Signal conditioning circuitry for the RCO and HDO pressure sensors INTRODUCTION The signal conditioning circuitry described below has been especially designed for First Sensors RCO series pressure sensors. However it can be applied to all sensors with a Common Mode voltage which is half of the supply voltage (VCM≈½ VS), e.g. First Sensors HDO series. The RCO and HDO pressure sensors are calibrated and temperature compensated for span and offset. They offer a very economical method to sense pressures from 10 mbar up to 10 bar. Many industrial systems have only one fairly unstable single voltage power supply available. This application note discusses a circuit which achieves nearly any required stable output signal by using unstable single or dual supply voltages. Further, it provides excellent performance and easy adjustment, requires little power and minimal board space and is low cost. POWER SUPPLY OPTIONS The universal circuitry shown in Figure 1 can be operated from single or dual power supplies. In either case certain voltage limitations do exist. These are mainly due to the output voltage swing, current consumption and power supply rejection of the used LT1014 quad amplifiers. Other amplifiers such as Rail to Rail versions can widen these limits. Single supply operation The circuitry requires a supply voltage of 7 to 30 V for correct operation. Dual supply operation When the circuitry is powered by dual supplies, the positive power supply must be between 7 and 30 V, while the negative voltage supply can be any negative voltage down to -22 V. However, the total voltage across the circuit must be limited to 30 V. GENERAL DESCRIPTION Referring to the schematic diagram of the universal signal conditioning circuitry shown in Figure 1, amplifier A1 is used to provide a regulated voltage for the pressure sensor. In this manner, the circuit becomes independent of supply variations, power supply noise and ripple. The voltage VB will be 10 V when jumper J4 is left open, and 5 V when J4 is connected. E / 11157 / 1 Note that in both cases the min. positive supply voltage must be at least 1.2 V higher than VB or the chosen maximum Vout. Amplifiers A3 and A4 are connected as an instrumentation amplifier and provide gain to the sensor output signal, Vin. Amplifier A2, in conjunction with potentiometer RO set the initial (zero-pressure) output voltage. The complete expression for the output voltage Vout is given by the following equation: R3 Vout = Vin 2 1 + R gain + VRef (1) where R gain = R S + R G (2) and VRef is the voltage as set by RO. Connection point V- can be connected to a negative power supply when available. The connection to Vallows the circuitry to run from a dual supply, thus giving the output the ability to swing to or below true ground. When a negative power supply is not used, the output of the circuitry at zero pressure will be 50 to 150 mV above ground. This small offset voltage can be further reduced to 30 to 80 mV by mounting a 2 kΩ resistor R5 between output and ground. Also, when a negative power supply is not used, jumper J5 must be connected such that V- is connected to ground. The polarity shown for the output voltage of the sensor is for a RCO gage/differential pressure sensor with the pressure applied to port B. By using jumpers J1, J2 and J3, a variety of combinations are possible for VRef , the output voltage that is to represent zero pressure. For example, it is possible to set the output voltage to change from 1 V at zero pressure to 5 V at full-scale pressure; or if dual supplies are available, to set the output to 0 V at zero pressure, to 5 V at full scale positive pressure and to -5 V for an equal but negative pressure (vacuum). Also, by using the proper jumper connections, it is possible to set the output to 5.0 V at zero pressure with swings to +10 V for positive pressure and to ground for negative pressure. Many other combinations are possible as well. 1/5 www.first-sensor.com www.sensortechnics.com Signal conditioning circuitry for the RCO and HDO pressure sensors Jumper J3 With jumper J3 open, the gain of A2 is unity. With J3 shorted, the gain is 3 V/V. This jumper is only useful when it is desired to set VRef higher than 2.5 V, which is the maximum possible by using only J1 and J2. For example, if it is required to set VRef between 3.75 V and 7.5 V, the best adjustment is accomplished by jumpering J1and J3. If it is desired