temperature:

5) Calibrate offset and FSO using the OFFSET TC and
FSO TC DACs, respectively.

6) Store calibration data in the test computer.

Repeat steps 5 and 6 for each required test tempera­ture.

0.1%-Accurate Signal Conditioner
for Piezoresistive Sensor Compensation

8 _______________________________________________________________________________________

MAX1457

CURRENT

SOURCE

BIAS

GENERATOR

OSCILLATOR

16-BIT DAC - OFFSET TC

16-BIT DAC - OFFSET

16-BIT DAC - FSO

16-BIT DAC - FSO TC

16-BIT DAC - FSO LIN

FSOTCDAC
OTCDAC
OFSTDAC
FSODAC

LINDAC

LINOUT

FSOTCOUT

VBBUF

A = 1

A = 1

V

BDRIVE

V

DD

A = 1

OP AMP

AMPOUT

V

SS

VOUT

NBIAS

R

OSC

1.5M

0.1µF

0.1µF

0.1µF

5 x 0.1µF

R

BIAS

400k

V

DD

FADJ
FOUT

SERIAL

EEPROM

INTERFACE

12-BIT ADC

AGND

SENSOR

0.1µF

+5V

+5V

V

DD

V

SS

5k*

+5V

0.1µF

0.1µF

*OPTIONAL PULL-UP RESISTOR

MCS

ECS

ECLK

EDI

EDO

LINDACREF

AMP+

AMP-

CS

CLK

DI

DO

V

DD

EEPROM

93C66 SO-8

ORG
V

SS

BDRIVE

ISRC

R

ISRC

50k

PGA

V

DD

R

STC

R

LIN

(OPTIONAL)

INM

INP

VOUT

Figure 5. Basic Ratiometric Output Configuration

MAX1457

0.1%-Accurate Signal Conditioner

for Piezoresistive Sensor Compensation

_______________________________________________________________________________________ 9

MAX1457

BIAS

GENERATOR

OSCILLATOR

16-BIT DAC - OFFSET TC
16-BIT DAC - OFFSET
16-BIT DAC - FSO

16-BIT DAC - FSO TC

16-BIT DAC - FSO LIN

FSOTCDAC

OTCDAC

OFSTDAC

FSODAC

LINDAC

LINOUT

FSOTCOUT

VBBUF

V

BDRIVE

R

B

R

D

OPTIONAL FEEDTHROUGH

CAPACITOR FOR

EMI/RFI PROTECTION

A = 1

A = 1

A = 1

OP AMP

AMPOUT

V

SS

R

C

R

OFST

VOUT

NBIAS

R

OSC

1.5M

R

A

50Ω

(TYP)

0.1µF

0.1µF

0.1µF

5 x 0.1µF

R

BIAS

400k

V

DD

FADJ

FOUT

SERIAL

EEPROM

INTERFACE

12-BIT ADC

AGND

SENSOR

0.1µF

0.1µF

*OPTIONAL PULL-UP RESISTOR

MCS

ECS

ECLK

EDI

EDO

LINDACREF

AMP+

AMP-

+5V

V

SS

EEPROM

93C66 SO-8

BDRIVE

ISRC

10µF

R

ISRC

50k

R

STC

50Ω

R

LIN

(OPTIONAL)

INM

0.1µF

INP

PGA

V

DD

VIN

VIN+

GND

REF02

VOUT

VIN-

CS

+5V

5k*

CLK

DI

DO

V

DD

ORG

V

SS

Figure 6. Basic 2-Wire 4–20mA Configuration

MAX1457

7) Perform curve-fitting to test data.

8) Based on a curve-fit algorithm, calculate up to 120
sets of offset and FSO correcting values.

9) Download correction coefficients to transducer
EEPROM.

10) Perform a final test.

The resulting transducer temperature errors are limited by
the following factors:

• Number of selected segments for compensation (up to

120).

• Accuracy of the curve fitting, which depends on the

algorithm used, the number of test temperatures, and
the sensor temperature error’s shape.

• Repeatability of the sensor performance. This will limit

the MAX1457’s accuracy.

Sensor Calibration and

Compensation Example

Calibration and compensation requirements for a sen­sor involve conversion of the sensor-specific perfor­mance into a normalized output curve. An example of
the MAX1457’s capabilities is shown in Table 1.

As shown in Table 1, a repeatable piezoresistive sensor
with an initial offset of 16.4mV and FSO of 55.8mV was
converted into a compensated transducer (utilizing the
piezoresistive sensor with the MAX1457) with an offset
of 0.500V and a span of 4.000V. Nonlinear sensor offset
and FSO temperature errors, which were on the order
of 4% to 5% FSO, were reduced to under ±0.1% FSO.
The graphs in Figure 8 show the output of the uncom­pensated sensor and the output of the compensated
transducer.

