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    Ee1320 sensors readout and signal conditioning Ee1320 sensors readout and signal conditioning Presentation Transcript

    • EE1320: Measurement ScienceLecture 3:Sensor Readout and Signal ConditioningDr. ir. Michiel Pertijs, Electronic Instrumentation LaboratoryMay 7, 2013 Delft University of Technology Challenge the future
    • Course program 2013week date topic4.1 Tu 23/4 #1 intro measurements and meas. systems Fr 26/4 #2 sensors4.3 Tu 7/5 #3 sensor readout and signal conditioning4.4 Tu 14/5 #4 instrumentation amplifiers We 15/5 intermediate test4.5 Tu 21/5 #5 analog-to-digital converters4.6 We 29/5 #6 measurement instruments I4.7 Tu 4/6 #7 measurement instruments II We 5/6 intermediate test4.8 Tu 11/6 tutorial4.11 We 3/7 final examLecturer: dr. ir. Michiel Pertijsroom HB 15.050, M.A.P.Pertijs@tudelft.nl, 015-2786823 Measurement Science (EE1320) – Lecture 3 2
    • Last time… Sensors DATA ACQUISITION measurement analog-to- object signal sensor digital conditioning conversiontransduction of informationfrom a (non-electrical) domainto the electrical domain• self generating / modulating• direct / tandem transduction Measurement Science (EE1320) – Lecture 3 3
    • Today: sensor readout and signal conditioning DATA ACQUISITION measurement analog-to- object sensor signal digital conditioning conversion• Why signal conditioning?• Readout of • thermocouples with a non-inverting amplifier • photodiodes with a transimpedance amplifier • capacitive sensors with a charge amplifier• Additionally: effects of non-ideal properties of opamps Measurement Science (EE1320) – Lecture 3 4
    • Overview study material• Regtien 12 (except 12.1.4, 12.1.5): • Properties of opamps and basic opamp circuits (recap) • Non-ideal properties of opamps• Regtien 13.1.1, 13.1.2 • Integrator en differentiator• Regtien 5.2: Signal models – equivalent error sources Measurement Science (EE1320) – Lecture 3 5
    • Importance of signal conditioning• Output signal of sensors is often not suitable for ADC input • too small - input range ADC not used effectively • too sensitive - load of sensor by ADC yields errors • too much interference / noise - ADC saturates without filtering • wrong format - e.g. resistive sensor on voltage-input• Signal conditioning adapts the sensor signal to the ADC • amplification (scaling) • buffering • filtering • conversion Measurement Science (EE1320) – Lecture 3 6
    • Importance of signal conditioning - without scaling Us Us,max Us t Do Us Do unused Do range 0101 sensor ADC t 0000 1111 Measurement Science (EE1320) – Lecture 3 7
    • Importance of signal conditioning - with scaling Uv,max Uv Us, Uv Us t Do How? Us Uv Do Do often usingsensor amplifier ADC t 0000 opamp circuits! 1111 Measurement Science (EE1320) – Lecture 3 8
    • Opamps – remember them? 1 kΩ The output voltage Uo is: 1 kΩ +10 V A –1 V B 2V1V C –2 V Uo –10 V D –10 V Measurement Science (EE1320) – Lecture 3 9
    • Opamps – remember them? 1 kΩ The output voltage Uo is: 1 kΩ +10 V A 1V B 2V Uo C –2 V 1V –10 V D –10 V Measurement Science (EE1320) – Lecture 3 11
    • Opamp – ideal behavior • Amplifies differential input voltage with very high I+ (ideally infinite) gain AUin A Uout U out = A ⋅ U in = A ⋅ (U + − U − ) I− A→∞ • Input currents are zero: I+ → 0 I− → 0 Regtien 12.1 Measurement Science (EE1320) – Lecture 3 13
