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19-1365; Rev 0; 5/98 MAX1450 Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors 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 Pin Configuration TOP VIEW INP 1 I.C. 2 I.C. 3 SOTC 4 SOFF 5 A1 6 A0 7 OFFTC 8 OFFSET 9 BBUF 10 MAX1450 20 INM 19 VSS 18 BDRIVE 17 ISRC 16 I.C. 15 VDD 14 OUT 13 A2 12 I.C. 11 FSOTRIM Features o 1% Sensor Signal Conditioning o Corrects Sensor Errors Using Coefficients Stored in External Trimmable Resistors, Potentiometers, or DACs o Compensates Offset, Offset TC, FSO, FSO TC, and FSO Linearity o Rail-to-Rail® Analog Output o Programmable Current Source for Sensor Excitation o Fast Signal-Path Settling Time (< 1ms) o Accepts Sensor Outputs from 10mV/V to 30mV/V o Fully Analog Signal Path Ordering Information PART TEMP. RANGE PIN-PACKAGE MAX1450CAP 0°C to +70°C 20 SSOP MAX1450C/D 0°C to +70°C Dice* MAX1450EAP -40°C to +85°C 20 SSOP * Dice are tested at TA = +25°C, DC parameters only. Rail-to-Rail is a registered trademark of Nippon Motorola, Ltd. Functional Diagram FSOTRIM VDD CURRENT SOURCE ISRC VDD BDRIVE INP INM + PGA - A=1 MAX1450 A2 A1 A0 OUT SOTC SOFF OFFTC OFFSET BBUF SSOP VSS ________________________________________________________________ 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. Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors MAX1450 ABSOLUTE MAXIMUM RATINGS Supply Voltage, VDD to 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 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 = 0, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS GENERAL CHARACTERISTICS Supply Voltage Supply Current ANALOG INPUT (PGA) VDD IDD TA = +25°C (Note 1) Input Impedance Input-Referred Offset Temperature Coefficient RIN (Notes 2, 3) Amplifier Gain Nonlinearity Output Step-Response Time 63% of final value Common-Mode Rejection Ratio Input-Referred Adjustable Offset Range CMRR From VSS to VDD (Note 4) Input-Referred Adjustable Full-Span Output Range (Note 5) SUMMING JUNCTION (Figure 1) MIN TYP MAX UNITS 4.5 5.0 5.5 V 2.8 3.5 mA 1.0 ±0.5 0.01 1 90 ±100 10 to 30 MΩ µV/°C %VDD ms dB mV mV/V Offset Gain ∆VOUT ∆VOFFSET 1.15 V/V Offset TC Gain ANALOG OUTPUT (PGA) Differential Signal Range Gain Minimum Differential Signal Gain Differential Signal Path Temperature Coefficient Output Voltage Swing Output Current Range Output Noise ∆VOUT ∆VOFFTC Eight selectable gains (Table 3) At any gain 5kΩ load to VSS or VDD, TA = +25°C No load, TA = TMIN to TMAX VOUT = (VSS + 0.25V) to (VDD - 0.25V), TA = +25°C DC to 10Hz, gain = 39, sensor impedance = 5kΩ 1.15 V/V 39 to 221 V/V 36 39 44 V/V VSS + 0.25 VSS + 0.05 -1.0 (sink) ±50 ppm/°C VDD - 0.25 V VDD - 0.05 1.0 (source) mA 500 µVRMS 2 _______________________________________________________________________________________ MAX1450 Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors ELECTRICAL CHARACTERISTICS (continued) (VDD = +5V, VSS = 0, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS CURRENT SOURCE Bridge Current Range IBDRIVE 0.1 0.5 2.0 mA Bridge Voltage Swing VBDRIVE VSS + 1.3 VDD 1.3 V Current-Source Gain AA ∆IBDRIVE/∆IISRC (Figure 2) 13 µA/µA Current-Source Input Voltage Range BUFFER (BBUF) Voltage Swing Current Drive VISRC No load VBDRIVE = 2.5V VSS + 1.3 VSS + 1.3 -100 VDD1.3 V VDD 1.3 V 100 µA Offset Voltage VOFS (VBDRIVE - VBBUF) at VBDRIVE = 2.5V, no load -20 20 mV 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. _______________________________________________________________________________________ 3 Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors MAX1450 PIN 1 2, 3, 12, 16 4 5 6 7 8 9 10 11 13 14 15 17 18 19 20 NAME INP I.C. SOTC SOFF A1 A0 OFFTC OFFSET BBUF FSOTRIM A2 OUT VDD ISRC BDRIVE VSS INM Pin Description FUNCTION Positive Sensor Input. Input impedance is typically 1MΩ. Rail-to-rail input range. Internally connected. Leave unconnected. Offset TC Sign Bit Input. A logic low inverts VOFFTC with respect to VSS. This pin is internally pulled to VSS via a 1MΩ (typical) resistor. Connect to VDD to add VOFFTC to the PGA output, or leave unconnected (or