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Biomedical Instrumentation

Biomedical Instrumentation. Prof. Dr. Nizamettin AYDIN naydin @ yildiz .edu.tr naydin @ ieee.org http:// www.yildiz .edu.tr/~naydin. Amplifiers and Signal Processing. Applications of Operational Amplifier In Biological Signals and Systems.

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Biomedical Instrumentation

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  1. Biomedical Instrumentation Prof. Dr. Nizamettin AYDIN naydin@yildiz.edu.tr naydin@ieee.org http://www.yildiz.edu.tr/~naydin

  2. Amplifiers and Signal Processing

  3. Applications of Operational AmplifierIn Biological Signals and Systems • The three major operations done on biological signals using Op-Amp: • Amplifications and Attenuations • DC offsetting: • add or subtract a DC • Filtering: • Shape signal’s frequency content

  4. Ideal Op-Amp • Most bioelectric signals are small and require amplifications Op-amp equivalent circuit: The two inputs are 1 and 2. A differential voltage between them causes current flow through the differential resistance Rd. The differential voltage is multiplied by A, the gain of the op amp, to generate the output-voltage source. Any current flowing to the output terminal vo must pass through the output resistance Ro.

  5. Inside the Op-Amp (IC-chip) 20 transistors 11 resistors 1 capacitor

  6. Ideal Characteristics • A =  (gain is infinity) • Vo = 0, when v1 = v2 (no offset voltage) • Rd =  (input impedance is infinity) • Ro = 0 (output impedance is zero) • Bandwidth =  (no frequency response limitations) and no phase shift

  7. Two Basic Rules • Rule 1 • When the op-amp output is in its linear range, the two input terminals are at the same voltage. • Rule 2 • No current flows into or out of either input terminal of the op amp.

  8. Inverting Amplifier o 10 V i -10 V 10 V Rf i i Slope = -Rf / Ri Ri - i o -10 V + (b) (a) • An inverting amplified. Current flowing through the input resistor Ri also flows through the feedback resistor Rf. • The input-output plot shows a slope of -Rf/ Riin the central portion, but the output saturates at about ±13 V.

  9. Summing Amplifier Rf R1 1 - R2 o 2 +

  10. Example 3.1 • The output of a biopotential preamplifier that measures the electro-oculogram is an undesired dc voltage of ±5 V due to electrode half-cell potentials, with a desired signal of ±1 Vsuperimposed. Design a circuit that will balance the dc voltage to zero and provide a gain of -10 for the desired signal without saturating the op amp.

  11. Answer 3.1 • We assume that vb, the balancing voltage at vi=5 V. For vo=0, the current through Rf is zero. Therefore the sum of the currents through Ri and Rb, is zero. +10 Rf Ri i 100 kW 10 kW i - i + b /2 +15V Rb o 0 Voltage, V 20 kW Time 5 kW + v b -15 V -10 o (a) (b)

  12. Follower ( buffer) • Used as a buffer, to prevent a high source resistance from being loaded down by a low-resistance load. In another word it prevents drawing current from the source. - o i +

  13. Noninverting Amplifier o i i 10 V Rf Ri Slope = (Rf + Ri )/ Ri -10 V 10 V i - o -10 V i +

  14. Differential Amplifiers • Differential Gain Gd • Common Mode Gain Gc • For ideal op amp if the inputs are equal then the output = 0, and the Gc = 0. • No differential amplifier perfectly rejects the common-mode voltage. • Common-mode rejection ratio CMMR • Typical values range from 100 to 10,000 • Disadvantage of one-op-amp differential amplifier is its low input resistance v3 v4

  15. Instrumentation Amplifiers Differential Mode Gain Advantages: High input impedance, High CMRR, Variable gain

  16. Comparator – No Hysteresis +15 R1 ui - v2 uo R1 uref + -15 R2 uo 10 V -10 V uref ui -10 V v1 > v2, vo = -13 V v1 < v2, vo = +13 V If (vi+vref) > 0 then vo = -13 V else vo = +13 V R1 will prevent overdriving the op-amp

