Freescale Semiconductor Application Note AN1097 Rev 3, 05/2005 Calibration-Free Pressure Sensor System by: Michel Burri, Senior System Engineer Geneva, Switzerland INTRODUCTION The MPX2000 series pressure transducers are semiconductor devices which give an electrical output signal proportional to the applied pressure. The sensors are a single monolithic silicon diaphragm with strain gauge and thin-film resistor networks on the chip. Each chip is laser trimmed for full scale output, offset, and temperature compensation. The purpose of this document is to describe another method of measurement which should facilitate the life of the design. The MPX2000 series sensors are available both as unported elements and as ported assemblies suitable for pressure, vacuum and differential pressure measurements in the range of 10 kPa through 200 kPa. The use of the on-chip A/D converter of the MC68HC05B6 HCMOS MCU makes possible the design of an accurate and reliable pressure measurement system. + Vs Pin 3 Rs1 RP Pin 2 + RIN VOUT Pin 4 SYSTEM ANALYSIS The measurement system is made up of the pressure sensor, the amplifiers, and the MCU. Each element in the chain has its own device-to-device variations and temperature effects which should be analyzed separately. For instance, the 8-bit A/D converter has a quantization error of about ±0.2 percent. This error should be subtracted from the maximum error specified for the system to find the available error for the rest of elements in the chain. The MPX2000 series pressure sensors are designed to provide an output sensitivity of 4.0 mV/V excitation voltage with full-scale pressure applied or 20 mV at the excitation voltage of 5.0 VDC. An interesting property must be considered to define the configuration of the system: the ratiometric function of both the A/D converter and the pressure sensor device. The ratiometric function of these elements makes all voltage variations from the power supply rejected by the system. With this advantage, it is possible to design a chain of amplification where the signal is conditioned in a different way. Thermistor Laser Trimmed On-Chip RS2 Pin 1 GND Figure 1. Seven Laser-Trimmed Resistors and Two Thermistors Calibrate the Sensor for Offset, Span, Symmetry and Temperature Compensation The op amp configuration should have a good commonmode rejection ratio to cancel the DC component voltage of the pressure sensor element which is about half the excitation voltage value VS. Also, the op amp configuration is important when the designer's objective is to minimize the calibration procedures which cost time and money and often don't allow the unit-to-unit replacement of devices or modules. One other aspect is that most of the applications are not affected by inaccuracy in the region from 0 kPa to 40 kPa. Therefore, the goal is to obtain an acceptable tolerance of the system from 40 kPa through 100 kPa, thus minimizing the inherent offset voltage of the pressure sensor. PRESSURE SENSOR CHARACTERISTICS Figure 2 shows the differential output voltage of the MPX2100 series at +25°C. The dispersion of the output voltage determines the best tolerance that the system may © Freescale Semiconductor, Inc., 2005. All rights reserved. achieve without undertaking a calibration procedure, if any other elements or parameters in the chain do not introduce additional errors. VOUT (mV) 20 VS = 5 VDCTA = 25°C The instrument amplifier does provide an excellent CMRR and a symmetrical buffered high input impedance at both noninverting and inverting terminals. It minimizes the number of the external passive components used to set the gain of the amplifier. Also, it is easy to compensate the temperature variation of the Full Scale Output of the Pressure Sensor by implementing resistors “Rf” having a negative coefficient temperature of -250 PPM/°C. The differential-mode voltage gain of the instrument amplifier is: FULL-SCALE 10 Avd = 5 V1-V2 2 Rf = 1+ Vs2-Vs4 Rg (1) OFFSET +VS 0 P (kPa) -5 0 20 40 60 80 100 Figure 2. Spread of the Output Voltage versus the Applied Pressure at 25°C The effects of temperature on the full scale output and offset are shown in Figure 3. It is interesting to notice that the offset variation is greater than the full scale output and both have a positive temperature coefficient respectively of +8.0 µV/degree and +5.0 µV excitation voltage. That means that the full scale variation may be compensated by