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Biomedical Instrumentation I

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  1. Biomedical Instrumentation I Chapter 7 Bioelectric Amplifiers from Introduction to Biomedical Equipment Technology By Joseph Carr and John Brown

  2. Bioelectric Amplifiers • Definition: An amplifier is used to process biopotential • Adjust gain • DC coupled: needed when signal is very slowly changing or is dc i.e. O2level may change pressure mmHg per min or hour • AC coupled: need to overcome electrode offset • Frequency Response: range that amplifier can work over i.e. ECG is 0.05 to 100 Hz • Low Frequency Response or High Frequency Response: frequencies where gain drops 3 dB below its mid frequency value. • Gain Types of Amplifiers: • Low Gain Amplifier: gain between 1 and 10 where unity or 1 is most common (used for Isolation, buffering and possibly impedance transformation between signal source and readout device) • Medium Gain Amplifier: gain between 10 and 1000 used for ECG, EMG etc. • High Gain Amplifier: gain greater than 1000 used for EEG

  3. Important Design Parameters • Noise: want to keep noise as low as possible so keep thermal and shot noise low by choosing proper components and shielding wires • Drift: want to keep drift at a minimum where drift is spurious changes in output signal caused by change in operating temperature rather than input signal chagnes • High Input Impedance: Needed with all three types of amplifiers because almost all bioelectric signal sources have high source impedance of 103 to 107Ώ • Engineering Rule of thumb is to have input impedance at least 1 order of magnitude higher than source impedance

  4. +V - Inverting Output Noninverting + -V Operational Amplifier • Circuit Diagram where power supply is bipolar (positive voltage to ground and negative voltage to ground) Inverting Input Produces an output signal 180o out of phase of the input signal Noninverting Input Produces an output Signal in phase with the input signal Inverting and Noninverting input have Same gain but different phase effects • +V and –V can be DC or AC (note this circuit has no ground terminal -> ground connection is between 2 power supplies) +V + - Ground + - Notice how –V is negative to ground -V

  5. Differential Signals: when E1 ≠ E2 then op amp will see a differential input of E2 –E1 thus output ~ gain (E2-E1) Common Mode Signals: Signal Voltages are common or the same to both inputs or E1 = E2 such that the input = 0 so the output = 0 Common Mode Rejection Ratio (CMRR): expression of how close an op amp is to ideal where the common mode signal has no effect on output terminal voltage +V E3 - E1 Output E2 + -V

  6. 741 Properties of an Ideal Operational Amplifier • Infinite Open Loop (no feedback) voltage gain Avol = infinity • Zero Output impedance (Zo = 0) • Infinite Input impedance (Zi= infinity) • Infinite Frequency Response • Zero noise Contribution • Both Inputs follow each other in feedback circuits

  7. Implications of Ideal Properties on Operational Amplifiers • Infinite open loop gain : gain without any feedback; thus the closed loop characteristics of the circuit are determined entirely by the properties of the feedback loop network and are independent of amplifying device • Output impedance = 0 : ideal voltage source • Input impedance = infinity : input terminals neither sink nor source any current thus don’t load any circuit to which they are connected • Input tend to follow each other: thus can treat inputs as if they were the same (if voltage is applied to noninverting input then we treating inverting input as if it were the same in other words have same voltage at other input)

  8. R1 - A Vinput + Voutput R1 R2 0 Vinput Voutput I1 I2 Inverting Followers R2 Noninverting Input is grounded thus inverting input is ground by definition of property 6 (slide6) and is called a virtual ground Kirrchhoff’s Current Law = Σ of all currents entering = Σ leaving the summation Ohm’s Voltage Law = V = IR Point A = Summing Junction R1 = Input Resistance R2 = Feedback Resistance Transfer Function =>

  9. 12KΏ - A Vinput + Voutput Example of Inverting Amplifier • What is the gain of the inverting amplifier if R2 = 120KΏ and R1 = 12KΏ 120KΏ Note: R1 and R2 determine the magnitude of the Gain and the sign tells if the output Is in phase (+) or 180o out of phase (-)

  10. R2 R1 - A + Voutput Vinput 0 Vinput Voutput I1 I2 Non Inverting Amplifier Noninverting Input is grounded thus inverting input is ground by definition of property 6 (slide6) and is called a virtual ground Kirrchhoff’s Current Law = Σ of all currents entering = Σ leaving the summation Ohm’s Voltage Law = V = IR Point A = Summing Junction R1 = Input Resistance R2 = Feedback Resistance R1 R2

  11. Non Inverting Amplifier R2 R1 - A + Voutput Vinput Calculate the voltage gain of the following noninverting amplifier when R2 = 10KΏ and R1 = 2 KΏ Can this circuit ever be an attenuator? (meaning have the gain be less than 1)

  12. Unity Gain Noninverting Followers • Connect output directly to input using a wire. Circuit is used for output buffering and impedance matching between a high source impedance and a low-impedance input circuit - A + Voutput Vinput

  13. Multiple Inverting Input Circuits } Rf Ohm’s Law: I = V/R R1 Vinput R2 - Kirrchhoff’s Current Law = Σ of all currents entering = Σ leaving the summation A R3 + Voutput R1 Vinput I1 R2 0 I2 R3 Rf Voutput I3 If

  14. Multiple Non Inverting Input Circuits } Rf Ohm’s Law: I = V/R R1 R2 - Kirrchhoff’s Current Law = Σ of all currents entering = Σ leaving the summation A R3 + Voutput Vinput R1 0 I1 R2 Vinput I2 R3 Rf Voutput I3 If

  15. Homework Problems • Read rest of Chapter 7 and begin Ch8 • Problems 1-10 • Problem 6 described on page 143. Where the input resistances (“R1 or Rin”) must be 10 times larger than the source impedance looking into the amplifier • Problem 7 and 8 refer to figure 7-19 not figure 7-20

  16. Review • Three key points on how to solve Op Amp Gain Equations • Derivation of Gain Equation • Inverting Op Amps • Non Inverting Op Amps