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Common Mode Rejection Ratio (CMRR): Which Components It Characterizes?

Common Mode Rejection Ratio (CMRR): Which Components It Characterizes?

Verified Sources
Nov 4, 2026

The common mode rejection ratio is a measure of how effectively a differential amplifier suppresses common-mode signals (the same voltage/noise appearing on both inputs) while amplifying only the differential component. In circuit terms, it is defined as the ratio of differential gain to common-mode gain: CMRR=∣AdAcm∣CMRR = \left|\dfrac{A_d}{A_{cm}}\right|.

Because instrumentation amplifiers are specifically designed to amplify small differential signals in the presence of large shared (common-mode) interference, CMRR is a key parameter of an instrumentation amplifier.

In practice, other blocks may mention related concepts, but the canonical, design-critical meaning of CMRR is tied to differential input stages—i.e., instrumentation amplifiers—not oscillators, and not the core “static” performance spec of a general ADC or power supply (unless they explicitly discuss input-referred common-mode rejection in a differential measurement context). 2

Mermaid concept map of what CMRR compares:

Footnotes

  1. Wikipedia — Common mode rejection ratio (CMRR) https://en.wikipedia.org/wiki/Common_mode_rejection_ratio - Definition and gain-ratio relationship for CMRR. ↩ ↩2

  2. Analog Devices (education/amp theory, instrumentation amplifier and CMRR usage) — Instrumentation amplifier basics & CMRR context https://www.analog.com/en/education/education-library/instrumentation-amplifiers.html - Explains instrumentation amplifiers and CMRR as a key parameter for rejecting common-mode signals. ↩

  3. Wikipedia — Power supply rejection ratio (PSRR) https://en.wikipedia.org/wiki/Power-supply_rejection_ratio - Distinguishes PSRR (supply rejection) from input common-mode rejection. ↩

CMRR (Common-Mode Rejection Ratio) Explained

Multiple-choice answer

Correct choice: (ii) Instrumentation amplifier.

Why: Instrumentation amplifiers are differential-input devices used to measure tiny signal differences under substantial common-mode interference. Their performance is therefore dominated by differential vs. common-mode gain behavior, which is exactly what CMRR quantifies.

Footnotes

  1. Analog Devices (education/amp theory, instrumentation amplifier and CMRR usage) — Instrumentation amplifier basics & CMRR context https://www.analog.com/en/education/education-library/instrumentation-amplifiers.html - Explains instrumentation amplifiers and CMRR as a key parameter for rejecting common-mode signals. ↩

What CMRR is measuring (and why it matters)

For a differential amplifier:

  • AdA_d is differential gain (how much the output responds to Vd=V+−V−V_d = V^+ - V^-)
  • AcmA_{cm} is common-mode gain (how much the output responds to the common component VcmV_{cm})

Thus CMRR=∣AdAcm∣CMRR = \left|\dfrac{A_d}{A_{cm}}\right| (often expressed in dB). High CMRR means common-mode interference is strongly rejected.

Key terms:

  • CMRR
  • Differential gain
  • Common-mode gain
  • Common-mode voltage

Footnotes

  1. Wikipedia — Common mode rejection ratio (CMRR) https://en.wikipedia.org/wiki/Common_mode_rejection_ratio - Definition and gain-ratio relationship for CMRR. ↩

How to reason about which component has a meaningful CMRR

  1. 1
    Step 1

    CMRR is defined for systems that compare two inputs and can reject a shared (common-mode) signal.

  2. 2
    Step 2

    Instrumentation amplifiers are explicitly used to reject common-mode noise while preserving differential signals.

  3. 3
    Step 3

    Oscillators do not operate as differential input amplifiers with a CMRR specification for input noise rejection.

  4. 4
    Step 4

    Unless a datasheet models a differential common-mode input and publishes common-mode rejection behavior, CMRR is not the primary ‘key parameter’ in the standard sense used for differential amplifiers.

  5. 5
    Step 5

    Therefore, the best match is (ii) instrumentation amplifier.

Why the other options are not “the key parameter” (in standard usage)

(i) Power supply

Power supplies are characterized primarily by regulation, ripple/noise, line/load transient response, etc. While supply noise can couple into circuits, that is typically handled through PSRR (power-supply rejection ratio) rather than CMRR.
So CMRR is not the headline “key parameter” for a generic power supply.

PSRR

(iii) Oscillator

Oscillators are evaluated by frequency stability, phase noise, tuning range, startup time, etc. Common-mode rejection ratio is not a standard core spec because oscillators are not specified as differential-input amplifiers with common-mode gain behavior in the same way.

(iv) ADC

ADC datasheets commonly emphasize:

  • resolution, sampling rate, ENOB/SNR/SINAD
  • input impedance, differential/non-differential input ranges
  • effective common-mode behavior when applicable

However, the most universal and explicitly defined “CMRR” specification is classically associated with differential amplifiers/instrumentation amplifiers.

ADC

Footnotes

  1. Wikipedia — Power supply rejection ratio (PSRR) https://en.wikipedia.org/wiki/Power-supply_rejection_ratio - Distinguishes PSRR (supply rejection) from input common-mode rejection. ↩

  2. Wikipedia — Phase noise / oscillator performance concepts (oscillators are typically characterized differently) https://en.wikipedia.org/wiki/Phase_noise - Shows typical oscillator specifications are phase noise/frequency stability rather than CMRR. ↩

  3. Analog Devices (education/amp theory, instrumentation amplifier and CMRR usage) — Instrumentation amplifier basics & CMRR context https://www.analog.com/en/education/education-library/instrumentation-amplifiers.html - Explains instrumentation amplifiers and CMRR as a key parameter for rejecting common-mode signals. ↩

Exam strategy

When a question asks about CMRR, interpret it via the definition: a ratio of differential gain to common-mode gain—so look for differential input amplifier blocks, especially instrumentation amplifiers.

Don’t confuse CMRR with PSRR

CMRR rejects common-mode at the inputs; PSRR rejects power-supply variations. Data sheets often publish both, but they belong to different coupling paths.

Conceptual mapping: where CMRR is central vs. secondary

CMRR is central when the block is explicitly a differential amplifier with defined differential/common-mode gains.

Where CMRR fits in signal-chain design

Differential acquisition

1. Front-end sensing

Small differential signals are measured in the presence of larger common-mode interference—CMRR matters."

Noise rejection & gain

2. Instrumentation amplification

Instrumentation amplifiers maximize differential gain while minimizing common-mode gain."

Signal conditioning

3. Anti-aliasing / filtering

Filters reduce bandwidth and interference but do not replace the need for input CMRR."

Digitization

4. ADC conversion

ADC focuses on SNR/ENOB; common-mode concerns are handled by front-end architecture and ADC input behavior."

CMRR fundamentals in one place

Knowledge Check

Question 1 of 3
Q1Single choice

The common mode rejection ratio (CMRR) is a key parameter of:

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