CHAPTER 2. EE 5344 Intro to MEMS - Interface Circuitry. ( x)

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1 Open Lp vs lsed Lp Gain HAPTE EE 53 Intr t MEMS - Interface ircuitry v () s G () s Y () s Y ( s) G( s) ( s) pen Lp n feedback v ( S ) - G () s Y () s e ( s) ( s) Y ( s) H ( s) Y ( s) G( s) e( s) () s H Slve: Y ( s) G( s) lsed-lp () s G() s H () s Feedback Sensr Signals Interfacing Prcessr takes the signal frm the sensr, cnditins it, and cnverts it t an understandable frmat. It may perfrm Analg t Digital cnversin. Mst sensrs are analg. Mst prcessing is dne in digital frm. mmnly measured electrical parameters:. esistance. apacitance 3. Inductance Measurement f esistance The prcessr measures the change in the resistance f the sensing element. ( ) linear resistance sensr (r ) Baseline resistance fractinal change Eamples: Blmeters, magnetresistrs /interface circuitry

2 Usually <. In the case f >>, a ptential divider may be used. Buffer ltage Divider System gain: A ( ) Sensitivity S d d ( ) Since S is nt cnstant, nt linear. S Linear nly fr << If is small <% If is large >> S [ ( ) ] S [ ] almst linear Make linear by making >> /interface circuitry

3 3 ltage dividers are nt very sensitive t small changes in. A better chice is a Wheatstne Bridge. 3 3 The bridge is nulled by adjusting such that 0. Then Hwever, fr small, it is cmmn t use the circuit in unbalanced mde, measure and amplify it. apacitance Measurement apacitance is the amunt f charge induced fr a given vltage change. dq if Linear d q planar, parallel-plate capacitr ε r : relative permittivity A: area A A ε ε r ε d: separatin f plates d d ε : permittivity f space If a phenmenn changes A, ε r r d, the can be measured t detect the phenmenn. 3/interface circuitry

4 δ d dε A d, d δε δa da ε, d d dd ε, A δd Eamples: I radiatin can change ε r Frce can change d. apacitive ltage Divider: ~ Mre accurate way is: Bridge. Series Arrangement: χ χ ~ χ jω slve jω Frm the real part: /interface circuitry

5 5 Frm the imaginary part: Parallel Arrangement: ~ ( jω ) ( jω ) jω Slve jω Frm the real part Frm the imaginary part: Inductance Measurement: Self Inductance L: Where dφ L N di N: Number f wire turns Φ : Magnetic flu i: urrent flwing thrugh the wire Magnetmtive frce ( mmf ) Ni eluctance L mmf Φ There d Ni d Φ di di N L L 5/interface circuitry

6 6 And dφ L N di N L Analgy Between Electric and Magnetic ircuits Electric I current emf Magnetic Φ magnetic flu F mmf Fr a slenid AArea f cre l L l l µ µ A µ A r µ permeability f free space µ r relative permeability f the cre Mst sensrs perate by changing airgap l r mving magnetic cre and changing µ This mdifies the L and there L, which is sensed by induced vltage. di L dt There is als mutual inductance between r mre cils: M 6/interface circuitry

7 7 Eample Linear ariable Differential Transfrmer (LDT) i p M L in L M L mtin di ( M p M ) dt Measures displacement Interface Electrnic ircuitry Prcessr r Pre-prcessr circuitry that biases the sensr and cnditins the sensr put. Functins f interface electrnics:. T amplify: Operatinal amplifiers. T reduce nise: Lw-, High-, r band pass filters. Sample and hld circuits, Integratrs. 3. T supply pwer: Biasing circuits fr mdulating sensrs. an be dc r pulse bias.. T cmpensate: Eamples are Linearizatin circuits, cmparatrs, calibratrs. 7/interface circuitry

8 8 I detectr radiatin I detectr in dark cmparatr Anther eample is a shutter in a radiatin detectr t blank the signal. 5. T cntrl Self-testing, checking battery levels etc. 6. T transmit Mst f the time, signal is digitized, multipleed fr transmissin. Eamples are: Frequency mdulatin: Signal can be cnverted t frequency mdulatr. Digitizatin: AD (analg-t-digital cnverters) cnvert the analg signal t digital, which is easier t transmit, manipulate and nise free. Multipleing: ery useful in the case f detectr arrays. (Fr eample, 030 arrays f piels in fcal plane array f I detectrs). 3 Sensr Output 8 MUX Address (6-bit) Prcessr 8/interface circuitry

9 9 Digital Transmissin: Serial Interfacing S-3 standard (Others are S- and S-3) Dual wire transmissin (simple) Line length < 35 feet Speed < 0 kbits/s in binary channel ntrls device at a time GPIB (IEEE 88) Interfacing General Purpse Instrument Bas, same as IEEE-88, called as HPIB by Hewlett-Packard. equires a specialized cable with 5 lines. Line length < 7 feet Speed < Mbits/s ntrls up t 0 devices at a time. Specialized Sensr Interfacing 9/interface circuitry