Technical Note 1.Constant Current Source For Fujikura pressure sensor, a constant current source as shown in Fig. 1 is recommended. A reference voltage (Vref), which is generated by zener diode or voltage reference device, is applied to the non-inverting terminal of the operational amplifier (A1). The inverting terminal voltage of A1 is equal to the Vref, then the constant current(I) is : I= Vref R2 ….. [1] The output voltage of A1(VA1) is : VA1 = Vref + I × Rb = I × ( R 2 + Rb) ….. [2] where Rb : Bridge resistance The supply voltage (Vcc) should be sufficiently higher than VA1. In a condition of no pressure (no mechanical stress) to the sensing chip, each output voltage of the bridge terminal(V1), (V2) is calculated by : V 1 = V 2 = Vref + I × Rb Rb = I × ( R2 + ) 2 2 ….. [3] Fig.2 shows a basic circuit for the type FPM. LM385-1.2 is a voltage reference which features 1.235V of Vref, and 150ppm/ of temperature drift. The voltage across the resistor R2 is equal to Vref. (i.e.1.235V) The recommended constant current for the FPM is 1.5mA. Therefore R2 is : R2 = 1.235V = 820Ω 1.5mA The specified bridge resistance of the FPM is 4,000-6,000 Ω , thus the maximum output voltage of A1(VA1) is : VA1 max = 1.5 × (820 + 6000) = 10.23 In this case, 12V DC is chosen for the supply voltage(Vcc), and a 10k Ω resistor is connected between the Vcc and the LM385-1.2 for 1mA operation. R1 = 12V − 1.235V = 10kΩ 1mA - Technical note of Fujikura pressure sensor Fujikura Ltd. Sensor Department 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan phone +81-3-5606-1072 E-mail : [email protected] 1 One part of the bridge connection of the FPM is opened. If offset calibration is required, a zero balance trimmer should be connected like Fig. 2. Or, a closed bridge connection, like Fig. 1, is also available, if zeroing should be done at the back end signal conditioning circuit. Vcc 12VDC R1 Vref ZD 10k + 1.235V I A1 - LM385 -1.2 VA1 + I=1.5mA A1 - Zero balance trimmer V1 V1 SENSOR SENSOR 1.235V Vref V2 V2 R2 820 Fig.1 Constant current source Fig. 2 Basic circuit for type FPM 2.Constant Voltage Source Fig.3 shows an example of constant voltage source. Please note that temperature characteristic by constant voltage source is much different from the one by constant current source. For further information, please refer to the following 4－2. 12VDC VR 5k *** Example for FPM sensor *** I SENSOR V' Vcc: Bridge resistance: I: R1: V': VR: V1 V2 12VDC 4,000 ~ 6,000 ohm 1.5mA 100 ohm 150mV 1,900 ~ 2,500 ohm or Vcc: VR: Bridge resistance: R1: I: 7.5VDC 0 ohm 4,000 ~ 6,000 ohm 0 ohm 1.9 ~ 1.3mA R1 Fig. 3 Basic circuit of constant voltage source - Technical note of Fujikura pressure sensor Fujikura Ltd. Sensor Department 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan phone +81-3-5606-1072 E-mail : [email protected] 2 3.Amplifier Circuit A differential amplifier shown in Fig 4 is provided to treat low-level and differential signal from a sensing element. The output voltage(Vout) is calculated by the following formula : Vout = R5 × (V 1 − V 2) ….. [4] R3 R5 SENSOR V1 R3 + Vout A2 - V2 R4 R3 = R4, R5 = R6 R6 Fig. 4 Simple differential amplifier Since the input resistance of the amplifier is not very high, an instrumentation amplifier shown in Fig.5 should be used for high precision measurement. Each output voltage of VA1, VA2, and Vout is calculated by : VA2 = V 1 + R5 × (V 1 − V 2) …..[5] VR1 VA3 = V 2 − R5 × (V 1 − V 2) VR1 Vout = …..[6] 2 × R3 R7 R7 × (VA2 − VA3) = × (1 + ) × (V 1 − V 2) R5 R5 VR1 …..[7] In case of positive level shift is required, add Vshift to non-inverting input terminal of the amplifier A4. Then Vout is: Vout = R7 2 × R3 × (1 + ) × (V 1 − V 2) + Vshift R5 VR1 …..[8] - Technical note of Fujikura pressure sensor Fujikura Ltd. Sensor Department 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan phone +81-3-5606-1072 E-mail : [email protected] 3 SENSOR V1 Vcc + R5 A2 - R7 Vshift R3 + A4 Vout R4 V2 A3 + R6 R8 R3 = R4, R5 = R6, R7 = R8 Fig. 5 Instrumentation amplifier How to design circuit for FPM-07PG is by the following manner : [FPM-07PG] Pressure range(gauge) : -0.492 to +0.492 kg/cm2 Sensor output(I=1.5mA) : -80 to + 80 mV DC (example) [Amplifier] Supply voltage(Vcc) : 12V DC Output voltage(Vout) : 1to9V DC Gain = 1 ~ 9V 8V = = 50 − 80 ~ +80mV 160mV R3=R4=R5=R6=R7=R8=10k VR1=2k : adjust to approx. 400 ohm Vref=1.235V (refer Fig.2) VR2=10k : adjust to approx. 