Chapter 4 Sensor interface circuits Prof. Dehan Luo

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1 Chapter 4 Sensor interface circuits Prof. Dehan Luo
第四章 传感器接口电路 Section One Review of circuit theory（第一节 电路理论复习） 1、Voltage, current and resistance 2、Capacitance and inductance 3、Complex number representations（复数表达） Section Two Measurement of resistance （第二节 电阻传感器测量电路） 1、Voltage dividers（分压器） 2、Wheatstone Bridge（惠斯登电桥） 3、Temperature compensation for strain gauges （应变片温度补偿） Section Three AC bridges （第三节 交流电桥测量电路） 1、Measurement of capacitance （电容传感器测量电路） 2、Measurement of inductance （电感传感器测量电路） Section Four operational amplifier circuits（第四节 运算放大器测量电路） 1、Inverting and non-inverting amplifier 2、Summing and differential amplifier 3、Integrating and differentiating amplifier Intelligent Sensors System School of Information Engineering

2 Section One Review of circuit theory
Chapter 4 Sensor interface circuits Prof. Dehan Luo Section One Review of circuit theory 第一节 电路理论复习 Voltage, current, resistance and power 1、Voltage The voltage between two points is the energy required to move a unit of positive charge from a lower to a higher potential. Voltage is measured in Volts (V) （电压是正电荷从低电位移到高电位所需要的能量，电压测量单位是伏特） 2、Current Current is the rate of electric charge through a point. The unit of measure is the Ampere or Amp (A) （电流是电荷通过某点的速率，电流测量单位是安培） Intelligent Sensors System School of Information Engineering

3 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Voltage, current, resistance and power （Cont.)（续） 3、Resistance Given a piece of conducting material connected to a voltage difference V, which drives through it a current I，the resistance is defined as（假设导电材料连接到电压差为V上，通过它的电流为I， 则该材料的电阻定义为） （1）As you will recall, this is known as Ohm’s Law （2）An element whose resistance is constant for all values of V is called an ohmic resistor（在电压范围内，电阻值为常数的 元件被称为欧姆电阻） （3）Series and parallel resistors…（电阻串联、并联….） 4、Power The power dissipated by a resistor is（电阻消耗的功率是） Intelligent Sensors System School of Information Engineering

4 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Voltage, current, resistance and power （Cont.)（续） 5、Kirchhoff’s Laws（基尔霍夫定律） （1）1st Law (for nodes) （第一基尔霍夫定律，节点定律） The algebraic sum of the currents into any node of a circuit is zero Or, the sum of the currents entering equals the sum of the currents leaving.Thus, elements in series have the same current flowing through them Intelligent Sensors System School of Information Engineering

5 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Voltage, current, resistance and power （Cont.)（续） 5、Kirchhoff’s Laws（基尔霍夫定律）（续） （2）2nd Law (for loops) （第二基尔霍夫定律，回路定律） The algebraic sum of voltages in a loop is zero.Thus, elements in parallel have the same voltage across them. Intelligent Sensors System School of Information Engineering

6 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Capacitors and inductors 1、A capacitor is an element capable of storing charge （1）The amount of charge is proportional to the voltage across the capacitor （电容器的电荷量正比于电容器两端的电压） Q=CV C is known as the capacitance (measured in Farads) （2）Taking derivatives（导出公式） （3）Therefore, a capacitor is an element whose rate of voltage change is proportional to the current through it（电容器是电压上升 速率正比于通过其电流的器件） Intelligent Sensors System School of Information Engineering

7 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Capacitors and inductors （Cont.) （续） 2、Similarly, an inductor is an element whose rate of current change is proportional to the voltage applied across it （电感中的电流增加的速率正比于加在它两端的电压） L is called the inductance and is measured in Henrys （L称为电感，单位为亨） Intelligent Sensors System School of Information Engineering

8 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Capacitors and inductors （Cont.) （续） 3、Frequency analysis Consider a capacitor driven by a sine wave voltage（电容器由正弦电压驱动） （1）The current through the capacitor is （2）Therefore, the current phase-leads the voltage by 900 and the ratio of amplitudes is（电流相位超前电压相位90度，幅值比为） （3）What happens when the voltage is a DC source? Intelligent Sensors System School of Information Engineering

9 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Voltages as complex numbers（电压表达的复数形式） 1、At this point it is convenient to switch to a complex-number representation of signals （在此，将信号转换成复数表达方式是方便的） Intelligent Sensors System School of Information Engineering

10 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Voltages as complex numbers（电压表达的复数形式）（Cont.)(续) 2、Applying this to the capacitor V(t)/I(t) relationship （电容器上电压电流的复数关系） Intelligent Sensors System School of Information Engineering

11 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Voltages as complex numbers（电压表达的复数形式）（Cont.)(续) 3、 Impedance Impedance (Z) is a generalization of resistance for circuits that have capacitors and inductors （阻抗是含有电容和电感电路的电抗的一般表达形式） Capacitors and inductors have reactance, while resistors have resistance Intelligent Sensors System School of Information Engineering

12 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Voltages as complex numbers（电压表达的复数形式）（Cont.)(续) 4、 Ohm’s Law generalized（欧姆定律通用形式） 5、 Impedance in series and parallel（串、并联电路） Intelligent Sensors System School of Information Engineering

13 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Voltages as complex numbers（电压表达的复数形式）（Cont.)(续) 6、 High-pass filter Intelligent Sensors System School of Information Engineering

14 Section Two Measurement of resistance
Chapter 4 Sensor interface circuits Prof. Dehan Luo Section Two Measurement of resistance 第二节 电阻传感器测量电路 1、Voltage dividers（分压器） 2、Wheatstone Bridge（惠斯登电桥） 3、Temperature compensation for strain gauges （应变片温度补偿） Intelligent Sensors System School of Information Engineering