    • Opamp – ideal behavior FEEDBACK NETWORK • Feedback network (with passive components) determines transfer I+ • Stable negative feedback ⇒Uin A Uin → 0 (“virtual ground”) I− Uout • Zero-conditions at the input: U in = U + − U − → 0 I+ → 0 I− → 0 Regtien 12.1 Measurement Science (EE1320) – Lecture 3 14
    • Thermocouple readout using a non-inverting amplifier • Thermocouple senses temperature difference: Metal a U ab = α ab ⋅ (T1 − T2 ) Metal b • Sensitivity determined by Seebeck coefficients Metal c αa and αb: α ab = α a − α b • Example: chromel/alumel (“type K” thermocouple) α ab = 40 µV/K Regtien 7.2.4 Measurement Science (EE1320) – Lecture 3 15
    • Thermocouple readout using a non-inverting amplifier • Given: • measurement range Metal a 0 ≤ T1 – T2 ≤ 100 K • ADC input range Metal b 0 ≤ Uo ≤ 1 V Metal a • chromel/alumel couple αab = 40 µV/K 1V• Determine the required gain G= = 250 40 µV/K ⋅ 100 K• Dimension the resistors R1 + R2 e.g. R1 = 249 kΩ, G= R2 R2 = 1 kΩ Regtien 12.1.3 Measurement Science (EE1320) – Lecture 3 16
    • Opamp offset voltage• Ideal opamp: Uout = 0 when Uin = 0• Actual opamps have an offset: Uout = 0 at Uin ≠ 0• Offset voltage Uos: input voltage Uin for which Uout = 0 Uout Uout typical values: • standard opamp: ±1 mV • precision opamp: < ±10 µV Regtien 12.2.2 Measurement Science (EE1320) – Lecture 3 17
    • Thermocouple readout with offset • Given: Metal a • opamp offset voltage |Uos| < 1 mV Metal b • chromel/alumel couple Metal a αab = 40 µV/K• Determine the maximum measurement error due to the offset voltage Measurement Science (EE1320) – Lecture 3 18
    • Combining sources of error • Sources of error can occur at many positions in the chain measurement analog-to-x object sensor signal digital y conditioning conversion ε1 ε2 ε3 ε5 ε4 • For instance: offset, distortion, crosstalk, cross sensitivity • How to determine their combined effect on the measurement? Regtien 5.2 Measurement Science (EE1320) – Lecture 3 20
    • Combining sources of error • Determine the effect of the individual sources on the output measurement analog-to- ∆y1x signal ∆ytotal … object sensor digital conditioning conversion ∆y5 ε1 ε2 ε3 ε5 ε4 • If the system is linear (or can be linearized), then: ∆y total = ∑ ∆y i for systematic sources of error (e.g. offset) ∆y total = ∑ ∆y i2 for uncorrelated stochastic 2 sources of error (e.g. noise) Regtien 5.2 Measurement Science (EE1320) – Lecture 3 21
    • Equivalent sources of error • Translate the output-referred error back to the input DATA ACQUISITION∆xeq measurement analog-to- ∆y1 signal ∆ytotal … object sensor digital conditioning conversion ∆y5 ε1 ε2 ε3 ε5 ε4 • ∆xeq is called the equivalent input error • Example: what’s the effect of ε3 on the measurement? • Determine output error due to ε3 ⇒ ∆y3 • Translate this back to the input ⇒ ∆xeq,3 • ε3 causes an equally large error as an error ∆xeq,3 at the input would Regtien 5.2 Measurement Science (EE1320) – Lecture 3 22
    • h⋅c E = h ⋅ν = λPhoto diode Typical sensitivity• Photons lead to a photocurrent Iph of a Si photodiode• Characteristic properties: • spectral sensitivity R [A/W] • dark current Idark [A] • quantum efficiency η [%]: electrons / photon Regtien 9.1.2 Measurement Science (EE1320) – Lecture 3 23
    • Photodiode readout using a transimpedance amplifier • transimpedance amplifier = current-voltage converter Uo = Rf ⋅ Iph paragraaf 6.2 Measurement Science (EE1320) – Lecture 3 24
