Reader feedback: Amplifier Question of the Week

Dear Mr. Masi: Greetings! As usual, informative and concise answer to the Jan. 9, “Question of the Week.” [What is the difference between an instrumentation amplifier and an op-amp?] You may wish to note a slight variation on emphasis, as follows: Thanks, Alon Harpaz, Electrical Engineer, Danaher Motion, Dover Instrument, Westboro, MA 01581-0200

02/01/2007


Process Control

Either an op-amplifier or an instrumentation amplifier can be suitable for summing functions when using a differential measurement sensor.


Dear Mr. Masi:

Greetings! As usual, informative and concise answer to the Jan. 9, “Question of the Week.” [What is the difference between an instrumentation amplifier and an op-amp?] You may wish to note a slight variation on emphasis, as follows:

  1. Operational amplifiers are characterized mostly by their very large gain, which (when put into the closed-loop equation of the properly constructed combined circuit) allows various other characteristics of the amplifier to be ignored; thus their input impedance, phase shift (within reason, of course), and other imperfections become negligible compared to the circuit’s main function (be it addition, multiplication, etc.)

  2. Instrumentation amplifiers are circuits (made either from discrete components or monolithic) that are characterized by extremely high input impedance and common-mode rejection ratio; the circuit copied from Wikipedia amply illustrates these characteristics. They are targeted to be used as signal conditioners for transducers, which have “fragile” outputs, such as low-level signals, high output impedance, floating relative to ground, etc.; thus the high input impedance and CMRR requirements.

 

  • Thanks,
    Alon Harpaz, Electrical Engineer, Danaher Motion, Dover Instrument, Westboro, MA 01581-0200





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