modifying the gain somewhere in the chain amplifier by components arranged to produce a negative TC of 250 PPM/°C. The dark area of Figure 3 shows the trend of the compensation which improves the full scale value over the temperature range. In the area of 40 kPa, the compensation acts in the ratio of 40/100 of the value of the offset temperature coefficient. VOUT (f) ∆T +85°C Positive Full Scale Variation -15°C Offset Variation 0 20 40 60 80 100 + V1 3 2 Rf RG 4 1 + V2 0V Figure 4. One Power Supply to Excite the Bridge and to Develop a Differential Output Voltage The major source of errors introduced by the op amp is offset voltages which may be positive or negative, and the input bias current which develops a drop voltage ∆V through the feedback resistance Rf. When the op amp input is composed of PNP transistors, the whole characteristic of the transfer function is shifted below the DC component voltage value set by the Pressure Sensor as shown in Figure 5. The gain of the instrument amplifier is calculated carefully to avoid a saturation of the output voltage, and to provide the maximum of differential output voltage available for the A/D Converter. The maximum output swing voltage of the amplifiers is also dependent on the bias current which creates a ∆V voltage on the feedback resistance Rf and on the Full Scale output voltage of the pressure sensor. P (kPa) Figure 3. Output Voltage versus Temperature. The Dark Area Shows the Trend of the Compensation OP AMP CHARACTERISTICS For systems with only one power supply, the instrument amplifier configuration shown in Figure 4 is a good solution to monitor the output of a resistive transducer bridge. AN1097 2 Sensors Freescale Semiconductor V1, V2 lib (nA) 5 VDC VCC 600 V2X 1/2 VCC UNIT 2 300 V1X V2 VEE UNIT 1 450 V1 0 5 10 15 150 20 VPS (mV) T (°C) 0 -50 -25 0 25 50 75 100 125 Figure 5. Instrument Amplifier Transfer Function with Spread of the Device to Device Offset Variation Figure 7. Input Bias Current versus Temperature Figure 5 shows the transfer function of different instrument amplifiers used in the same application. The same sort of random errors are generated by crossing the inputs of the instrument amplifier. The spread of the differential output voltage (V1-V2) and (V2x-V1x) is due to the unsigned voltage offset and its absolute value. Figure 6 and Figure 7 show the unit-to-unit variations of both the offset and the bias current of the dual op amp MC33078. MCU CONTRIBUTION As shown in Figure 5, crossing the instrument amplifier inputs generated their mutual differences which can be computed by the MCU. +VS + Vio (mV) 3 +2 2 UNIT 1 Rg +1 4 UNIT 2 1 0V -2 -50 -25 0 25 50 75 V2 P UNIT 3 -1 Rf + 0 V1 - 100 125 T (°C) Figure 6. Input Offset Voltage versus Temperature To realize such a system, the designer must provide a calibration procedure which is very time consuming. Some extra potentiometers must be implemented for setting both the offset and the Full Scale Output with a complex temperature compensation network circuit. The new proposed solution will reduce or eliminate any calibration procedure. Figure 8. Crossing of the Instrument Amplifier Input Using a Port of the MCU Figure 8 shows the analog switches on the front of the instrument amplifier and the total symmetry of the chain. The residual resistance RDS(on) of the switches does not introduce errors due to the high input impedance of the instrument amplifier. With the aid of two analog switches, the MCU successively converts the output signals V1, V2. Four conversions are necessary to compute the final result. First, two conversions of V1 and V2 are executed and stored in the registers R1, R2. Then, the analog switches are commuted in the opposite position and the two last conversions of V2x and V1x are executed and stored in the registers R2x and R1x. Then, the MCU computes the following equation: AN1097 Sensors Freescale Semiconductor 3 RESULT = (R1-R2) + (R2x-R1x) (2) The result is twice a differential conversion. As demonstrated below, all errors from the instrument amplifier MC74HC4053 MPX2100AP are cancelled. Other averaging techniques may be used to improve the result, but the appropriated algorithm is always determined by the maximum bandwidth of the input signal and the required accuracy of the system. +5V MC33078 + - 3 VRH V1 CH1 Rf 2 Pressure Sensor System Rg 4 1 Rf + - VDD I/O MC68HC05B6 P CH2 V2 VRL VSS 0V Figure 9. Two Channel Input and