    • Photodiode readout using a transimpedance amplifier • Given: • spectral sensitivity Rspec = 0.5 A/W • sensitive area A = 1 mm2 • light intensity range 0 ≤ P ≤ 1 W/m2 • ADC input range 0 ≤ Uo ≤ 1 V• Determine the required feedback resistance Regtien 12.1.1 Measurement Science (EE1320) – Lecture 3 25
    • Opamp offset current and bias current• Ideal opamp: I+ → 0, I− → 0• Practical opamps require bias current Ibias• Moreover, input currents are not exactly equal: offset current: Ios = Ibias1 – Ibias2 Ibias1 values vary strongly depending on opamp type • MOS: pA ~ nA Ibias2 • bipolar: nA ~ µA often highly temperature dependent! Regtien 12.2.2 Measurement Science (EE1320) – Lecture 3 27
    • Modeling offset and bias current with equivalent sources Uos Ibias1 Ibias2 ⇔ Ibias2 Ibias1 Practical opamp Ideal opamp with offset and without offset and bias currents bias currents Regtien 12.2.2 Measurement Science (EE1320) – Lecture 3 28
    • Photodiode readout with offset and bias current • Given: • dark current photodiode Idark ≤ 5 nA • opamp offset voltage |Uos| < 1 mV • opamp bias current Ibias < 10 nA • opamp offset current |Ios| < 1 nA• Determine maximum offset voltage • Rf = 2MΩ at the output (in mV)• Determine the equivalent input offset in terms of the photocurrent (in nA) Measurement Science (EE1320) – Lecture 3 29
    • Readout of capacitive sensors using a charge amplifier• Capacitive sensor: Cs = ε0 ⋅ ε r ⋅ A ∆A d ∆d• Cs changes due to • displacement ∆d, ∆A • change in dielectric constant ∆εr• Example: humidity sensor porous electrode moisture-sensitive polymer Regtien 7.2.3, 20.2.2 (Fig. 20.16) Measurement Science (EE1320) – Lecture 3 31
    • Readout of capacitive sensors using a charge amplifier • Charge amplifier • resembles an inverting amplifier • Transfer: Zf jωCs C Uo = − Ui = − Ui = − s Ui Zs jωCf Cf • Requires AC voltage Ui (ω > 0) Regtien 13.1.1 Measurement Science (EE1320) – Lecture 3 32
    • Readout of capacitive sensors with offset and bias current • What is wrong?? Regtien 13.1.1 Measurement Science (EE1320) – Lecture 3 33
    • Readout of capacitive sensors with offset and bias current • “Tamed” integrator: feedback resistance Rf • DC transfer determined by Rf: ∆Uo = −U os + (Ibias + Ios ) ⋅ Rf 1 • For ω > transfer Rf Cf Cs determined by Cf: U o = − Ui Cf Regtien 13.1.1 Measurement Science (EE1320) – Lecture 3 35
    • Differential readout of capacitive sensors Why differential structure??• Differential structure ⇒ Uo = 0 for x = 0• Differential structures eliminate offsets and even-order non-linearities. Measurement Science (EE1320) – Lecture 3 36
    • Summary• Signal conditioning needed to adjust sensor output signal to input of ADC• Opamps are suitable building blocks for sensor readout • non-inverting amplifier (example: thermocouples) • transimpedance amplifier (example: photodiodes) • charge amplifier (example: capacitive sensors)• Opamp’s non-ideal properties influence the transfer • offset voltage, bias current, offset current • modeled with equivalent sources at input of opamp • can be translated to equivalent input- or output-referred errors Measurement Science (EE1320) – Lecture 3 37
    • What’s next?• Study: • Regtien chapter 12, sections 5.2, 13.1.1, 13.1.2• Practice • See the exercises on Blackboard!• Questions, things unclear? Let me know! M.A.P.Pertijs@tudelft.nl Next time: instrumentation amplifiers! Measurement Science (EE1320) – Lecture 3 38