connect to VSS) to subtract VOFFTC from the PGA output. Offset Sign Bit Input. A logic low inverts VOFFSET with respect to VSS. This pin is internally pulled to VSS via a 1MΩ (typical) resistor. Connect to VDD to add VOFFSET to the PGA output, or leave unconnected (or connect to VSS) to subtract VOFFSET from the PGA output. PGA Gain-Set Input. Internally pulled to VSS via a 1MΩ (typical) resistor. Connect to VDD for a logic high or VSS for a logic low. PGA Gain-Set LSB Input. Internally pulled to VSS via a 1MΩ (typical) resistor. Connect to VDD for a logic high or VSS for a logic low. Offset TC Adjust. Analog input summed with PGA output and VOFFSET. Input impedance is typically 1MΩ. Rail-to-rail input range. Offset Adjust Input. Analog input summed with PGA output and VOFFTC. Input impedance is typically 1MΩ. Rail-to-rail input range. Buffered Bridge-Voltage Output (the voltage at BDRIVE). Use with correction resistor RSTC to correct for FSO tempco. Bridge Drive Current-Set Input. The voltage on this pin sets the nominal IISRC. See the Bridge Drive section. PGA Gain-Set MSB Input. Internally pulled to VSS via a 11kΩ (typical) resistor. Connect to VDD for a logic high or VSS for a logic low. PGA Output Voltage. Connect a 0.1µF capacitor from OUT to VSS. Positive Supply Voltage Input. Connect a 0.1µF capacitor from VDD to VSS. Current-Source Reference. Connect a 50kΩ (typical) resistor from ISRC to VSS. Sensor Excitation Current Output. This pin drives a nominal 0.5mA through the bridge. Negative Power-Supply Input. Negative Sensor Input. Input impedance is typically 1MΩ. Rail-to-rail input range. ______________ 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 VSS to VDD. Pins A0, A1, and A2 set the PGA gain anywhere from 39V/V to 221V/V (in steps of 26). A2 A1 A0 INP PGA INM OFFTC SOTC ± Σ OUT A=1 ± OFFSET SOFF Figure 1. Signal-Path Functional Diagram 4 _______________________________________________________________________________________ MAX1450 Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors 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 (VOFFSET) and offset temperature-compensation voltage (VOFFTC) add or subtract from the PGA output depending on their respective sign bits, offset sign (SOFF), and offset TC sign (SOTC). VOFFSET and VOFFTC can range in magnitude from VSS to 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 VSS and VDD while 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 RISRC, sets the nominal current, IISRC which sets the FSO (refer to Figure 3 for sensor terminology.) IISRC is additionally modulated by components from the external resistor RSTC and the optional resistor RLIN. RSTC is used to feed back a portion of the buffered bridge-excitation voltage (VBBUF), 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 RLIN 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 (VOUT at PMIN) is 0.5V, and the desired FSO voltage (VOUT(PMAX) - VOUT(PMIN)) is 4V; thus the FS output voltage (VOUT at PMAX) 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- FSOTRIM BBUF OUT VDD MAX1450 IISRC IBDRIVE ≈ 13 (IISRC) VBDRIVE A=1 BBUF RSTC IISRC BDRIVE INP (EXTERNAL) RLIN (OPTIONAL) INM (EXTERNAL) RISRC (EXTERNAL) SENSOR Figure 2. Bridge Drive Circuit _______________________________________________________________________________________ 5 MAX1450 Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors 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 RISRC RISRC programs the nominal sensor excitation current and is placed between ISRC and VSS. Use a variable resistor with a nominal starting value of: RISRC ≈ 13 x Rb(T1) ≈ 13(5kΩ) = 65kΩ where Rb(T1) is the sensor input impedance at temperature T1 (usually +25°C). Selecting RSTC RSTC compensates the FSO TC errors and is placed between BBUF and ISRC. Use a variable resistor with a nominal starting value of the following: R STC ≈ RISRC x 500ppm/°C TCR − TCS ≈ 65kΩ x 500ppm/°C = 65kΩ 2600ppm/ oC − − 2100ppm/ oC This approximation works best for bulk, micromachined, silicon piezoresistive sensors (PRTs). Negative values for RSTC indicate unexpected sensor behavior that cannot be compensated by the MAX1450 without additional external circuitry. 