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Modern Instrumentation PHYS 533/CHEM 620

Modern Instrumentation PHYS 533/CHEM 620. Lecture 5 Special Purpose Circuits Amin Jazaeri Fall 2007. Unity-Gain Buffer. Unity-Gain Buffer. The input impedance now is very large and equal to:. The output impedance is very small and equal to:. Unity-Gain Buffer.

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Modern Instrumentation PHYS 533/CHEM 620

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  1. Modern InstrumentationPHYS 533/CHEM 620 Lecture 5 Special Purpose Circuits Amin Jazaeri Fall 2007

  2. Unity-Gain Buffer

  3. Unity-Gain Buffer The input impedance now is very large and equal to: The output impedance is very small and equal to:

  4. Unity-Gain Buffer • The value of the voltage follower is to serve in impedance matching. • One can use this circuit to connect, say, a capacitive sensor or, an electronic microphone. • If amplification is necessary, the voltage follower may be followed by an inverting or noninverting amplifier

  5. R R + - VB - R V1 V2 VA + - Vout + R Rs Vcm R R Difference Gain: Instrumentation Amplifier

  6. Instrumentation Amplifier • The gain of an amplifier of this type is: In a commercial instrumentation amplifier all resistances are internal and produce a gain usually around 100. Rs is external and can be set by the user to obtain the gain required.

  7. Instrumentation Amplifier • The output of the instrumentation amplifier is The main use of this amplifier is to obtain an output proportional to difference between inputs. Important in differential sensors, especially when one sensor is used to sense the stimulus and an identical sensor is used for reference (such as when temperature compensation is needed)

  8. Instrumentation Amplifier • Each of the inputs of the amplifier has the high impedance of the Op-Amp used. • The output impedance is low (inverting amp.) • The main problem in a circuit of this type is that the CMRR depends on the matching of the resistances (R, R2 and R3) in each section of the circuit. • These are internal and are adjusted during production to obtain the required CMRR.

  9. Instrumentation Amplifier • The out put voltage can be expressed in terms of Differential and Common mode gains • The Common Mode Rejection Ratio (CMRR) is defined as: • Actual instrumentation Amps can achieve CMRRs of about 105 or larger.

  10. Log and Antilog Amplifiers • The key to obtaining the desired nonlinear output from an op-amp circuit is the use of an appropriate non-linear feedback element. • The current through a diode is expressed as: • Where kT/q is approximately 0.025V

  11. Log Amplifier

  12. Antilog Amplifier

  13. Constant Current Source

  14. Voltage to Current Converter

  15. Voltage to Current Converter

  16. Differentiator Like the inverting amp:

  17. Integrator Like the inverting amp:

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