19-1342; Rev 1; 8/98 KIT ATION EVALU E L B A IL AVA 0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation ____________________________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 • Calibration and Compensation: Computation and storage (in an external EEPROM) of calibration and compensation coefficients determined from transducer pretest data. PART TEMP. RANGE Pin Configurations appear at end of data sheet. Functional Diagram • 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. 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. PIN-PACKAGE MAX1457CWI 0°C to +70°C 28 Wide SO MAX1457CCJ 0°C to +70°C 32 TQFP MAX1457C/D 0°C to +70°C Dice* Ordering Information continued at end of data sheet. Note: Contact the factory for customized solutions. *Dice are tested at TA = +25°C. VDD VDD ISRC BIAS GENERATOR NBIAS OSCILLATOR FADJ FOUT MAX1457 BDRIVE INP PGA INM VOUT LINDAC FSOTCDAC OTCDAC OFSTDAC FSODAC VDD AGND VSS 12-BIT ADC MCS ECS ECLK EDI EDO SERIAL EEPROM INTERFACE 16-BIT DAC - FSO 16-BIT DAC - OFFSET 16-BIT DAC - OFFSET TC 16-BIT DAC - FSO TC 16-BIT DAC - FSO LINEARITY 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. _______________Ordering Information VDD LINDACREF AMP+ A=1 LINOUT A=1 FSOTCOUT VBDRIVE A=1 VBBUF AMPOUT AMPVSS ________________________________________________________________ Maxim Integrated Products 1 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. MAX1457 ________________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). MAX1457 0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation 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 = +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 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 = +25°C, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS GENERAL CHARACTERISTICS Supply Voltage VDD Supply Current IDD 4.5 RBIAS = 400kΩ, fCLK = 100kHz (Note 1) ANALOG INPUT (PGA) Input Impedance RIN Input-Referred Offset Tempco (Notes 2, 3) Common-Mode Rejection Ratio CMRR 5.5 V 2.6 mA 1 MΩ 1 MΩ ±0.5 µV/°C 0.01 %VDD fCLK = 100kHz, to 63% of final value 1 ms From VSS to VDD 90 dB Amplifier Gain Nonlinearity Output Step-Response Time 5 2.0 Input-Referred Adjustable Offset Range (Note 4) ±100 mV Input-Referred Adjustable Full-Span Output Range (Note 5) 5 to 30 mV/V 54 to 306 V/V ANALOG OUTPUT (PGA) Differential Signal Gain Range Minimum Differential Signal Gain TA = TMIN to TMAX 49 54 Differential Signal Gain Tempco 60 ±50 Output Voltage Swing 5kΩ load to VSS or VDD VSS + 0.25 VDD - 0.25 No load VSS + 0.02 VDD - 0.02 Output Current Range VOUT = (VSS + 0.25V) to (VDD - 0.25V) Output Noise Gain = 54, DC to 10Hz, sensor impedance = 5kΩ, full-span output = 4V V/V ppm/°C -1.0 (sink) 1.0 (source) 0.0025 V mA %FSO CURRENT SOURCE Bridge Current Range IBR Bridge Voltage Swing VBR VSS + 1.3 VDD - 1.3 V VISRC VSS + 1.3 VDD - 1.3 V Current-Source Reference Input Voltage Range 0.1 0.5 2.0 mA DIGITAL-TO-ANALOG CONVERTERS DAC Voltage Resolution Reference voltage = 5.000V Differential Nonlinearity Output filter capacitor = 0.1µF, fCLK = 100kHz 200 µV 2 LSB DAC Resolution 2 _______________________________________________________________________________________ 16 Bits 0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation (VDD = +5V, VSS = 0V, TA = +25°C, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS ANALOG-TO-DIGITAL CONVERTER ADC Differential Nonlinearity VBR = 2.5V to 3.5V, fCLK = 100kHz Conversion Time fCLK = 100kHz 2 LSB 160 ms ADC Resolution 12 Bits OUTPUTS (LINDAC, FSOTCDAC) Voltage Swing RBIAS = 400kΩ (no load) Current Drive RBIAS = 400kΩ, VIN = 2.5V, VOUT = 2.5V ±20mV -50 50 µA (VIN - VOUT) at VIN = 2.5V, RBIAS = 400kΩ (no load) -20 20 mV Offset Voltage VOFS VSS + 1.3 VDD - 1.3 V UNCOMMITTED OP AMP Input Common-Mode Voltage Range Open-Loop Gain CMR AV Offset Voltage (as unity-gain follower) Output Voltage Swing Output Current Range VSS + 1.3 RBIAS = 400kΩ RBIAS = 400kΩ, VIN = 2.5V (no load) VDD - 1.2 60 -20 dB 20 5kΩ load to VSS or VDD VSS + 0.25 VDD - 0.25 No load VSS + 0.02 VDD - 0.02 VOUT = (VSS + 0.25V) to (VDD - 0.25V) -1.0 (sink) V 1.0 (source) mV V mA 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 13µA 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. _______________________________________________________________________________________ 3 MAX1457 ELECTRICAL CHARACTERISTICS (continued) MAX1457 0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation ______________________________________________________________Pin Description PIN 4 NAME FUNCTION SO TQFP 1 28 INP Positive Sensor Input. Input impedance >1MΩ. Rail-to-rail input range. 2 29 INM Negative Sensor Input. Input impedance >1MΩ. Rail-to-rail input range. 3 30 AMP+ Positive Input of General-Purpose Operational Amplifier 4 31 AMP- Negative Input of General-Purpose Operational Amplifier 5 1 AMPOUT 6 2 BDRIVE 7 3 VOUT — 4, 16, 22, 32 N.C. Not internally connected. 8 5 ISRC Current-Source Reference. Connect a 50kΩ resistor from ISRC to VSS. 9 6 FSOTCOUT 10 7 VBBUF Buffered Bridge Voltage (the voltage at BDRIVE). Leave unconnected if unused. 11 8 LINOUT Buffered FSO Linearity DAC Output. Use a resistor (RLIN) greater than 100kΩ, from LINOUT to ISRC to correct second order FSO nonlinearity errors. Leave unconnected if not correcting second order FSO nonlinearity errors. 12 9 LINDACREF 13 10 LINDAC 14 11 VSS 15 12 OTCDAC OFFSET TC DAC Output Voltage. Connect a 0.1µF capacitor from OTCDAC to VSS. 16 13 FSODAC FSO DAC Output Voltage. Connect a 0.1µF capacitor from FSODAC to VSS. 17 14 FSOTCDAC FSO TC DAC Output Voltage. Connect a 0.1µF capacitor from FSOTCDAC to VSS. 18 15 OFSTDAC OFFSET DAC Output Voltage. Connect a 0.1µF capacitor from OFSTDAC to VSS. 19 17 EDO 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. 20 18 EDI 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. 21 19 ECLK CMOS Logic-Level Clock Output for external EEPROM. High impedance when MCS is low. 22 20 ECS 23 21 FOUT Frequency Output. Internal oscillator output signal. Normally left open. 24 23 FADJ Frequency Adjust. Connect to VSS with a 1.5MΩ resistor (ROSC) to set internal oscillator frequency to 100kHz. Connect a 0.1µF bypass capacitor from FADJ to VSS. 25 24 MCS Master Chip Select. The MAX1457 is selected when MCS is high. Leave unconnected for normal operation (internally pulled up to VDD with 1MΩ resistor). External 5kΩ pull-up may be required in noisy environments. 