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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