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19-1342; Rev 1; 8/98
KIT ATION EVALU ILABLE AVA
0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation
________________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 MicroWireTM and SPITM, 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.
VDD ISRC
____________________________Features
o High Accuracy (within 0.1% of sensor's repeatable errors) o Compensates Offset, Offset TC, FSO, FSO TC, Temperature/Pressure Nonlinearity o Rail-to-Rail(R) Analog Output for Calibrated, Temperature-Compensated Pressure Measurements o Programmable Sensor Excitation Current o SPI/MicroWire-Compatible Serial Interface o Fast Signal-Path Settling Time (<1ms) o Accepts Sensor Outputs from 5mV/V to 30mV/V o Pin-Compatible with MCA7707
MAX1457
_______________Ordering Information
PART TEMP. RANGE PIN-PACKAGE MAX1457CWI 0C to +70C 28 Wide SO MAX1457CCJ 0C to +70C 32 TQFP MAX1457C/D 0C to +70C Dice* Ordering Information continued at end of data sheet. Note: Contact the factory for customized solutions. *Dice are tested at TA = +25C.
Pin Configurations appear at end of data sheet.
Functional Diagram
VDD BIAS GENERATOR NBIAS FADJ FOUT VOUT LINDAC FSOTCDAC OTCDAC OFSTDAC FSODAC A=1 LINOUT A=1 FSOTCOUT VBDRIVE A=1 VBBUF
MAX1457
OSCILLATOR BDRIVE INP PGA INM VDD
AGND 16-BIT DAC - FSO 16-BIT DAC - OFFSET 16-BIT DAC - OFFSET TC 16-BIT DAC - FSO TC 16-BIT DAC - FSO LINEARITY VDD VSS 12-BIT ADC
_______________________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.
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.
MCS ECS ECLK EDI EDO LINDACREF AMP+ AMPSERIAL EEPROM INTERFACE
AMPOUT VSS
________________________________________________________________ Maxim Integrated Products
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For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 408-737-7600 ext. 3468.
0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation MAX1457
ABSOLUTE MAXIMUM RATINGS
Supply Voltage, VDD to VSS......................................-0.3V to +6V All other pins ....................................(VSS - 0.3V) to (VDD + 0.3V) Continuous Power Dissipation (TA = +70C) 28-Pin Wide SO (derate 12.50mW/C above +70C) ..........1W 32-Pin TQFP (derate 11.1mW/C above +70C)...........889mW Operating Temperature Ranges MAX1457C_ _ ......................................................0C to +70C MAX1457A_ _ .................................................-40C to +125C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10sec) .............................+300C
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.
ELECTRICAL CHARACTERISTICS
(VDD = +5V, VSS = 0V, TA = +25C, unless otherwise noted.) PARAMETER GENERAL CHARACTERISTICS Supply Voltage Supply Current ANALOG INPUT (PGA) Input Impedance Input-Referred Offset Tempco Amplifier Gain Nonlinearity Output Step-Response Time Common-Mode Rejection Ratio Input-Referred Adjustable Offset Range Input-Referred Adjustable Full-Span Output Range ANALOG OUTPUT (PGA) Differential Signal Gain Range Minimum Differential Signal Gain Differential Signal Gain Tempco Output Voltage Swing Output Current Range Output Noise CURRENT SOURCE Bridge Current Range Bridge Voltage Swing Current-Source Reference Input Voltage Range DAC Voltage Resolution Differential Nonlinearity DAC Resolution 2 _______________________________________________________________________________________ IBR VBR VISRC 0.1 VSS + 1.3 VSS + 1.3 0.5 2.0 VDD - 1.3 VDD - 1.3 mA V V 5k load to VSS or VDD No load VOUT = (VSS + 0.25V) to (VDD - 0.25V) Gain = 54, DC to 10Hz, sensor impedance = 5k, full-span output = 4V VSS + 0.25 VSS + 0.02 -1.0 (sink) 0.0025 TA = TMIN to TMAX 49 54 to 306 54 50 VDD - 0.25 VDD - 0.02 1.0 (source) 60 V/V V/V ppm/C V mA %FSO CMRR fCLK = 100kHz, to 63% of final value From VSS to VDD (Note 4) (Note 5) RIN (Notes 2, 3) VDD IDD RBIAS = 400k, fCLK = 100kHz (Note 1) 4.5 5 2.0 1 1 0.5 0.01 1 90 100 5 to 30 5.5 2.6 V mA M M V/C %VDD ms dB mV mV/V SYMBOL CONDITIONS MIN TYP MAX UNITS
DIGITAL-TO-ANALOG CONVERTERS Reference voltage = 5.000V Output filter capacitor = 0.1F, fCLK = 100kHz 200 2 16 V LSB Bits