26 25 NBIAS Bias Setting Pin. Connect to VDD with a 400kΩ resistor (RBIAS). Connect a 0.1µF bypass capacitor from NBIAS to VSS. 27 26 AGND Mid-Supply Reference for Analog Circuitry. Connect a 0.1µF capacitor from VSS to AGND. 28 27 VDD 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 0.1µF capacitor from VOUT to VSS. High impedance when MCS is low. Buffered FSO TC DAC Output. Tie to ISRC with a resistor (RSTC ≥ 50kΩ). Reference Input to FSO Linearity DAC. Normally tied to VOUT. FSO Linearity DAC Output Voltage. Connect 0.1µF capacitor from LINDAC to VSS. Negative Power Supply Input 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. Positive Power-Supply Input. Connect a 0.1µF capacitor from VDD to VSS. _______________________________________________________________________________________ 0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation 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). 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. FULL-SPAN OUTPUT (FSO) VOLTAGE 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) FULL-SCALE (FS) • FSO TC (including nonlinearities) • Offset OFFSET • Offset TC (including nonlinearities) PRESSURE • 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. Figure 1. Typical Pressure-Sensor Output TO/FROM EXTERNAL EEPROM VDD ECS RSTC ECLK EDO EDI TEMPERATUREDEPENDENT VOLTAGE DAC REFERENCE VOLTAGE 16 VBR FSO TC DAC EEPROM INTERFACE IBR T 12 16 ADC OFFSET TC BDRIVE VBR OUTPUT PGA Σ A=1 Figure 2. Simplified Diagram of Temperature Error Correction _______________________________________________________________________________________ 5 MAX1457 _______________Detailed Description MAX1457 0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation 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., ≅ 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. 6 PRESSURE SMALL NONLINEARITY ERROR TEMPERATURE b) RESULTANT ERROR AFTER LINEAR COMPENSATION PRESSURE TEMPERATURE c) RESULTANT ERROR AFTER MULTISLOPE COMPENSATION Figure 3. Multislope Temperature Compensation _______________________________________________________________________________________ 0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation _____________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. VDD IBR RLIN 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 = 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 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. 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 FSO LIN DAC • Three MicroWire/SPI interface lines: EDI (data-in), EDO (data-out), and ECLK (clock) 111...1 16 BIT VBR PGA VOUT 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 Figure 4. Pressure Nonlinearity Correction _______________________________________________________________________________________ 7 MAX1457 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. MAX1457 0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation +5V RSTC RLIN (OPTIONAL) RISRC 50k CURRENT SOURCE VDD ISRC RBIAS 400k VDD BIAS GENERATOR NBIAS OSCILLATOR FOUT 0.1µF FADJ BDRIVE INP 0.1µF VOUT PGA INM VDD AGND VSS +5V VDD ORG VSS CS CLK DI DO 16-BIT DAC - FSO 16-BIT DAC - OFFSET 16-BIT DAC - OFFSET TC 16-BIT DAC - FSO TC 16-BIT DAC - FSO LIN EEPROM 93C66 SO-8 A=1 12-BIT ADC +5V 5k* MCS ECS ECLK EDI EDO SERIAL EEPROM INTERFACE VOUT LINOUT A=1 FSOTCOUT A=1 VBBUF 5 x 0.1µF VBDRIVE VDD LINDACREF AMP+ 0.1µF 0.1µF ROSC 1.5M LINDAC FSOTCDAC OTCDAC OFSTDAC FSODAC 0.1µF SENSOR 0.1µF OP AMP AMPOUT AMP- 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., +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 