Asics for MEMS

1. Asics for MEMS BRILLANT Grégory 2th of October 2006

2. Overview • Smart Sensor Interface Electronics • Equivalent circuit representation of electromachanical transducers

3. Smart Sensor Interface Electronics

4. Overview • Object Oriented Design • Parasitic Effects • Analog to Digital Conversion • High accuracy over a wide Dynamic range • Presentation of two systems

5. Introduction • Information processing systems need sensors to acquire physical, mechanical and chemical information • Sensors are inescapable in applications such as smart cars or smart homes • But:They are • Solution:Smart Sensor Systems combine : • Sensors • Signal conditioning • ADC • Bus interfacing • And self testing, auto-calibration, data evaluation and identification, … EXPENSIVE

6. Object oriented Design of sensor system • When designing sensor systems, traditional Top/Down and Bottom/Up approach are limited • Interdisciplinary and open characters of sensor subsystems • Long design time and inflexible designs • Solution:Object oriented Design. • The result of the object-oriented design is a detail description how the system can be built, using objects • Save costs and speed up the design • If it’s possible to implement the sensor element and its interface on a same chip, we speak about Smart Sensor

7. Parasitic effects in sensing elements • Excitation signals for sensing elements are usually square-wave (and not sinusoidal). Care has to be taken to avoid undesired electro-physical interaction • Electrical excitation of a resistive temperature sensor causes self heating →measurement errors • In conductivity sensors the excitation can cause electrolysis → corrosion

8. Parasitic effects in sensing elements • Sensing elements deliver an electrical output representing the measurement • But: there are parasitic electrical effects • Capacitive humidity sensors are often shunted by a parasitic resistive component • Resistive sensors are often shunted by parasitic capacitors • The various components are founded by analyzing impedance measurement at various frequencies. • Small-sized, low power integrated circuit must be used to achieve this measurements • The use of additional sensor elements improve the reliability

9. Parasitic effects in sensing elements • Connecting wires and cables can affect the measurement • Solution:two-port measurements 4-wire technique is applied to measure a low-ohmic sensor. The interface chip delivers an excitation current and the voltage over the sensor is measured using a high impedance input amplifier The dual case is applied to measure a high-ohmic sensor admittance

10. Analog to digital conversion • The sensor signal is often converted to a voltage signal → Standard ADC can be used • Capacitive sensing element: A/D converter requires an analog input voltage → Problem: Complication of the front end ADC design because of the introduction of: • Many additional transfer parameters • Biasing quantities • Conversion steps

11. Analog to digital conversion • Solution: sample and hold, quantization, digital filtering and ∑Δ conversion can be implemented in the DSP microcontroller • DSP or microcontroller are well equipped to measure frequency or time interval → using of period-modulated signal

12. High accuracy over a wide Dynamic range: errors • Two kinds of errors: • Systematic errors: inaccuracy of the system parameters → calibrating • Random errors: interferences, noise and instability → filtering, separating common mode and differential mode signal,… • Calibration: sensor-under-test is compared to another one of superior quality • Trimming: sensor behavior is altered permanently to make its characteristics match the nominal as close as possible

13. High accuracy over a wide Dynamic range: chopping • But: calibration and trimming have to performed under certain conditions with respect to the temperature, supply voltage and time conditions during sensor operation • Solution:Chopping techniques. Reduce random errors, noise, low frequency interferences and offset • Implementation:switches interchange the wires of a signal source at a high frequency ≠ Common chopper: +,-,+,-,… Improved chopper: +,-,-,+,+,… This sampling sequence result in a filter operation applied to the interferences

14. High accuracy over a wide Dynamic range: auto calibration • Two signal approach: measure of a reference signal S1 in exactly the same way as the input signal Sx • Ratio Sx/S1 or difference Sx-S1 is used to eliminates errors • Three signal approach: more accurate. Measure of two reference signals. • (Sx-S1)/(S2-S1) is used.

15. High accuracy over a wide Dynamic range: amplification • During auto-calibration, the signals are processed in an identical way • The system should be linear or well characterized over the full signal range → this poses a problem when the signal are not in the same range of magnitude • To achieve a high accuracy, signals should have a high dynamic range, but that is not often the case • Amplification or division by a scaling factor A

16. High accuracy over a wide Dynamic range: amplification • Problem: realize A without loosing precision • Dynamic feedback instrumentation amplifier can solve this problem • Resistive load K=u+v+w+z • Dynamic feed back is made by rotation of the resistor chain • Mismatches between resistors are critical • 6*K switches

17. High accuracy over a wide Dynamic range: amplification • DEM amplifier can also be a solution • Possible implementation: switched-capacitors • The rotation of the capacitors at each clock cycle can almost eliminates the effect of capacitor mistmatching

18. High accuracy over a wide Dynamic range: division • Instead amplify the smallest signals, division of the strongest signals can also be applied • One possible realization • A resistive voltage divider (Nr resistors) combined with a capacitive voltage divider (Nc capacitors) • Division ratio: NcNr

19. Universal sensor interface • The Universal Sensor Interface is a complete front-end for sensor systems • The output is based on a period modulator oscillator • The USI converts the signals of sensing elements into period-modulated signals → microcontroller and DSP compatible • Signal processing in the USI • The input signal is selected by the multiplexer • Chopped signal conversion • Period length conversion

20. A wide range voltage processor • Example: measurement system for thermocouple voltages • Two measured signals: thermocouple voltage Vx and reference junction temperature Tj • Voff is measured to allow offset compensation • All algorithmic signal processing is performed by the microcontroller. The voltages are firstly converted to the time domain

21. Conclusion • In the smart sensor systems presented, measurement techniques are implemented using a limited number of low-cost, low-power integrated circuits only. • By applying synchronous detection, auto calibration and advanced chopping, high immunity is obtained for interfering signals, 1/f noise and parameter drift. • The dynamic range of the signals can be extended using dynamic amplifiers and dynamic dividers.