  17. Comparator – With Hysteresis uo R1 With hysteresis 10 V ui - uo -10 V R1 10 V uref + - uref ui R2 R3 -10 V • Reduces multiple transitions due to mV noise levels by moving the threshold value after each transition. Width of the Hysteresis = 4VR3

  18. Rectifier o 10 V R D1 D2 -10 V 10 V (1-x)R xR i - -10 V i + (b) R D4 D3 - + (a) • Full-wave precision rectifier: • For i > 0, D2 and D3 conduct, whereas D1 and D4 are reverse-biased.Noninverting amplifier at the top is active xR (1-x)R v o D2 - + i (a)

  19. Rectifier o 10 V R D1 D2 -10 V 10 V (1-x)R xR i - -10 V i + (b) R D4 D3 - + (a) • Full-wave precision rectifier: • For i < 0, D1 and D4 conduct, whereas D2 and D3 are reverse-biased. Inverting amplifier at the bottom is active xRi R i v o D4 - + (b)

  20. One-Op-Amp Full Wave Rectifier Ri = 2 kW Rf = 1 kW i v o D - RL = 3 kW + (c) • For i < 0, the circuit behaves like the inverting amplifier rectifier with a gain of +0.5. For i > 0, the op amp disconnects and the passive resistor chain yields a gain of +0.5.

  21. Logarithmic Amplifiers Rf/9 Ic Rf Ri - i o + (a) • Uses of Log Amplifier • Multiply and divide variables • Raise variable to a power • Compress large dynamic range into small ones • Linearize the output of devices (a) A logarithmic amplifier makes use of the fact that a transistor's VBE is related to the logarithm of its collector current. For range of Ic equal 10-7 to 10-2 and the range of vo is -.36 to -0.66 V.

  22. Logarithmic Amplifiers v o 10 V -10 V 10 V i 1 -10 V 10 (b) (a) With the switch thrown in the alternate position, the circuit gain is increased by 10. (b) Input-output characteristics show that the logarithmic relation is obtained for only one polarity; 1 and 10 gains are indicated. VBE Rf/9 Ic VBE 9VBE Rf Ri - i o + (a)

  23. Integrators for f < fc A large resistor Rf is used to prevent saturation

  24. A three-mode integrator With S1 open and S2 closed, the dc circuit behaves as an inverting amplifier. Thus o = ic and o can be set to any desired initial conduction. With S1 closed and S2 open, the circuit integrates. With both switches open, the circuit holds o constant, making possible a leisurely readout.

  25. Differentiators • A differentiator • The dashed lines indicate that a small capacitor must usually be added across the feedback resistor to prevent oscillation.

  26. Active Filters- Low-Pass Filter Rf Ri • A low-pass filter attenuates high frequencies - ui uo + (a) |G| Rf/Ri 0.707 Rf/Ri freq fc = 1/2RiCf

  27. Active Filters (High-Pass Filter) • A high-pass filter attenuates low frequencies and blocks dc. Rf Ci Ri - ui uo + (b) |G| Rf/Ri 0.707 Rf/Ri freq fc = 1/2RiCf

  28. Active Filters (Band-Pass Filter) Cf • A bandpass filter attenuates both low and high frequencies. Ci Rf Ri - ui uo + (c) |G| Rf/Ri 0.707 Rf/Ri freq fcL = 1/2RiCi fcH = 1/2RfCf

  29. Frequency Response of op-amp and Amplifier • Open-Loop Gain • Compensation • Closed-Loop Gain • Loop Gain • Gain Bandwidth Product • Slew Rate

  30. Input and Output Resistance - Ro uo Rd ud ii + io Aud - ui RL + CL Typical value of Ro = 40  Typical value of Rd = 2 to 20 M

  31. Phase Modulator for Linear variable differential transformer LVDT + - + -

  32. Phase Modulator for Linear variable differential transformer LVDT + - + -

  33. Phase-Sensitive Demodulator Used in many medical instruments for signal detection, averaging, and Noise rejection

  34. The Ring Demodulator • If vc is positive then D1 and D2 are forward-biased and vA = vB. So vo = vDB • If vc is negative then D3 and D4 are forward-biased and vA = vc. So vo = vDC

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