4.5 VOLTAGE (V) FULL-SPAN OUTPUT (FSO) 0.5 OFFSET FULL-SCALE (FS) PMIN PMAX PRESSURE Figure 3. Typical Pressure-Sensor Output 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: SensorFSO = S x VBDRIVE x ∆P = 1.5mV/V psi x 2.5V x 10 psi = 0.0375V where S is the sensor sensitivity at T1, VBDRIVE is the sensor excitation voltage (initially 2.5V), and ∆P is the maximum pressure differential. Table 1. Sensor Information 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 S(p) Offset Tempco -1030 ppmFSO/°C Sensitivity Linearity Error as % FSO BSLF (Best StraightLine Fit) 0.1% FSO BSLF PMIN Minimum Input Pressure 0 PSI PMAX Maximum Input Pressure 10 PSI Table 2. Compensation Components/Values PARAMETER DESCRIPTION RISRC RSTC APGA OFFTC RLIN Resistor that programs the nominal sensor excitation current Resistor that compensates FSO TC errors Programmable-gain amplifier gain Offset TC correction voltage, including its respective sign bit Resistor that corrects FSO linearity errors (optional) 6 _______________________________________________________________________________________ Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors MAX1450 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 A PGA ≈ OUTFSO SensorFSO ≈ 4V = 106V/V 0.0375V 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: OTC Correction = ∆VOUT(T) ∆VBDRIVE(T) x 1.15 which can be approximated by: OTC Correction ≈ OTC x FSO x (∆T) TCS x VBDRIVE x 1.15 x (∆T) ≈ −1030ppm / °C x 4V = 0.68 −2100 x 2.5V x 1.15 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 VDD; for negative values, connect SOTC to VSS. Select the Offset TC resistor divider (ROTCA and ROTCB, Figure 4) using the following equation: OTC Correction = ROTCA ROTCA + ROTCB 0.17 = ROTCA ROTCA + ROTCB where 500kΩ ≥ (ROTCA + ROTCB) ≥ 100kΩ. Choose ROTCB = 100kΩ and ROTCA = 20kΩ. Transfer Function The following transfer function (linearity correction not included) is useful for data modeling or for developing compensative algorithms: VOUT = VBDRIVE x  VS x PGA + 1.15 x  VOFFTC VDD   + 1.15 x VOFFSET  where VBDRIVE = VDD + VDD RISRC RSTC 1 +1 AA x Rb(T) RSTC (AA = current source gain) FSO Calibration Perform FSO calibration at room temperature with a fullscale sensor excitation. 1) At +25°C (or T1), set VFSOTRIM to 2.5V. Adjust RISRC until VBBUF = 2.5V. 2) Adjust VOFFSET until the room temperature offset voltage is 0.5V (see OFFSET Calibration section). 3) Measure the full-span output (measuredVFSO). 4) Calculate VBIDEAL(25°C) using the following equation: VBIDEAL(25o C) =  [ [ ] [ ] ] VFSOTRIM 1+ desiredVFSO − measuredVFSO measuredVFSO   Note: If VBIDEAL(25°C) is outside the allowable bridge voltage swing of (VSS + 1.3V) to (VDD - 1.3V), readjust the PGA gain setting. If VBIDEAL(25°C) is too low, decrease the PGA gain setting by one step and return to Step 1. If VBIDEAL(25°C) is too high, increase the PGA gain setting by one step and return to Step 1. _______________________________________________________________________________________ 7 Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors MAX1450 5) Set VFSOTRIM = VBIDEAL(25°C). Adjust RISRC until VBBUF = VBIDEAL(25°C). 6) Readjust VOFFSET 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 (RSTC). The value of RSTC 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 VBIDEAL(T2). While at T2, adjust RSTC until VBBUF = VBIDEAL(T2). 