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[FAQ] Common Mode Voltage Range vs. Input Voltage Range for a Comparator

Intellectual 1790 points

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What is the difference between Input voltage range and common mode voltage range for a comparator?

  • A common definition of input common mode voltage range (CMVR) that is used for Operational Amplifiers (Op-Amps) is the average voltage of the inverting and non-inverting input voltages. This is acceptable for an Op-Amp because they use negative feedback and the inputs are typically kept within 1 mV of each other.

    Comparators very rarely have inputs so close in voltage value. Most use cases for comparators have a large differential between the input pins. While the average of the two inputs could be in the common-mode range, one of the inputs could have a value outside of the input range spec.

    An example would be a comparator with an input voltage limit of 3.5V, and one input is a 4V DC signal and the other is a 2V DC signal. Under the Op-Amp definition, the inputs are valid with an average of 3V. However, this is violation as both input voltages must be under 3.5V and the 4V signal violates the input range specification.

    The Absolute Maximum input voltage range specifies the voltage range where the input ONLY remains undamaged and performance is not warrantied when outside the specified input operating range.

    To summarize, input common mode voltage range specifies the voltage range for each input for proper device operation. Do NOT “average” input voltages for comparators.

    Please reference this PowerPoint slide:

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