0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation
ELECTRICAL CHARACTERISTICS (continued)
(VDD = +5V, VSS = 0V, TA = +25C, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
MAX1457
ANALOG-TO-DIGITAL CONVERTER ADC Differential Nonlinearity Conversion Time ADC Resolution OUTPUTS (LINDAC, FSOTCDAC) Voltage Swing Current Drive Offset Voltage UNCOMMITTED OP AMP Input Common-Mode Voltage Range Open-Loop Gain Offset Voltage (as unity-gain follower) Output Voltage Swing Output Current Range CMR AV RBIAS = 400k RBIAS = 400k, VIN = 2.5V (no load) 5k load to VSS or VDD No load VOUT = (VSS + 0.25V) to (VDD - 0.25V) -20 VSS + 0.25 VSS + 0.02 -1.0 (sink) VSS + 1.3 60 20 VDD - 0.25 VDD - 0.02 1.0 (source) VDD - 1.2 V dB mV V mA VOFS RBIAS = 400k (no load) RBIAS = 400k, VIN = 2.5V, VOUT = 2.5V 20mV (VIN - VOUT) at VIN = 2.5V, RBIAS = 400k (no load) VSS + 1.3 -50 -20 VDD - 1.3 50 20 V A mV VBR = 2.5V to 3.5V, fCLK = 100kHz fCLK = 100kHz 2 160 12 LSB ms Bits
Note 1: Circuit of Figure 5 with current source turned off. This value is adjustable through a bias resistor and represents the IC current consumption. This excludes the 93C66 EEPROM average current, which is approximately 13A at a refresh rate of 3Hz (fCLK = 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 voltage 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.
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0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation MAX1457
______________________________________________________________Pin Description
PIN SO 1 2 3 4 5 6 7 -- 8 9 10 11 12 13 14 15 16 17 18 19 TQFP 28 29 30 31 1 2 3 4, 16, 22, 32 5 6 7 8 9 10 11 12 13 14 15 17 NAME INP INM AMP+ AMPAMPOUT BDRIVE VOUT N.C. ISRC FSOTCOUT VBBUF LINOUT LINDACREF LINDAC VSS OTCDAC FSODAC FSOTCDAC OFSTDAC EDO FUNCTION Positive Sensor Input. Input impedance >1M. Rail-to-rail input range. Negative Sensor Input. Input impedance >1M. Rail-to-rail input range. Positive Input of General-Purpose Operational Amplifier Negative Input of General-Purpose Operational Amplifier Output of General-Purpose Operational Amplifier. High impedance when MCS is low. Sensor Excitation Current. This pin drives a nominal 0.5mA through the sensor. PGA Output Voltage. Connect a 0.1F capacitor from VOUT to VSS. High impedance when MCS is low. Not internally connected. Current-Source Reference. Connect a 50k resistor from ISRC to VSS. Buffered FSO TC DAC Output. Tie to ISRC with a resistor (RSTC 50k). Buffered Bridge Voltage (the voltage at BDRIVE). Leave unconnected if unused. Buffered FSO Linearity DAC Output. Use a resistor (RLIN) greater than 100k, from LINOUT to ISRC to correct second order FSO nonlinearity errors. Leave unconnected if not correcting second order FSO nonlinearity errors. Reference Input to FSO Linearity DAC. Normally tied to VOUT. FSO Linearity DAC Output Voltage. Connect 0.1F capacitor from LINDAC to VSS. Negative Power Supply Input OFFSET TC DAC Output Voltage. Connect a 0.1F capacitor from OTCDAC to VSS. FSO DAC Output Voltage. Connect a 0.1F capacitor from FSODAC to VSS. FSO TC DAC Output Voltage. Connect a 0.1F capacitor from FSOTCDAC to VSS. OFFSET DAC Output Voltage. Connect a 0.1F capacitor from OFSTDAC to VSS. 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. 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. CMOS Logic-Level Clock Output for external EEPROM. High impedance when MCS is low. 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. Frequency Output. Internal oscillator output signal. Normally left open. Frequency Adjust. Connect to VSS with a 1.5M resistor (ROSC) to set internal oscillator frequency to 100kHz. Connect a 0.1F bypass capacitor from FADJ to VSS. Master Chip Select. The MAX1457 is selected when MCS is high. Leave unconnected for normal operation (internally pulled up to VDD with 1M resistor). External 5k pull-up may be required in noisy environments. Bias Setting Pin. Connect to VDD with a 400k resistor (RBIAS). Connect a 0.1F bypass capacitor from NBIAS to VSS. Mid-Supply Reference for Analog Circuitry. Connect a 0.1F capacitor from VSS to AGND. Positive Power-Supply Input. Connect a 0.1F capacitor from VDD to VSS.
