Instrumentation Amplifiers Based on Two op Amps

Instrumentation Amplifiers Based on Two op Amps

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An instrumentation amplffier is an electronic circuit that simultaneously yields: high input impedance; high common mode rejection; high stable gain that can be adjusted by a single resistor and without a trade-off between gain and bandwidth (as h...

An instrumentation amplffier is an electronic circuit that simultaneously yields: high input impedance; high common mode rejection; high stable gain that can be adjusted by a single resistor and without a trade-off between gain and bandwidth (as happens in op amps); low value and low drift for offset voltage and currents; and low output impedance. Commercially available units exhibiting these performances are in the form of monolithic, hyhrid, or modular ICs. Hybrid and modular models are based on two basic topologies called "two-op-amp instrumentation amplifler" and "three-op-amp instru- mentation amplifier."

The analysis ofthese basic circuits is ofgreat interest because they can be implemented by using inexpensive discrete components. The performance is adequate for many applications and can sometimes be built quicker and at lower cost than when using integrated units.

Figure 3.39 shows the basic structure for a two-op-amp-based instrumen- tation amplifier. We consider the op amps ideal, then repeat the same steps leading to (3.47-3.50). Then the necessary condition to obtain an inflnite CMRR is also that expressed by (3.53). The output voltage is then

instrumentation amplifiers based on two op amps 1.png

instrumentation amplifiers based on two op amps 2.png

Therefore, although it is also necessary to match four resistors, now by means of Rs it is possible to change the gain without affecting the matching of those four-critical resistors. A shortcoming of this circuit is the possible saturation of the first op amp when the common mode input signal is large. To avoid that saturation, the following condition must be fulfilled:

instrumentation amplifiers based on two op amps 3.png

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