Increase Common-Mode Range For Fully Differential Amplifiers

Aug. 4, 2003
Fully differential amplifiers have grown in popularity because of their low distortion, excellent noise rejection, and the simplicity of interfacing them with differential analog-to-digital converters (ADCs). This application shows how to increase the...

Fully differential amplifiers have grown in popularity because of their low distortion, excellent noise rejection, and the simplicity of interfacing them with differential analog-to-digital converters (ADCs). This application shows how to increase the common-mode range of a fully differential amplifier by using a few external resistors (Fig. 1).

The circuit uses a voltage-divider technique to reduce the voltage at the inputs of the differential amplifier. This method was adapted from a technique employed on a monolithic instrumentation amplifier (INA148) for use with the fully differential amplifier. Implementing the given configuration, the amplifier's common-mode range is approximately ±90 V, with a differential gain of approximately 1.0. Even wider common-mode ranges could be achieved using a larger R1/R2 ratio.

Three important considerations when executing this topology are determining the gain, the maximum common-mode range, and the common-mode rejection. Modeling the differential amplifier as a voltage-controlled voltage source is the basis of the analysis (Fig. 2).

Assuming the resistors are equivalent in both halves of the amplifier simplifies the analysis (i.e., R1a = R1b = R1). Equation 1 gives the gain of the amplifier.

This simplified equation will always yield a zero common-mode output. Performing a more detailed analysis that uses unique values for all resistances produces a far more complex equation.

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