20 21 22 23 24
18 19 20 21 23
EDI ECLK ECS FOUT FADJ
25
24
MCS
26 27 28
25 26 27
NBIAS AGND VDD
4
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0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation MAX1457
_______________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 3V (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.
PRESSURE OFFSET
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
FULL-SPAN OUTPUT (FSO)
FULL-SCALE (FS)
Figure 1. Typical Pressure-Sensor Output
TO/FROM EXTERNAL EEPROM VDD RSTC ECS TEMPERATUREDEPENDENT VOLTAGE ECLK EDO EDI
DAC REFERENCE VOLTAGE
VBR EEPROM INTERFACE IBR BDRIVE VBR PGA T ADC 12
16
FSO TC DAC
16 OFFSET TC
OUTPUT
A=1
Figure 2. Simplified Diagram of Temperature Error Correction
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0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation 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).
PRESSURE
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).
SMALL NONLINEARITY ERROR
TEMPERATURE
a) UNCOMPENSATED SENSOR ERROR
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., 1C). 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.
PRESSURE
SMALL NONLINEARITY ERROR TEMPERATURE
b) RESULTANT ERROR AFTER LINEAR COMPENSATION
PRESSURE
TEMPERATURE
c) RESULTANT ERROR AFTER MULTISLOPE COMPENSATION
Figure 3. Multislope Temperature Compensation
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0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation
Pressure Nonlinearity Correction
The MAX1457 corrects pressure nonlinearity in an analog fashion by providing a resistive feedback path (resistor RLIN 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. 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 RA. With RA = 50, a 0.2V to 1.0V range would be required during the calibration procedure. If needed, the PGA output can be divided using resistors RB and 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.
MAX1457
_____________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.
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 transducer's EEPROM
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.
VDD
RLIN
FSO LIN DAC 111...1 16 BIT
IBR
VBR PGA VOUT
Figure 4. Pressure Nonlinearity Correction
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0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation MAX1457
+5V RSTC RBIAS 400k NBIAS 0.1F FADJ BDRIVE INP 0.1F INM VDD AGND SENSOR 0.1F VSS +5V +5V EEPROM 93C66 SO-8 VDD ORG VSS CS CLK DI DO 5k* MCS ECS ECLK EDI EDO LINDACREF AMP+ 0.1F AMPSERIAL EEPROM INTERFACE 16-BIT DAC - FSO 16-BIT DAC - OFFSET 16-BIT DAC - OFFSET TC 16-BIT DAC - FSO TC 16-BIT DAC - FSO LIN 12-BIT ADC A=1 PGA VOUT LINDAC FSOTCDAC OTCDAC OFSTDAC FSODAC LINOUT OSCILLATOR FOUT ROSC 1.5M VOUT
RLIN (OPTIONAL) RISRC 50k CURRENT SOURCE VDD BIAS GENERATOR
VDD
0.1F
ISRC
0.1F
5 x 0.1F
A=1 VBDRIVE A=1
FSOTCOUT
VBBUF
VDD OP AMP AMPOUT
MAX1457
VSS *OPTIONAL PULL-UP RESISTOR
Figure 5. Basic Ratiometric Output Configuration
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., +25C): 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
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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.