to set VRef at (or near) ground, jumpers J1 and J3 should be left open and J2 should be shorted. JUMPER CONNECTIONS Jumper J4 This jumper simply controls the non-inverting gain of amplifier A1. With J4 open, the gain is 4 V/V and the voltage at the top of the bridge will be approximately 10 V. Leaving J4 open is the proper connection for all applications where V+ is 12 V or higher. For applications where V+ is between 8 V and 12 V, J4 should be connected. This will give A1 a gain of 2 V/V and hence VB will be at approximately 5 V. Jumper J5 This jumper is used when operation is from a single supply. When a negative power supply is not used, jumper J5 must be included such that V- will be eliminated from operation. When jumper J5 is connected, V- will be connected to ground. Jumpers J1 and J2 This network is a voltage divider, with 2.5 V at the top of the divider. Since R1 and RO are 10 kΩ, the range of the wiper arm voltage is easy to determine. With jumpers J1 and J2 open, the wiper arm will range from 0.83 V to 1.67 V and hence, this is also the range of adjustment for voltage VRef assuming jumper J3 is open. If J1 is shorted, the range of adjustment is now from 1.25 V to 2.5 V. If J2 is shorted and J1 left open, the range is from proximately 0.0 V to 1.25 V. R1 J1 RO Zero adjust V+ A Z1 R1, R2, R4 R3 C1 RO RS, RG R5 + A2 R1 + + 2.5 A1 R1 J2 R4 V R2 R2 R3 R R2 J4 R S G Rgain J3 R2 Gain adjust Ref R4 + - C1 R4 - Z1 LT1014DN LT1004CZ-2.5 10 kOhm SIP resistor array 100 kOhm SIP resistor array 10 µF / 35 V Tantalum 10 kOhm multiturn pot select see text 2 kOhm resistor R3 V B R3 GND + RCO Sensor A3 - R3 - A4 + Vout + Vin V- R5 J5 Figure 1: Signal conditioning circuitry for the RCO series pressure sensors E / 11157 / 1 2/5 www.first-sensor.com www.sensortechnics.com Signal conditioning circuitry for the RCO and HDO pressure sensors For the best adjustment range we will select RS=3.24 kΩ and RG to be a 500 Ω multiturn cermet pot. DESIGN EXAMPLE 1 Consider the need to measure a positive pressure from 0 to 100 psi and provide a 1 V to 6 V output, given a single 15 V supply. Since the zero pressure output voltage VR is to be 1 V, this is easily accommodated by leaving jumpers J1, J2, and J3 open but jumper J5 shorted. R5 is not connected. Solution We select a RCOP100D... sensor for this application. Pressure is applied to port B. Since a 15 V supply is available, J4 will be left open which will provide 10 V to the sensor. From the RCO data sheet, the output span for 100 psi input pressure is typ. 100 mV when operating from a 12 V supply. Because the span is ratiometric to the supply voltage, the output for VB=10 V supply will be 83.33 mV. Since the desired output span is 5 V (from 1 V to 6 V) the voltage gain required is 60 V/V. From equation (1) we get for the gain AV = The final circuit design is shown in Fig. 2. Adjustment procedure 1. With zero pressure applied, adjust RO until Vout is 1.00 V. 2. Apply 100 psig and adjust RO until the output is 6.00 V. 3. Repeat 1. and 2. if required. R3 ∆Vout = 2 1 + ∆Vin R gain For R3=100 kΩ this equation is then solved for R gain = 3.448 kΩ A Z1 R1, R2, R4 R3 C1 RO RS RG R1 RO Zero adjust V+ = 15 V R1 + A2 - + + 2.5 - Z1 A1 R1 R4 R4 V R2 Gain adjust Ref R3 + - C1 R2 LT1014DN LT1004CZ-2.5 10 kOhm SIP resistor array 100 kOhm SIP resistor array 10 µF / 35 V Tantalum 10 kOhm multiturn pot 3.24 kOhm 500 Ohm multiturn pot R R2 R S G Rgain R2 R3 V = 10 V B GND + RCO Sensor R3 A3 - R3 - A4 + + Vin Vout = 1...6 V Figure 2: RCO signal conditioning circuitry for 1 V to 6 V output for 0 to 100 psi input pressure E / 11157 / 1 3/5 www.first-sensor.com www.sensortechnics.com Signal conditioning circuitry for the RCO and HDO pressure sensors To allow for approximately ±5 % gain adjustment from nominal, choose RS = 3.74 kΩ and RG to be a 500 Ω multiturn pot. DESIGN EXAMPLE 2 A dialysis machine must measure a pressure of ±500 mmHg. It is desired that the zero pressure output voltage is nominally at 5.0 V and provide an output change of ±2.5 V for this pressure input. A +12 V supply is available. Since the zero pressure output required is 5.0 V, jumpers J1, J3 and J5 will be connected and jumper J2 will be left open. R5 is not connected. In this manner, the wiper arm voltage of RO can range from 1.25 V to 2.5 V and amplifier A2 provides a gain of 3 V/V. Thus, VRef can be adjusted from 3.75 V to 7.5 V. Solution We select a RCOP015D... sensor for this application. Pressure is applied to port B. Again, J4 will be left open to provide 10 V to the top of the sensor. From the RCO data sheet, the span at 12 V supply and 15 psi is typ. 90 mV. This equates to 0.5 mV/V per psi. Thus, at 10 V and 500 mmHg (9.67 psi) the output from the sensor bridge will be Vbridge = The final circuit design is shown in Fig. 3. Adjustment procedure 1. With zero pressure applied, adjust RO until Vout is 5.00 V. 2. Apply +500 mmHg to port B and adjust RO until the output is 7.50 V. 3. Apply +500 mmHg to port A and check if the output is 2.50 V. 4. Repeat 1. to 3. if required. 