15 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Voltage dividers（分压器） Assumptions Interested in measuring the fractional change in resistance x of the sensor: RS=R0(1+x) R0 is the sensor resistance in the absence of a stimuli（ R0为无激励时的传感器电阻） Load resistor expressed as RL=RK for convenience The output voltage of the circuit is Questions What if we reverse RS and RL? How can we recover RS from Vout? Intelligent Sensors System School of Information Engineering

16 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Voltage dividers （Cont.)（分压器）（续） What is the sensitivity of this circuit? For which RL do we achieve maximum sensitivity? This is, the sensitivity is maximum when RL=R Intelligent Sensors System School of Information Engineering

17 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Wheatstone bridge 1、A circuit that consists of two dividers A reference voltage divider (left) A sensor voltage divider 2、Wheatstone bridge operating modes （a）Null mode（零模式，即平衡电桥） R4 adjusted until the balance condition is met: Advantage: measurement is independent of fluctuations in VCC （b）Deflection mode （偏差模式，即不平衡电桥） The unbalanced voltage Vout is used as the output of the circuit Intelligent Sensors System School of Information Engineering

18 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Wheatstone bridge (Cont.)（续） 3、 Assumptions（假设） （1）Want to measure sensor fractional resistance changes RS=R0(1+x) （2）Bridge is operating near the balance condition （电桥工作于接近平衡条件） 4、The output voltage becomes（输出电压为） Intelligent Sensors System School of Information Engineering

19 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Wheatstone bridge (Cont.)（续） 5、 What is the sensitivity of the Wheatstone bridge?（惠斯登电桥灵敏度） The sensitivity of the Wheatstone bridge is the same as that of a voltage divider You can think of the Wheatstone bridge as a DC offset removal circuit（惠斯登电桥可认为是直流偏置消除电路） So what are the advantages, if any, of the Wheatstone bridge? Intelligent Sensors System School of Information Engineering

20 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Temperature compensation for strain gauges （应变片温度补偿） 1、 Strain gauges are quite sensitive to temperature （1）A Wheatstone bridge and a dummy strain gauge may be used to compensate for this effect（惠斯登电桥与补偿应变片可用来补偿温度效应） Intelligent Sensors System School of Information Engineering

21 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Temperature compensation for strain gauges （应变片温度补偿）（续） （2） The “active” gauge RA is subject to temperature (x) and strain (y) stimuli（测量应变片感受温度X和应变Y） （3）The dummy gauge RD, placed near the “active”gauge, is only subject to temperature （测量应变片置于测量应变片附近，只感受温 度X和） （4）The gauges are arranged according to the figures below （5） The effect of (1+y) on the right divider cancels out （右侧分压器上的（1+y）被消除） Intelligent Sensors System School of Information Engineering

22 Section Three AC bridges
Chapter 4 Sensor interface circuits Prof. Dehan Luo Section Three AC bridges 第三节 交流电桥测量电路 1、Measurement of capacitance （电容传感器测量电路） 2、Measurement of impedance （电感传感器测量电路） Intelligent Sensors System School of Information Engineering

23 Chapter 4 Sensor interface circuits Prof. Dehan Luo AC bridges （交流电桥）
1、The structure of the Wheatstone bridge can be used to measure capacitive and inductive sensors （惠斯登电桥可用于电 容传感器和店感传感器的测量） （1）Resistance replaced by generalized impedance （2）DC bridge excitation replaced by an AC source 2、The balance condition becomes（平衡条件为） which yields two equalities, for real and imaginary components （实部与实部，虚部与虚部分别相等） Intelligent Sensors System School of Information Engineering

24 Chapter 4 Sensor interface circuits Prof. Dehan Luo
AC bridges（Cont.) （交流电桥）(续) 3、There is a large number of AC bridge arrangements These are named after their respective developer Intelligent Sensors System School of Information Engineering

25 Section Four operational amplifier circuits
Chapter 4 Sensor interface circuits Prof. Dehan Luo Section Four operational amplifier circuits 第四节 运算放大器测量电路 1、Inverting and non-inverting amplifier 2、Summing and differential amplifier 3、Integrating and differentiating amplifier 4、Current-voltage conversion Intelligent Sensors System School of Information Engineering

26 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Inverting and non-inverting amplifiers 1、Non-inverting amplifier 2、Inverting amplifier Intelligent Sensors System School of Information Engineering

27 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Summing and differential amplifier （求和放大器和差动放大器） 1、Summing amplifier 2、 Differential amplifier Intelligent Sensors System School of Information Engineering

28 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Integrating and differentiating amplifier（积分与微分放大器） （1）、 Integrating amplifier （2）、 Differentiating amplifier Intelligent Sensors System School of Information Engineering

29 Chapter 4 Sensor interface circuits Prof. Dehan Luo
Current-voltage conversion （电流—电压转换器） （1） Current to-voltage （2） Intelligent Sensors System School of Information Engineering

30 Chapter 4 Sensor interface circuits Prof. Dehan Luo References
1、P. Horowitz and W. Hill, 1989, The Art of Electronics, 2nd Ed., Cambridge University Press, Cambridge, UK 2、D. C. Ramsay, 1996, Principles of Engineering Instrumentation, Arnold, London, UK 3、F. F. Mazda, 1987, Electronic instruments and measurement techniques, Cambridge Univ. Pr., New York 4、A. J. Diefenderfer, 1972, Principles of electronic instrumentation, W. B. Sanuders Co., Philadelphia, PA. 5、R. Pallas-Areny and J. G. Webster, 1991, Sensors and Signal Conditioning, Wiley, New York 6、J. W. Gardner, 1994, Microsensors. Principles and Applications, Wiley, New York. Intelligent Sensors System School of Information Engineering