19-1365; Rev 0; 5/98 Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors Features ♦ 1% Sensor Signal Conditioning ♦ Corrects Sensor Errors Using Coefficients Stored in External Trimmable Resistors, Potentiometers, or DACs ♦ Compensates Offset, Offset TC, FSO, FSO TC, and FSO Linearity ♦ Rail-to-Rail® Analog Output ♦ Programmable Current Source for Sensor Excitation ♦ Fast Signal-Path Settling Time (< 1ms) ♦ Accepts Sensor Outputs from 10mV/V to 30mV/V ♦ Fully Analog Signal Path 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 Ordering Information PART TEMP. RANGE PIN-PACKAGE MAX1450CAP 0°C to +70°C 20 SSOP MAX1450C/D MAX1450EAP 0°C to +70°C -40°C to +85°C Dice* 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 Pin Configuration TOP VIEW INP 1 20 INM I.C. 2 19 VSS I.C. 3 18 BDRIVE SOTC 4 SOFF 5 17 ISRC MAX1450 16 I.C. A1 6 15 VDD A0 7 14 OUT OFFTC 8 13 A2 OFFSET 9 12 I.C. BBUF 10 ISRC MAX1450 CURRENT SOURCE VDD A2 A1 A0 BDRIVE INP + INM - PGA OUT SOTC SOFF OFFTC OFFSET A=1 BBUF 11 FSOTRIM 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. MAX1450 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. MAX1450 Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors 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 MIN TYP MAX UNITS GENERAL CHARACTERISTICS Supply Voltage VDD Supply Current IDD 4.5 TA = +25°C (Note 1) 5.0 5.5 V 2.8 3.5 mA ANALOG INPUT (PGA) Input Impedance RIN Input-Referred Offset Temperature Coefficient (Notes 2, 3) Amplifier Gain Nonlinearity Output Step-Response Time Common-Mode Rejection Ratio CMRR 1.0 MΩ ±0.5 µV/°C 0.01 %VDD 63% of final value 1 ms From VSS to VDD 90 dB Input-Referred Adjustable Offset Range (Note 4) ±100 mV Input-Referred Adjustable Full-Span Output Range (Note 5) 10 to 30 mV/V SUMMING JUNCTION (Figure 1) Offset Gain ∆VOUT ∆VOFFSET 1.15 V/V Offset TC Gain ∆VOUT ∆VOFFTC 1.15 V/V 39 to 221 V/V ANALOG OUTPUT (PGA) Differential Signal Range Gain Eight selectable gains (Table 3) Minimum Differential Signal Gain Differential Signal Path Temperature Coefficient 36 At any gain 39 44 ±50 ppm/°C 5kΩ load to VSS or VDD, TA = +25°C VSS + 0.25 VDD 0.25 No load, TA = TMIN to TMAX VSS + 0.05 VDD 0.05 Output Current Range VOUT = (VSS + 0.25V) to (VDD - 0.25V), TA = +25°C -1.0 (sink) Output Noise DC to 10Hz, gain = 39, sensor impedance = 5kΩ Output Voltage Swing 2 V/V V 1.0 (source) 500 _______________________________________________________________________________________ mA µVRMS Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors MAX1450 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 0.5 2.0 mA VDD 1.3 V CURRENT SOURCE Bridge Current Range IBDRIVE 0.1 Bridge Voltage Swing VBDRIVE VSS + 1.3 Current-Source Gain AA Current-Source Input Voltage Range ∆IBDRIVE/∆IISRC (Figure 2) VISRC 13 µA/µA VSS + 1.3 VDD1.3 V BUFFER (BBUF) Voltage Swing No load VSS + 1.3 VDD 1.3 V Current Drive VBDRIVE = 2.5V -100 100 µA (VBDRIVE - VBBUF) at VBDRIVE = 2.5V, no load -20 20 mV Offset Voltage Note 1: Note 2: Note 3: Note 4: Note 5: VOFS Contact factory for high-volume applications requiring less than 1.5mA. All electronics temperature errors are compensated together with the sensor errors. The sensor and the MAX1450 must always be at the same temperature during calibration and use. This is the maximum allowable sensor offset at minimum gain (39V/V). 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 Description PIN NAME FUNCTION 1 INP Positive Sensor Input. Input impedance is typically 1MΩ. Rail-to-rail input range. 2, 3, 12, 16 I.C. Internally connected. Leave unconnected. 4 SOTC 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. 5 SOFF 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. 6 A1 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. 7 A0 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. 8 OFFTC Offset TC Adjust. Analog input summed with PGA output and VOFFSET. Input impedance is typically 1MΩ. Rail-to-rail input range. 9 OFFSET Offset Adjust Input. Analog input summed with PGA output and VOFFTC. Input impedance is typically 1MΩ. Rail-to-rail input range. 10 BBUF Buffered Bridge-Voltage Output (the voltage at BDRIVE). Use with correction resistor RSTC to correct for FSO tempco. 11 FSOTRIM Bridge Drive Current-Set Input. The voltage on this pin sets the nominal IISRC. See the Bridge Drive section. 13 A2 14 OUT PGA Output Voltage. Connect a 0.1µF capacitor from OUT to VSS. 15 VDD Positive Supply Voltage Input. Connect a 0.1µF capacitor from VDD to VSS. 17 ISRC Current-Source Reference. Connect a 50kΩ (typical) resistor from ISRC to VSS. 18 BDRIVE 19 VSS Negative Power-Supply Input. 20 INM Negative Sensor Input. Input impedance is typically 1MΩ. Rail-to-rail input range. 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. Sensor Excitation Current Output. This pin drives a nominal 0.5mA through the bridge. ______________ 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). 4 A2 A1 A0 OFFTC SOTC ± INP PGA Σ A=1 INM ± OFFSET SOFF Figure 1. Signal-Path Functional Diagram _______________________________________________________________________________________ OUT Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors 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 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. 