0.1%-Accurate Signal Conditioner

for Piezoresistive Sensor Compensation

10 ______________________________________________________________________________________

MAX1457

EDO

V

OUT

V

DD

MCS

MODULE 1

EEPROM

EDI

ECLK

ECS

V

SS

V

SS

V

DD

V

DD

V

SS

TEST

OVEN

MAX1457

EDO

V

OUT

MCS

MODULE 2

EEPROM

EDI

ECLK

EDI
EDO

VOUT

DIGITAL

MULTIPLEXER

+5V

ECS1

ECS[1:N], MCS[1:N]

ECS2MCS1

MCS2

ECS N MCS N

ECLK

ECS

MAX1457

EDO

V

OUT

MCS

MODULE N

EEPROM

EDI

ECLK

ECS

• • •

• • •

• • •

• • •

• • •

• • •

• • •

DVM

Figure 7. Automated Test System Concept

______________________________________________________________________________________ 11

MAX1457

0.1%-Accurate Signal Conditioner

for Piezoresistive Sensor Compensation

V

OUT

(mV)

V

OUT

(V)

PRESSURE (kPa)

UNCOMPENSATED RAW SENSOR OUTPUT

0

40

80

120

160

0 4020 60 80 100

PRESSURE (kPa)

0 4020 60 80 100

COMPENSATED TRANSDUCER

0

1

2

3

4

5

ERROR (% FSO)

ERROR (% FSO)

TEMPERATURE (°C)

UNCOMPENSATED SENSOR TEMPERATURE ERROR

-20

-10

10

0

20

30

-50 50

FSO

FSO

0 100 150 -50 500 100 150

TEMPERATURE (°C)

COMPENSATED TRANSDUCER ERROR

-0.15

-0.10

-0.05

0

0.05

0.10

0.15

OFFSET

TA = +25°C
17mV ≤ V

OUT

≤ 73mV

TA = +25°C

0.5V ≤ V

OUT

≤ 4.5V

OFFSET

Figure 8. Comparison of an Uncompensated Sensor (left) and a Compensated Transducer (right)

Typical Uncompensated Input (Sensor)

Offset . . . . . . . . . . . . . . . . . . . . . . . . . .±100% FSO

FSO . . . . . . . . . . . . . . . . . . . . . .20mV/V to 30mV/V

Offset TC . . . . . . . . . . . . . . . . . . . . . . . . .20% FSO

Offset TC Nonlinearity . . . . . . . . . . . . . . . .4% FSO

FSO TC . . . . . . . . . . . . . . . . . . . . . . . . . .-20% FSO

FSO TC Nonlinearity . . . . . . . . . . . . . . . . . .5% FSO

Typical Compensated Transducer Output

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

V

OUT

. . . . . . . . . . . . . . . .ratiometric to VDDat 5.0V

Offset at +25°C . . . . . . . . . . . . . . .0.500V ± 200µV

FSO at +25°C . . . . . . . . . . . . . . . . .4.000V ±200µV

Offset Accuracy Over

Temperature Range . . . . . . . . . .±4mV (0.1% FSO)

FSO Accuracy Over

Temperature Range . . . . . . . . ..±4mV (0.1% FSO)

Table 1. MAX1457 Sensor Calibration and Compensation

MAX1457 Evaluation

___________________ Development Kit

To expedite the development of MAX1457-based trans­ducers and test systems, Maxim has produced a
MAX1457 evaluation kit (EV kit). First-time users of the
MAX1457 are strongly encouraged to use this kit.

The kit is designed to facilitate manual programming of
the MAX1457 with a sensor. It includes the following:

1) Evaluation board (EV board) with a silicon pressure
sensor, ready for customer evaluation.

2) Design/applications manual, which describes in detail
the architecture and functionality of the MAX1457.
This manual was developed for test engineers familiar
with data acquisition of sensor data and provides
sensor-compensation algorithms and test proce­dures.

3) MAX1457 communication software, which enables
programming of the MAX1457 from a computer key­board (IBM compatible), one module at a time.

4) Interface adapter and cable, which allows the con­nection of the EV board to a PC parallel port.

MAX1457

0.1%-Accurate Signal Conditioner

for Piezoresistive Sensor Compensation

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.

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.

TRANSISTOR COUNT: 17534
SUBSTRATE CONNECTED TO V

SS

Chip Information

Ordering Information (continued)

Pin Configurations

PART

MAX1457AWI
MAX1457ACJ -40°C to +125°C

-40°C to +125°C

TEMP. RANGE PIN-PACKAGE

28 Wide SO
32 TQFP

TOP VIEW

TOP VIEW

AMP-

AMP+

INM

293031

MAX1457

SS

OTCDAC

TQFP

INP

28

13

FSODAC

VDDAGND

27

14

FSOTCDAC

26

15

OFSTDAC

25

1611 12

NBIAS

N.C.

24 MCS

FADJ

23
22

N.C.

21

FOUT

20

ECS

19

ECLK

18

EDI

17

EDO

INP

INM
AMP+
AMP-

AMPOUT

BDRIVE

VOUT

ISRC

FSOTCOUT

VBBUF

LINOUT

LINDACREF

LINDAC

V

28
27
26
25
24
23
22
21
20
19
18
17
16
15

V

DD

AGND
NBIAS
MCS
FADJ
FOUT
ECS
ECLK
EDI
EDO
OFSTDAC
FSOTCDAC
FSODAC
OTCDAC

BDRIVE

VOUT

N.C.

ISRC

FSOTCOUT

VBBUF

1
2
3
4
5

MAX1457

6
7
8

9
10
11
12
13
14

SS

N.C.

32

1AMPOUT
2
3

4

5
6

7
8LINOUT

10

9

V

LINDAC

LINDACREF

SO