8 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. _______________________________________________________________________________________ 0.1µF ORG VSS VDD CS CLK DI DO EEPROM 93C66 SO-8 SENSOR RSTC ISRC AMP- LINDACREF AMP+ ECS ECLK EDI EDO MCS 5k* AGND INM INP SERIAL EEPROM INTERFACE 12-BIT ADC VSS +5V VDD RLIN (OPTIONAL) BDRIVE +5V 0.1µF *OPTIONAL PULL-UP RESISTOR 0.1µF RISRC 50k 10µF PGA MAX1457 VDD A=1 VSS OP AMP VBDRIVE A=1 A=1 OSCILLATOR BIAS GENERATOR 16-BIT DAC - FSO 16-BIT DAC - OFFSET 16-BIT DAC - OFFSET TC 16-BIT DAC - FSO TC 16-BIT DAC - FSO LIN AMPOUT VBBUF FSOTCOUT LINOUT LINDAC FSOTCDAC OTCDAC OFSTDAC FSODAC VOUT FOUT FADJ NBIAS RB ROFST RC 5 x 0.1µF ROSC 1.5M 0.1µF RBIAS 400k RA 50Ω (TYP) RD GND 0.1µF VOUT VIN REF02 0.1µF VIN- OPTIONAL FEEDTHROUGH CAPACITOR FOR EMI/RFI PROTECTION VIN+ MAX1457 50Ω 0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation Figure 6. Basic 2-Wire 4–20mA Configuration _______________________________________________________________________________________ 9 ECS[1:N], MCS[1:N] MCS1 ••• MCS2 ECS2 MODULE 1 MODULE 2 EDO +5V VDD ECS EEPROM EEPROM ECLK EDI MODULE N VDD MCS ECS ECLK EDI EDO VOUT VSS MCS N MCS MAX1457 MCS ECS ECS N ECLK EDI EDO VOUT VSS MAX1457 ECS1 EEPROM DIGITAL MULTIPLEXER MAX1457 MAX1457 0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation VDD 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. 10 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 MAX1457 Table 1. MAX1457 Sensor Calibration and Compensation Typical Compensated Transducer Output Temperature Range . . . . . . . . . . .-40°C to +125°C VOUT . . . . . . . . . . . . . . . .ratiometric to VDD at 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) 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 UNCOMPENSATED RAW SENSOR OUTPUT COMPENSATED TRANSDUCER 5 160 TA = +25°C 17mV ≤ VOUT ≤ 73mV TA = +25°C 0.5V ≤ VOUT ≤ 4.5V 4 VOUT (V) VOUT (mV) 120 80 40 3 2 1 0 0 0 20 40 60 PRESSURE (kPa) 80 100 0 UNCOMPENSATED SENSOR TEMPERATURE ERROR 40 60 PRESSURE (kPa) 80 100 COMPENSATED TRANSDUCER ERROR 30 0.15 0.10 20 ERROR (% FSO) OFFSET ERROR (% FSO) 20 10 0 FSO 0.05 OFFSET 0 -0.05 FSO -10 -0.10 -20 -0.15 -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) ______________________________________________________________________________________ 11 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. Chip Information TRANSISTOR COUNT: 17534 SUBSTRATE CONNECTED TO VSS Ordering Information (continued) PART TEMP. RANGE MAX1457AWI MAX1457ACJ PIN-PACKAGE -40°C to +125°C -40°C to +125°C 28 Wide SO 32 TQFP 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. Pin Configurations TOP VIEW INP 1 28 VDD N.C. AMP- AMP+ INM INP VDD AGND NBIAS TOP VIEW INM 2 27 AGND 32 31 30 29 28 27 26 25 AMP+ 3 26 NBIAS AMPOUT 1 24 MCS AMP- 4 25 MCS BDRIVE 2 23 FADJ AMPOUT 5 24 FADJ VOUT 3 22 N.C. N.C. 4 21 FOUT ISRC 5 FSOTCOUT 6 19 ECLK VBBUF 7 18 EDI LINOUT 8 17 EDO MAX1457 23 FOUT VOUT 7 22 ECS ISRC 8 21 ECLK FSOTCOUT 9 MAX1457 20 ECS 17 FSOTCDAC LINDAC 13 16 FSODAC VSS 14 15 OTCDAC 9 10 11 12 13 14 15 16 N.C. LINDACREF 12 OFSTDAC 18 OFSTDAC FSODAC LINOUT 11 FSOTCDAC 19 EDO OTCDAC VBBUF 10 VSS 20 EDI LINDAC BDRIVE 6 LINDACREF MAX1457 0.1%-Accurate Signal Conditioner for Piezoresistive Sensor Compensation TQFP SO 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.