Unit 2 Lect 2 preprocessing for sensor interfacing

lecture ppt

1. Preprocessing of Sensors

2. P r actical Applic a tion: V o ltage Di v ision Fo r mula A Wheatstone bridge is a measuring instrument used to measure an unknown electrical resistanc e . Unknown Resistor V a ri a ble r e sistor R x is the unknown resistance to be measure d ; R 1 , R 2 and R 3 are resistors of known resistance and the resistance of R 3 is adjustabl e . The W h e at s ton e B r id ge

3. With strain gage is connected, P r ac ti cal Applic atio n - V o ltage Di v ision Fo r mula -  

4. P r ac ti cal Applic atio n - Strain Gauge - V o ltage Di v ision Fo r mula -   The output of the bridge changes nonlinearly with applied force. H o wever, R is several hundred ohms, while ∆R is typically 0.01 Ω, a factor of 10,000 times smaller. So, we assume that   Small strain produced measurable voltage 

5. OPERATIONAL AMPLIFIER (Op Amp) Historically an Op Amp was designed to perform such mathematical operations as addition, subtraction, integration and differentiation. Hence the name Operational Amplifier.

6. ICs

7. Fairchild Semiconductors µA 702 (1963) µA 709 (1965) µA 741 (1968) µA 748 (1968) National Semiconductors LM101 (1967) LM101A (1968) LM107 (1968) LM324 (1974)

8. 741 Op-Amp –Internal Circuitry

9. Op Amp – A multistage Amplifier Differential Amplifier stage High gain CE amplifier stage Class B push pull emitter follower Differential Amplifier More stages of gain Class B Push pull Amplifier

10. Op Amp Equivalent Circuit

11. Ideal Op Amp Z in = ∞ Z out = 0 A V = ∞ Practical Op Amp Z in = 2MΩ Z out = 100Ω A V = 10 5

12. Voltage Transfer Characteristic Range where we operate the op amp as an amplifier. V in

13. APPLICATIONS

14. Non-Inverting Amplifier

15. Inverting Amplifier

16. Op-Amp Summing Amplifier

17. Op-Amp Differential Amplifier If R 1 = R 2 and R f = R g :

18. Op-Amp Integrator V o = -

19. Op-Amp Differentiator

20. Low-pass Filter (active) Cutoff frequency This works because the capacitor needs time to charge .

21. High pass filter (active)

22. Applications of Op-Amps Electrocardiogram (EKG) Amplification Need to measure difference in voltage from lead 1 and lead 2 60 Hz interference from electrical equipment

23. Simple EKG circuit Uses differential amplifier to cancel common mode signal and amplify differential mode signal Realistic EKG circuit Uses two non-inverting amplifiers to first amplify voltage from each lead, followed by differential amplifier Forms an “instrumentation amplifier”

24. Strain Gauge Use a Wheatstone bridge to determine the strain of an element by measuring the change in resistance of a strain gauge ( No strain) Balanced Bridge R #1 = R #2 ( Strain) Unbalanced Bridge R #1 ≠ R #2

25. Half-Bridge Arrangement Using KCL at the inverting and non-inverting terminals of the op amp we find that  ε ~ V o = 2 Δ R( R f /R 2 ) R + Δ R R f + - + V __ + V cc - V cc - + R f V ref R R - Δ R R Op amp used to amplify output from strain gauge

26. Op Amps Applications Audio amplifiers Speakers and microphone circuits in cell phones, computers, mpg players, boom boxes, etc. Instrumentation amplifiers Biomedical systems including heart monitors and oxygen sensors. Power amplifiers Analog computers Combination of integrators, differentiators, summing amplifiers, and multipliers

27. THANK YOU