2) Perform FSO Calibration 3) Perform FSO TC Compensation 4) Perform OFFSET TC Compensation 5) Perform OFFSET Calibration 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. 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- VDD FSOTRIM MAX1450 IISRC IBDRIVE ≈ 13 (IISRC) VBDRIVE RSTC (EXTERNAL) IISRC A=1 BDRIVE BBUF INP BBUF OUT RLIN (OPTIONAL) (EXTERNAL) INM RISRC (EXTERNAL) SENSOR Figure 2. Bridge Drive Circuit _______________________________________________________________________________________ 5 MAX1450 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 (V OFFTC) add or subtract from the PGA output depending on their respective sign bits, offset sign (SOFF), and offset TC sign (SOTC). V OFFSET and VOFFTC can range in magnitude from VSS to VDD. 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 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 SENSOR DESCRIPTION PARAMETER 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: RSTC ≈ ≈ RISRC x 500ppm/ °C TCR − TCS 65kΩ x 500ppm/ °C 2600ppm/ oC − − 2100ppm/ oC = 65kΩ 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. TYPICAL VALUE Rb(T) Input/Output Impedance 5kΩ at +25°C TCR Input/Output Impedance Tempco 2600ppm/°C S(T) Sensitivity 1.5mV/V psi at +25°C TCS Sensitivity Tempco -2100ppm/°C O(T) Offset 12mV/V at +25°C OTC Offset Tempco -1030 ppmFSO/°C S(p) Sensitivity Linearity Error as 0.1% FSO % FSO BSLF (Best StraightBSLF Line Fit) PMIN Minimum Input Pressure 0 PSI PMAX Maximum Input Pressure 10 PSI 4.5 Table 2. Compensation Components/Values PARAMETER DESCRIPTION FULL-SPAN OUTPUT (FSO) VOLTAGE (V) MAX1450 Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors RISRC Resistor that programs the nominal sensor excitation current RSTC Resistor that compensates FSO TC errors APGA Programmable-gain amplifier gain FULL-SCALE (FS) 0.5 OFFSET PMIN Offset TC correction voltage, including its respective sign bit RLIN Resistor that corrects FSO linearity errors (optional) PMAX PRESSURE Figure 3. Typical Pressure-Sensor Output 6 OFFTC _______________________________________________________________________________________ Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors 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 OUTFSO SensorFSO 4V ≈ = 106V/V 0.0375V APGA ≈ 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 4 V = 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 (R OTCA 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 VOFFTC VS x PGA + 1.15 x + 1.15 x VOFFSET VDD VDD V + DD RISRC RSTC where VBDRIVE = 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 V FSOTRIM 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) = [ ] [ desiredVFSO − measuredVFSO VFSOTRIM 1 + 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 V BIDEAL(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 MAX1450 Table 3. PGA Gain Settings MAX1450 Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors 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). 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. 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. VOFFTC = 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. Connect OFFTC to a resistor divider between BBUF and V SS . The divided-down V BBUF 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. RSTC (VBDRIVE(T1) − VBDRIVE(T2) ) x 1.15 + Current OFFTC OFFSET TC Compensation VDD VOFFSET(T1) − VOFFSET(T2) VDD RFSOB RLIN (OPTIONAL) RFSOA FSOTRIM RISRC 0.1µF VDD CURRENT SOURCE VDD ISRC VDD A2 A1 A0 BDRIVE INP 0.1µF INM OUT PGA 0.1µF OUT VDD SOTC SOFF ROTCB VDD OFFTC SENSOR OFFSET ROTCA ROFFB MAX1450 A=1 BBUF ROFFA VSS Figure 4. Basic Ratiometric Output Configuration 8 _______________________________________________________________________________________ Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors 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) 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 UNCOMPENSATED ERROR (RLIN REMOVED) A B C where S(p) is the sensitivity linearity error as % best straight-line fit (BSLF). Ideally, this variable resistor should be disconnected during temperature error compensation. If this is not possible, set it to the maximum available value. First measure the magnitude of the uncorrected error (R LIN = maximum value), then choose an arbitrary value for 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. D E F OVERCOMPENSATED ERROR (RLIN TOO SMALL) PRESSURE Figure 5. Effect of RLIN on Linearity Corrections _______________________________________________________________________________________ 9 MAX1450 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. 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 COMPENSATED TRANSDUCER ERROR 30 0.8 0.6 20 10 ERROR (% SPAN) 0.4 ERROR (% SPAN) MAX1450 Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors FSO 0 OFFSET FSO 0.2 0 -0.2 -0.4 -10 OFFSET -0.6 -20 -0.8 -50 0 50 100 150 -50 0 TEMPERATURE (°C) 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 ______________________________________________________________________________________ Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors SSOP.EPS ______________________________________________________________________________________ 11 MAX1450 Package Information MAX1450 Low-Cost, 1%-Accurate Signal Conditioner for Piezoresistive Sensors NOTES 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.