4.2k (Vshift=5V) *All resistors should be 1/4W and 1% tolerance *This circuit does not include temperature compensation resistor. - Technical note of Fujikura pressure sensor Fujikura Ltd. Sensor Department 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan phone +81-3-5606-1072 E-mail : [email protected] 4 Output characteristic Vout (V) V1-V2 (mV) 9 +100 7 5 +/-0 3 1 -100 -0.492 +/-0 +0.492 Gauge pressure (kg/cm2) 4.Temperature Compensation Piezoresistive pressure sensor has two temperature characteristics - Temperature Sensitivity of Offset and Temperature Coefficient of Span output. For high precision measurement, temperature compensation is recommended as below. 4-1 Temperature Sensitivity of Offset (TSO) TSO is the temperature drift behavior of bridge output at free of pressure. It is caused by : - Difference of thermal expansion coefficient of each components of the sensor - …silicon chip, glass pedestal, bonding resin, and package. Difference of thermal expansion coefficient of each material of the sensing chip …silicon substrate, oxidation layer and aluminum pattern. - Variety of thermal coefficient of the four piezoresistors of the bridge. - Variety of resistance value of the four piezoresistors of the bridge. In order to compensate TSO, a resistor such as metal film type, is connected in parallel to the bridge. Fig.6 shows a connection diagram, and the resistance is determined in the following manner, - Technical note of Fujikura pressure sensor Fujikura Ltd. Sensor Department 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan phone +81-3-5606-1072 E-mail : [email protected] 5 CC SENSOR Rp1 Rp2 Vout Rp1, Rp2: Low temperature drift resistor (Metal film type or similar type is recommended) Fig. 6 Connection for TSO compensation - V(p,t) refers to the output voltage in pressure "p" and temp "t" - "p0" refers to a no pressure condition. i.e. V(p0) is the offset voltage. - "tc" refers to the lower temp. of the operating temp. range, and "th" the higher. (1) Measure the output voltage at the following 10 points : Output Rp1 Rp2 V1=V(p0,tc) open open V2=V(p0,tc) 500k open V3=V(p0,tc) 1M open V4=V(p0,tc) open 500k V5=V(p0,tc) open 1M V6=V(p0,th) open open V7=V(p0,th) 500k open V8=V(p0,th) 1M open V9=V(p0,th) open 500k V10=V(p0,th) open 1M (2) Calculate the temperature drift of the offset at each point. TCV1=V6-V1 TCV2=V7-V2 TCV3=V8-V3 - Technical note of Fujikura pressure sensor Fujikura Ltd. Sensor Department 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan phone +81-3-5606-1072 E-mail : [email protected] 6 TCV4=V9-V4 TCV5=V10-V5 (3) Plot a temperature characteristic line as in Fig.7. TCV+ Rp1=500k Rp2=open 1M open open open open 1M TCV5 TCV4 open 500k TCV3 TCV2 TCV1 cross point Fig. 7 Rp1, Rp2 vs. TCV (4) The most suitable condition is the cross point of the line and X-axis. For example in Fig.7, Rp1 must be open (i.e.Rp1 is not needed) and Rp2 must be approximately 750k or so. 4-2 Temperature Coefficient of Span outpuut (TCS) TCS is the correlation between operating temperature and bridge output voltage. It is caused by the issues as below, (a) Impurity concentration of piezoresistors (b) Crystal direction of piezoresistors (c) Resistance value of external load resistor The following describes the detail of the items (a), (c). 4-2-1 Impurity concentration of piezoresistors TCS characteristic mainly depends on the impurity concentration of piezoresistors. Driven by a constant current source, the correlation is shown in Fig.8. There are two points of impurity concentration that can optimize the TCS. - Technical note of Fujikura pressure sensor Fujikura Ltd. Sensor Department 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan phone +81-3-5606-1072 E-mail : [email protected] 7 -3 TSC x 10 (/degC) 1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 18 10 19 10 20 10 21 10 atom/ cm3 Fig. 8 Relation between temperature coefficient of the pressure sensitivity and impurity surface concentration Fig.9 shows a temperature drift of pressure sensitivity at 2 × 10 atoms / cm 20 3 of impurity concentration. It shows that constant current source can make the temperature drift better than constant voltage source. An external load resistor is useful if further compensation is required. (Please refer to 4-2-2.) In case of constant voltage source, the temperature drift shows linear characteristic, therefore, a