22. Equivalent circuit representation of electromachanical transducers

23. Overview • Lumped-parameter electromechanical systems • Elementary Lumped-parametertransducers • Equivalent circuit representation • Coupling of the transducersto the outside world • Some examples of transducers

24. Introduction • A transducer is a device that converts one type of energy to another, or responds to a physical parameter. A transducer is in its fundamental form a passive component. • Electomechanical transducers are used to convert electrical energy into mechanical energy and vice –versa • Example: microphone in which a sound pressure is converted into an electrical signal • Equivalent circuit approach: the electrical and mechanical portions of the transducers are represented by electrical equivalents → single representation of device that operate in more than one energy domain.

25. Lumped-parameter electromechanical systems • Lumped parameter (or discrete) system: physical properties (mass, capacitance, inductance,…) are concentrated or lumped into single physical elements • The parameters which involve ordinary differential equations are called linear lumped parameters. • Lumped-parameter modeling is valid as long as the wavelength of the signal is greater than all dimensions of the system • Example: basic configuration of an electrostatic transducer

26. Energy exchange • Exchange of energy of a transducer and the outside world is achieved trough ports: pair of conjugate dynamic variables, the effort variable and the flow

27. Elementary Lumped-parametertransducers: configurations • Linear transducers are mathematically more easiest to study • Linear behavior is achieved for small signal variations around equilibrium levels • Four basics electromechanical lumped-parameter transducers: • Transverse electrostatic transducer • In-plane electrostatic transducer • Electromagnetic transducer • Electrodynamics transducer

28. Elementary Lumped-parametertransducers: equations • Characteristic equations: linear relations between small-signal variations of the port variable around the bias point • Matrix representations • Matrix B: effort variable as a function of state variable • Matrix T:relates the effort-flow variables at the electrical port directly to those at the mechanical port

29. Elementary Lumped-parametertransducers: equations • The coupling factor k represents the electromechanical energy conversion in lossless transducers • A coupling factor of 0 means no interactions • A state of equilibrium exists for 0<k<1 • Typical values for k are between 0.05 and 0.25

30. Equivalent circuit representation • There is an analogy in the mathematical descriptions between electric and mathematical phenomena • A series connection in the mechanical circuit becomes parallel in the equivalent electrical circuit

31. Equivalent circuit representation • The construction of the equivalent networks starts with the splinted transfer matrix of the electrostatic transducers • Center matrix: transducer • Left matrix: electrical admittance • Right matrix: mechanical impedance • Each of the constituent can be represented by an equivalent network

32. Equivalent circuit representation • There is no one way to decompose a matrix • But: each decomposition has its own network representation • The choice of which circuit to use is dictated by the application

33. Coupling of the transducersto the outside world • The exchange of energy of the transducer and the outside world is done trough ports • Laws of equilibrium link the transducer via their port relation to the external elements • Electrical parts: Kirchoff’s voltage and current laws • Mechanical parts: Newton law (∑Fi=0) and continuity of space (∑Ui=0, U: incremental velocity) • The mechanical laws are directly obtained by invoking the Kirchoff’s laws to the mechanical part in the equivalent circuit representation

34. Examples of lumped-parameterselectromecanical sytems • Condenser Microphone • In-plane parallel microresonators • Vibration sensors • Electromechanical feedback

35. Condenser microphone, force and displacement transducers • Operating principles: • the force to be measured is exerted on the mass. • The motion of the mass is converted into an electrical signal, a current, which flows in part through a resistors → production of an output voltage. • This voltage is a measure for the applied force or displacement.

36. Condenser microphone, force and displacement transducers

37. Condenser microphone, force and displacement transducers • If the applied force is the result of an acoustic pressure, the transducer can be used as an electrostatic or condenser microphone

38. In-plane parallelmicroresonators • In-plane parallel microresonators using electrostatic interdigitated structures for excitation and detection of the vibrational motion are used as transducing elements in a wide variety of applications

39. Vibration sensors • Vibration sensors are employed for measurements on moving vehicles, on buildings, or on machinery or as seismic pickups • The basic principle of vibration measurements is simply to measure the relative displacement of a mass connected to the vibrating body. • The transducer detects the mass displacement Xm relative to the displacement Xin of the vibrating body

40. Electromechanical feedback • Electromechanical feedback (or force balancing) is often employed for applications requiring a great accuracy • The system measures the force Fm exerted directly on the mass • The upper capacitor senses the induced mass displacement resulting in a change of the plate charge

41. Electromechanical feedback • The output voltage of the charge amplifier Va is next amplified by a high-gain (servo) amplifier T • The output voltage Vout is fed back to the lower capacitor. • This generates an electrostatic force which is proportional to the relative mass displacement and which always opposes motion of the mass from the rest position. • This way, the mass itself is kept very close to the zero-displacement position

42. Conclusion • The majority if the circuits presents 3 different parts: • An electrical part • An electromechanical coupling part • A mechanical part • The equivalent circuits can be used to determine the frequency and the transient response of the transducer • The equivalent circuit theory applied to the study of transducer characteristics is a basis for further investigations