90 mV ⋅ 10 V ⋅ 9.67 psi = 48.35 mV 12 V ⋅ 15 psi Because it is required that a 500 mmHg pressure provide an output voltage change of 2.5 V, the gain required is 51.7 V/V. Again, using the gain equation (1) with R3=100 kΩ and solving for Rgain gives The 2.5 V output signal at -500 mmHg can be checked by applying +500 mmHg on port A of the RCOP015D... sensor. Thus, no vacuum is required. R gain = 4.024 kΩ RO Zero adjust V+ = 12 V A Z1 R1, R2, R4 R3 C1 RO RS RG + R1 - A2 + + 2.5 A1 R1 R4 V R2 Gain adjust Ref R4 + - C1 R4 - Z1 R2 LT1014DN LT1004CZ-2.5 10 kOhm SIP resistor array 100 kOhm SIP resistor array 10 µF / 35 V Tantalum 10 kOhm multiturn pot 3.74 kOhm 500 Ohm multiturn pot R3 R R2 R S G Rgain R2 R3 V = 10 V B GND + RCO Sensor A3 - R3 R3 - A4 + + Vin Vout = 5 ±2.5 V Figure 3: RCO signal conditioning circuitry for 5 V ±2.5 V output for ±500 mmHg input pressure E / 11157 / 1 4/5 www.first-sensor.com www.sensortechnics.com Signal conditioning circuitry for the RCO and HDO pressure sensors The span for 0 to 125 mbar pressure will be 5.0 V-1.25 V=3.75 V. The gain required is 275.9 V/V. Using the gain equation (1) with R3=100 kΩ gives DESIGN EXAMPLE 3 In a portable respiration equipment a pressure of -25 mbar to +125 mbar needs to be measured. It is desired that the output voltage from the pressure sensor feeds an A/D converter working from a 5 V supply and with an analog input voltage of 0.5 to 5 V for the full pressure range. A 9 V battery supply is available. R gain = 0.730 kΩ We choose a 680.1 Ω resistor for RS and a 100 Ω multiturn pot for RG. Since the zero pressure output needs to be 1.25 V, jumpers J1, J2, J3 and J5 are left open. R5 is not connected. While the wiper arm voltage can range from 0.83 V to 1.67 V and amplifier A2 provides a unity gain, the output offset voltage can be set to 1.25 V. Solution We selected a RCOP001D... device for this application and accept a slightly worse linearity in the pressure range above 1 psi (69 mbar). Pressure is applied to port B. J4 needs to be closed for providing a 5 V supply to the top of the sensor. From the RCO data sheet the span for 0 to 1 psi is typ. 18 mV at 12 V supply. Thus, at 5 V and 125 mbar the output from the sensor bridge will be Vbridge = The final circuit design is shown in Fig. 4. Adjustment procedure 1. With zero pressure applied, adjust RO until Vout is 1.25 V. 2. Apply +125 mbar to port B and adjust RO until the output is 5.00 V. 3. Apply +25 mbar to port A and check if the output is 0.50 V. 4. Repeat 1. to 3. if required. 18 mV ⋅ 5 V ⋅ 125 mbar = 13.59 mV 12 V ⋅ 69 mbar Because it is required that 125 mbar pressure provide 5 V output and -25 mbar provide 0.5 V the output voltage at zero pressure needs to be (5000 mV − 500 mV ) ⋅ 25 mbar + 500 mV Voffset = The 0.5 V output signal at -25 mbar can be checked by applying +25 mbar on port A of the RCOP001D... sensor. Thus, no vacuum is required. 125 mbar + 25 mbar Voffset = 1.25 V A Z1 R1, R2, R4 R3 C1 RO RS RG R1 RO Zero adjust V+ =9V R1 + A2 - + + 2.5 - Z1 A1 R1 R4 R4 V Gain adjust Ref R3 + - C1 LT1014DN LT1004CZ-2.5 10 kOhm SIP resistor array 100 kOhm SIP resistor array 10 µF / 35 V Tantalum 10 kOhm multiturn pot 680.1 Ohm 100 Ohm multiturn pot R R2 R S G Rgain R2 R3 V=5V B R3 GND + RCO Sensor A3 - R3 - A4 + + Vout Vin = 0.5...5 V Figure 4: RCO signal conditioning circuitry for 0.5 V to 5 V output for -25 to +125 mbar input pressure This information is based on a Sensym application note. First Sensor reserves the right to make changes to any products herein. First Sensor does not assume any liability arising out of the application or use of any product or circuit described herein, neither does it convey any license under its patent rights nor the rights of others. E / 11157 / 1 5/5 www.first-sensor.com www.sensortechnics.com