3) Do not adjust VOFFSET or VOFFTC. OFFSET TC Compensation Connect OFFTC to a resistor divider between BBUF and VSS. The divided-down VBBUF is then fed into OFFTC and the appropriate polarity (designating whether VOFFTC should be added or subtracted from the PGA output) is selected with SOTC. 1) At T2, remeasure the offset at VOUT. 2) Use the following equation to determine the magnitude of VOFFTC(T2), and adjust ROTCA accordingly. If VOFFTC is negative, connect SOTC to VSS. If VOFFTC 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. ( ) VOFFTC = VOFFSET(T1) − VOFFSET(T2) VBDRIVE(T1) − VBDRIVE(T2) x 1.15 + Current OFFTC where Current OFFTC is the voltage at pin OFFTC. Note that the magnitude of VOFFTC 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. VDD RFSOB RFSOA 0.1µF SENSOR RSTC VDD RISRC RLIN (OPTIONAL) FSOTRIM VDD CURRENT SOURCE ISRC VDD BDRIVE INP INM PGA A2 A1 A0 OUT SOTC SOFF OFFTC OFFSET MAX1450 A = 1 BBUF VSS 0.1µF VDD OUT 0.1µF VDD ROTCB VDD ROTCA ROFFB ROFFA Figure 4. Basic Ratiometric Output Configuration 8 _______________________________________________________________________________________ MAX1450 Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors OFFSET Calibration Accomplish offset calibration by applying a voltage to the OFFSET pin (SOFF determines the polarity of VOFFSET). This voltage is generated by a resistor-divider between VDD and VSS (ROFFA and ROFFB in Figure 4). To calibrate the offset, set VOFFSET to 0 and perform a minimum pressure input reading at room temperature. If the output voltage (VOFFZERO) is greater than 0.5V, connect SOFF to VSS; if VOFFZERO is less than 0.5V, connect SOFF to VDD. Adjust VOFFSET until VOUT = 0.5V. Note that the magnitude of VOFFSET is directly 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 (VOUT) to ISRC to modulate the current source. If a bridge current is constant with applied pressure, sensor linearity remains unaffected. If, with a constant bridge 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 RLIN. To correct pressure linearity errors, use the following equation to determine the appropriate range for RLIN: ( ) RLIN ≈ 2 RISRC x RSTC RISRC + RSTC x S(p) where S(p) is the sensitivity linearity error as % best straight-line fit (BSLF). Ideally, this variable resistor should be disconnected during temperature error compensation. If this is not possible, set it to the maximum available value. First measure the magnitude of the uncorrected error (RLIN = maximum value), then choose an arbitrary value for RLIN (approximately 50% of maximum value). Measuring the new linearity error establishes a linear relationship between the amount of linearity correction and the value of RLIN. Note that if pressure linearity correction is to be performed, it must occur after temperature compensation is 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 RLIN. Ratiometric Output Configuration Ratiometric output configuration provides an output that is proportional to the power-supply voltage. When used with ratiometric A/D converters, this output provides digital pressure values independent of supply voltage. Most automotive and some industrial applications require ratiometric outputs. The MAX1450 has been designed to provide a 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. LINEARITY ERROR A B C D UNCOMPENSATED ERROR (RLIN REMOVED) E F OVERCOMPENSATED ERROR (RLIN TOO SMALL) PRESSURE Figure 5. Effect of RLIN on Linearity Corrections _______________________________________________________________________________________ 9 MAX1450 Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors Table 4. MAX1450 Calibration and Compensation 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 VOUT ...................................................Ratiometric to VDD at 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) UNCOMPENSATED SENSOR ERROR 30 COMPENSATED TRANSDUCER ERROR 0.8 ERROR (% SPAN) ERROR (% SPAN) 20 10 FSO 0 OFFSET 0.6 0.4 FSO 0.2 0 -0.2 -10 -20 -50 0 50 100 150 TEMPERATURE (°C) -0.4 OFFSET -0.6 -0.8 -50 0 50 100 150 TEMPERATURE °(C) Figure 6. Comparison of an Uncalibrated Sensor and a Temperature-Compensated Transducer Chip Information TRANSISTOR COUNT: 1364 SUBSTRATE CONNECTED TO VSS 10 ______________________________________________________________________________________ SSOP.EPS MAX1450 Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors Package Information ______________________________________________________________________________________ 11 Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors NOTES MAX1450 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.

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