One Output Port Are Used by the MCU SYSTEM CALCULATION Sensor out 2 Vs2 = a (P) + of2 Sensor out 4 Vs4 = b (P) + of4 Amplifier out 1 V1 = Avd (Vs2 + OF1) Amplifier out 2 V2 = Avd (Vs4 + OF2) Inverting of the amplifier input V1x = Avd (Vs4 + OF1) V2x = Avd (Vs2 + OF2) Delta = V1-V2 1st differential result = Avd * (Vs2 of OF1) - Avd * (Vs4 + OF2) Deltax = V2x-V1x 2nd differential result = Avd * (Vs2 + OF2) - Vdc * (Vs4 + OF1) Adding of the two differential results VoutV = Delta + Deltax = Avd*Vs2 + Avd*OF2 + Avd*OF2 - Avd*OF1 + Avd*OF1 - Avd*OF2 + Avd*OF2 - Avd*OF1 = 2 * Avd * (Vs2 -Vs4) = 2 * Avd * [(a (P) + of2) - (b (P) + of4)] = 2 * Avd * [V(P) + Voffset] There is a full cancellation of the amplifier offset OF1 and OF2. The addition of the two differential results V1-V2 and V2x-V1X produce a virtual output voltage VoutV which becomes the applied input voltage to the A/D converter. The result of the conversion is expressed in the number of counts or bits by the ratiometric formula shown below: 255 is the maximum number of counts provided by the A/D converter and VRH-VRL is the reference voltage of the ratiometric A/D converter which is commonly tied to the 5.0 V supply voltage of the MCU. When the tolerance of the full scale pressure has to be in the range of ± 2.5 percent, the offset of the pressure sensor may be neglected. That means the system does not require any calibration procedure. The equation of the system transfer is then: count = 2 * Avd * V(P) * 51/V where: Avd is the differential-mode gain of the instrument amplifier which is calculated using the equation (1). Then with Rf = 510 kΩ and Rg = 9.1 kΩ Avd = 113. The maximum counts available in the MCU register at the Full Scale Pressure is: count (Full Scale) = 2 * 113 * 0.02 V * 51/V = 230 knowing that the MPX2100AP pressure sensor provides 20mV at 5.0 excitation voltage and 100 kPa full scale pressure. The system resolution is 100 kPa/230 that give 0.43 kPa per count. 255 count = VoutV * VRH-VRL AN1097 4 Sensors Freescale Semiconductor +5V VDD FINE CAL. VRH I/O CH1 MC68HC05B6 P CH2 VRL VSS 0V Figure 10. Full Scale Output Calibration Using the Reference Voltage VRH-VRL When the tolerance of the system has to be in the range of ±1percent, the designer should provide only one calibration procedure which sets the Full Scale Output (counts) at 25°C 100 kPa or under the local atmospheric pressure conditions. +5V MC74HC4053 MPX2100AP MC33078 + 1/3 MC74HC4053 V1 3 Rf 2 Rg 4 1 Rf + VRH P1 I/O MC68HC05B6 CH1 P2 V2 VRL Pressure Sensor System VDD VSS 0V Figure 11. One Channel Input and Two Output Ports are used by the MCU Due to the high impedance input of the A/D converter of the MC68HC05B6 MCU, another configuration may be implemented which uses only one channel input as shown in Figure 11. It is interesting to notice that practically any dual op amp may be used to do the job but a global consideration must be made to optimize the total cost of the system according to the requested specification. When the Full Scale Pressure has to be sent with accuracy, the calibration procedure may be executed in different ways. For instance, the module may be calibrated directly using Up/Down push buttons. The gain of the chain is set by changing the VRH voltage of the ratiometric A/D converter with the R/2R ladder network circuit which is directly driven by the ports of the MCU. (See Figure 12.) Using a communication bus, the calibration procedure may be executed from a host computer. In both cases, the setting value is stored in the EEPROM of the MCU. The gain may be also set using a potentiometer in place of the resistor Rf. But, this component is expensive, taking into account that it must be stable over the temperature range at long term. AN1097 Sensors Freescale Semiconductor 5 + 5.0 V RO 2R I/O VDD VRH 2R R 2R R 2R R 2R R/2R Ladder Network P3 P2 BUS MC68HC05B6 P1 + 5.0 V P0 CH1 CH2 VRL VSS UP DOWN 0V Figure 12. R/2R Ladder Network for an MCU Table 1. Pressure Conversion Table Unity mbar Torr 1 0.01 7.5 10-3 - - - - 100 1 0.75 - - 0.0102 0.014 1 N/m2 = 1 Pascal 1 mbar atm at=kp/cm2 Pa mWS psi 1 Torr = 1 mmHg 133.32 1.333 .1 - - - 0.019 1 atm (1) 101325 1013.2 760 1 1.033 10.33 14.69 1 at = 1 kp/cm2(2) 98066.5 981 735.6 0.97 1 10 14.22 1 m of water 9806.65 98.1 73.56 0.097 0.1 1 1.422 1 lb/sqin = 1 psi 6894.8 68.95 51.71 0.068 - - 1 NOTES: 1. Normal atmosphere 2. 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