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50 REF02 RSTC 10F VOUT GND RBIAS 400k 0.1F VIN VIN+
RLIN (OPTIONAL) VDD BIAS GENERATOR 0.1F FADJ BDRIVE OSCILLATOR FOUT VOUT ROSC 1.5M PGA +5V AGND 0.1F VSS +5V 12-BIT ADC LINOUT 5k* MCS FSOTCOUT 5 x 0.1F VBBUF RD VBDRIVE A=1 ECS ECLK EDI EDO SERIAL EEPROM INTERFACE RB AMPOUT A=1 A=1 LINDAC FSOTCDAC OTCDAC OFSTDAC FSODAC INP INM 0.1F NBIAS RISRC 50k VDD ISRC
ORG VSS LINDACREF AMP+ OP AMP AMP-
CS CLK DI DO
0.1F
MAX1457
VSS
16-BIT DAC - FSO 16-BIT DAC - OFFSET 16-BIT DAC - OFFSET TC 16-BIT DAC - FSO TC 16-BIT DAC - FSO LIN
Figure 6. Basic 2-Wire 4-20mA Configuration
OPTIONAL FEEDTHROUGH CAPACITOR FOR EMI/RFI PROTECTION ROFST RC RA 50 (TYP) VIN-
0.1F
SENSOR
EEPROM 93C66 SO-8
VDD
MAX1457
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*OPTIONAL PULL-UP RESISTOR
0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation
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0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation MAX1457
ECS[1:N], MCS[1:N] ECS1 MCS1 ECS2 *** MCS2 ECS N MCS N
DIGITAL MULTIPLEXER
MODULE 1
MCS
MODULE 2
MCS
MODULE N
MCS
MAX1457
MAX1457
ECS EEPROM ECLK EDI EDO +5V VDD
ECS EEPROM ECLK EDI EDO VDD
ECS EEPROM ECLK EDI EDO VDD *** ***
VOUT VSS
VOUT VSS
VOUT VSS
DVM
VOUT ECLK EDI EDO
*** *** *** ***
TEST OVEN
Figure 7. Automated Test System Concept
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.
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MAX1457
0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation MAX1457
Table 1. MAX1457 Sensor Calibration and Compensation
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 . . . . . . . . . . .-40C to +125C VOUT . . . . . . . . . . . . . . . .ratiometric to VDD at 5.0V Offset at +25C . . . . . . . . . . . . . . .0.500V 200V FSO at +25C . . . . . . . . . . . . . . . . .4.000V 200V Offset Accuracy Over Temperature Range . . . . . . . . . .4mV (0.1% FSO) FSO Accuracy Over Temperature Range . . . . . . . . ..4mV (0.1% FSO)
UNCOMPENSATED RAW SENSOR OUTPUT
160 TA = +25C 17mV VOUT 73mV 120 VOUT (mV) 4 5
COMPENSATED TRANSDUCER
TA = +25C 0.5V VOUT 4.5V
80
VOUT (V) 0 20 40 60 PRESSURE (kPa) 80 100
3
2 1
40
0
0 0 20 40 60 PRESSURE (kPa) 80 100
UNCOMPENSATED SENSOR TEMPERATURE ERROR
30 0.15 0.10 OFFSET ERROR (% FSO) 10 ERROR (% FSO) 0.05
COMPENSATED TRANSDUCER ERROR
20
OFFSET 0 -0.05 FSO -0.10 -0.15
0
FSO
-10
-20 -50 0 50 100 TEMPERATURE (C) 150
-50
0
50 100 TEMPERATURE (C)
150
Figure 8. Comparison of an Uncompensated Sensor (left) and a Compensated Transducer (right)
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0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation MAX1457
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.
Chip Information
TRANSISTOR COUNT: 17534 SUBSTRATE CONNECTED TO VSS
Ordering Information (continued)
PART MAX1457AWI MAX1457ACJ TEMP. RANGE -40C to +125C -40C to +125C PIN-PACKAGE 28 Wide SO 32 TQFP
Pin Configurations
TOP VIEW
INP 1 INM 2 AMP+ 3 AMP- 4 AMPOUT 5 BDRIVE 6 VOUT 7 ISRC 8 FSOTCOUT 9 VBBUF 10 LINOUT 11 LINDACREF 12 LINDAC 13 VSS 14 28 VDD 27 AGND 26 NBIAS 25 MCS 24 FADJ AMPOUT BDRIVE VOUT N.C. ISRC FSOTCOUT VBBUF 19 EDO 18 OFSTDAC 17 FSOTCDAC 16 FSODAC 15 OTCDAC LINOUT 8 9 LINDACREF 10 LINDAC 11 VSS 12 OTCDAC 13 FSODAC 14 FSOTCDAC 15 OFSTDAC 16 N.C. 17 EDO 1 2 3 4 5 6 7
TOP VIEW
NBIAS 25 24 MCS 23 FADJ 22 N.C. 21 FOUT 20 ECS 19 ECLK 18 EDI AGND 26 AMP+ AMPN.C. INM VDD 27 INP 28
32
31
30
29
MAX1457
23 FOUT 22 ECS 21 ECLK 20 EDI
MAX1457
SO
TQFP
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 (c) 1998 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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