software compensation by micro-processor is useful. - Technical note of Fujikura pressure sensor Fujikura Ltd. Sensor Department 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan phone +81-3-5606-1072 E-mail : [email protected] 8 Constant current (CC) % Constant voltage (CV) % 4 0.4 CV CC 2 0.2 0 0 -2 -0.2 -4 -0.4 -10 0 10 20 30 40 50 Fig. 9 Temperature drift of pressure sensitivity for voltage and current drive 4-2-2 Compensation by an external load resistor A circuit with external load resistor is shown in Fig.10(a). When an external signal conditioning circuit, such as an amplifier or micro-processor, is connected to the sensor's output terminals, the input impedance of the device is considered as the load resistor RL. CC SENSOR Rb RL SV' RL SV' SV Fig. 10(a) Circuit with load resistor RL Fig. 10(b) Equivalent circuit Showing an example of correlation between TCS and RL, Fig. 11 indicates that there is the best RL value to minimize the TCS property. However, please note that the availability of the compensation is limited by a condition that “bare” TCS property is - Technical note of Fujikura pressure sensor Fujikura Ltd. Sensor Department 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan phone +81-3-5606-1072 E-mail : [email protected] 9 originally positive to the temperature. TCS (%FS) 0.6 RL=open 0.4 RL=50k 0.2 0 RL=20k -0.2 -0.4 RL=10k -0.6 -10 0 10 20 30 40 50 Fig. 11 Example of the relation between RL and TCS Fig.10(b) shows the equivalent circuit of Fig.10(a), and the RL value is calculated by the following formula : RL = where SVc × Rbh − SVh × Rbc SVh − SVc …..[9] SVc : Span output voltage at lower temp. of operating temp. range. SVh : Span output voltage at higher temp. of operating temp. range. Rbc : Bridge resistance at lower temp. of operating temp. Rbh : Bridge resistance at higher temp. of operating temp. Please note that the RL reduces sensor span output voltage as the following formula : SV ' = RL × SV Rb + RL …..[10] - Technical note of Fujikura pressure sensor Fujikura Ltd. Sensor Department 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan phone +81-3-5606-1072 E-mail : [email protected] 10 ＊ ＊＊＊＊APPENDX 1＊＊＊＊＊ [Analog output] 12VDC 10k Vref LM385 -1.2 1.235V + I=1.5mA A1 2 - 3 V1 12VDC + 10k A2 6 SENSOR FPM-**PG(R) 10k Vshift 10k - 200 10k 1 + 2k 5 A4 Output 1 ~ 5V - 820 10k V2 A3 10k 10k + [For pressure switch] NPN open collector Output input 1 - 5V + A5 10k - 12VDC 10k [For current output] 12VDC input 1 ~ 5V + A5 - 250 10k Iout = input/250 = 4 ~ 20mA RLmax = 200 - Technical note of Fujikura pressure sensor Fujikura Ltd. Sensor Department 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan phone +81-3-5606-1072 E-mail : [email protected] 11 ＊＊＊＊＊APPENDIX 2＊＊＊＊＊ [Barometric pressure monitor] 12VDC 10k Vref LM385 -1.2 1.235V + I=1.5mA A1 - 5 6 1 V1 SENSOR FPM-15PA(R) 12VDC + 10k A2 4 - 10k Vshift 10k 10k + 2k 2 A4 Output 4 ~ 8V - 820 10k - 10k 10k A3 V2 + Vout (V) 8 7 6 5 4 -680 0 +680 1,033g/cm2.abs - Technical note of Fujikura pressure sensor Fujikura Ltd. Sensor Department 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan phone +81-3-5606-1072 E-mail : [email protected] 12 ＊＊＊＊＊APPENDIX 3＊＊＊＊＊ [Vacuum monitor] 6 5 4 1 2 3 Pressure Sensor XFPM-100KPGV Q3 Q2 Q1 PIC16C711-04/P RA2 4.7k 5V RA1 RA3 RA0 RA4 OSC1 MCLR OSC2 Vss Vdd RB0 RB7 RB1 RB6 RB2 RB5 RB3 RB4 15pF 680pF 0.01uF XT 4MHz 15pF 5V 0.1uF 5V 4.7uF 300 a b c d e f g -99~0kPa.gauge LED1~LED3 7 segment display Common cathode Q1~Q3 Small signal NPN transistor 4.7k RA1 4.7k RA2 4.7k RA3 - Technical note of Fujikura pressure sensor Fujikura Ltd. Sensor Department 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan phone +81-3-5606-1072 E-mail : [email protected] 13 ＊＊＊＊＊APPENDIX 4＊＊＊＊＊ [Barometric pressure monitor] 5V PIC16F84-10/P 5V 22pF RA2 0.1uF RA1 RA3 RA0 RA4 OSC1 MCLR OSC2 4.7k XT 4MHz MAX-187 4.7uF Vss Vdd RB0 RB7 5V 22pF Vdd SCLK Vin CS RB7 RB1 RB6 SHDN Dout RB2 RB5 REF GND RB3 RB4 CS 0.1uF 4.7uF 4.7uF 30 0 5V a 680pF Pressure Sensor XFPM-115KPA 150 ~ 1150hPa : 0.2 ~ 4.7V b c d e f g 0.01uF 6 5 4 LED1~LED4 7 segment Display Common cathode 1 2 3 hPa Q1~Q3 Small signal NPN transistor 4.7k RA0 4.7k RA1 4.7k RA2 4.7k RA3 1st October , 2011 - Technical note of Fujikura pressure sensor Fujikura Ltd. Sensor Department 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan phone +81-3-5606-1072 E-mail : [email protected] 14