Common-Mode Rejection and CMRR: From Device to Installed RF System
Common-Mode Rejection and CMRR: From Device to Installed RF System
Common-mode voltage, common-mode current, amplifier CMRR, system rejection and choke impedance are related—but they are not interchangeable. Define the ports, reference and measured response before trusting a number in dB.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
Joeri's practical rule is simple: “common mode” is not one specification. It can name a stimulus at a balanced input, a current on a cable exterior, a conversion caused by asymmetry or the impedance inserted by a choke. The cure—and the measurement—depends on which one is present.
Define the Two Input Modes First
For two input voltages measured to the same declared reference, a common voltage convention is:
vd = v1 − v2
vcm = (v1 + v2)/2
Differential voltage is the difference between the conductors. Common-mode voltage is their average relative to the chosen third reference: chassis, shield, ground plane, enclosure, fixture or another return structure.
Current conventions must also be stated. In this article, common-mode current is the sum icm = i1 + i2, using consistent current directions at the port. Instruments and mixed-mode standards may normalize modal currents and reference impedances differently. The result can be valid under either convention, but values cannot be copied between them without checking the definitions.
CMR and CMRR Are Response Terms
Common-mode rejection, or CMR, describes how well a device or system prevents a common-mode stimulus from creating an unwanted response. Engineers also use CMR as a numeric dB abbreviation, so it is unsafe to assume that CMR is only qualitative.
For a linear differential amplifier, let Ad be differential-input gain to a declared output quantity. Let Acm be common-input gain to that same output quantity under compatible conditions. One familiar definition is:
CMRR = |Ad/Acm|
CMRRdB = 20 log10|Ad/Acm|
The ratio is dimensionless. Analog Devices notes that semiconductor terminology is not fully consistent: some documents call the ratio CMRR and its dB form CMR, while others use CMRR for both. Read the test definition, not just the heading.
If Ad = 1 V/V and CMRR is 60 dB at one declared frequency and operating point, then |Acm| = 0.001 V/V. A 100 mV common-mode test signal would ideally produce 100 µV of the declared unwanted output through that linear path. This example says nothing about another frequency, source impedance, common-mode bias or installed cable.
A Datasheet CMRR Is Not a System Guarantee
Device CMRR is measured at defined pins and conditions. Installed-system rejection includes everything before and around those pins:
- the antenna or sensor's two source impedances;
- transformer, balun or coupling-network asymmetry;
- cable length, shield exterior, mast and enclosure return paths;
- connector, protection and bias-network parasitics;
- board layout and component-ratio error;
- the amplifier's common-mode range and linearity; and
- the output cable, receiver and measurement fixture.
A common field that creates exactly the same voltage at the amplifier pins can be rejected according to the amplifier's CMRR. A field that couples unequally into the antenna legs creates a differential input before the amplifier. The amplifier treats that converted component like any other differential signal; high intrinsic CMRR cannot remove it.
This is the boundary that matters in active receive systems. The complete antenna may show poor common-mode rejection even when the active device has excellent bench CMRR, because the dominant conversion happens in the sensor, feed transition, cable or installation.
Source-Impedance Imbalance Converts Common Mode
Two inputs can see the same incident common-mode voltage and still develop a differential error if their source impedances or input admittances differ. The conversion may come from unequal resistance, capacitance, inductance or coupling to the surroundings.
That effect is frequency-dependent. A tiny capacitance difference may be irrelevant at audio frequency and decisive at HF. Texas Instruments' INA148 documentation gives a device-specific example in which only 500 Ω of source-impedance imbalance reduces CMR to about 66 dB. That is not a universal threshold; it demonstrates why source and circuit impedances must travel with the result.
Matching only DC resistance is therefore insufficient for RF. Measure or model the complex impedance of both paths, including protection capacitance, transformer leakage, connector geometry and the sensor's coupling to mast, earth and nearby conductors.
Frequency and Level Bound the Result
CMRR commonly falls as frequency rises because device gain, resistor ratios and parasitic path symmetry change, but the curve need not be smooth or monotonic. Resonances, fixture limits and calibration residuals can create peaks and nulls. Use the complete CMR-versus-frequency curve over the required band.
Also state:
- common-mode bias and signal amplitude;
- differential input amplitude and gain setting;
- supply voltage, load and temperature;
- common-mode and differential source impedances;
- the input common-mode range and output headroom; and
- the measurement noise floor and dynamic-range limit.
A small-signal CMRR number does not apply after an input junction, protection device or amplifier stage clips. Once the common-mode signal drives the circuit nonlinear, rectification, recovery and intermodulation replace the linear gain-ratio model.
Common-Mode Current Is a Different Measurement
On coax, the wanted transmission-line mode carries equal-and-opposite currents on the centre conductor and the inner surface of the shield. A different current can flow on the shield exterior with a return through the antenna, mast, station wiring, earth or capacitance to the surroundings.
A calibrated current probe around the complete coax largely cancels the wanted pair and responds to the residual whole-cable current. That reading is not amplifier CMRR. It establishes current at one position within the probe's frequency and calibration limits. It does not by itself reveal the source, return path, differential conversion, radiation or pattern effect.
ITU-T K.136 defines converted common-mode current as asymmetrical current produced from differential current by cable or network unbalance, and requires control of unwanted current on a shielded line's external conductor in receiver tests. The fixture boundary matters because the test cable can otherwise become part of the result.
Common-Mode Impedance Is Not CMRR
A choke's common-mode impedance is a complex circuit quantity:
Zch,cm(f) = Rcm(f) + jXcm(f)
It is measured in ohms, not as a rejection ratio. The choke changes common-mode current only after it is placed inside the full source, load and return network. Its parasitic capacitance, resonance, winding imbalance, differential insertion loss, voltage, current and temperature limits also matter.
High choke impedance can reduce one exterior-current path yet leave another parallel return. High receiver common-mode input impedance can reduce current while allowing a large common-mode voltage. Neither quantity says how much common stimulus becomes differential output. That conversion is a response measurement.
Use Mixed-Mode S-Parameters Carefully
At RF, a four-port VNA measurement can convert physical single-ended S-parameters into logical differential and common modes. The notation keeps stimulus and response explicit:
-
Sdd21: differential output from differential input; -
Sdc21: differential output from common input; -
Scd21: common output from differential input; and -
Scc21: common output from common input.
A test may define an RF rejection ratio from |Sdd21/Sdc21|, but it is not automatically identical to a low-frequency op-amp CMRR specification. Declare logical-port mapping, mixed-mode reference impedances, terminations, calibration or de-embedding plane and the exact output quantity.
Keysight's balanced-measurement documentation calculates these modes from complex physical-port data. Magnitude and phase both matter; a magnitude-only subtraction can erase the conversion mechanism or create a false null.
A Bench CMRR Test
Measure the differential and common gains separately at the same declared output:
- Define ports and reference. Mark the two inputs, common reference, output, load and calibration plane.
- Characterize the source. Measure amplitude, phase and complex source impedance presented to each input.
-
Apply a differential stimulus. Drive the input pair according to the chosen differential convention and measure
Ad. -
Apply a common stimulus. Drive both inputs equally relative to the same reference without changing their source impedances, and measure
Acm. - Sweep frequency and level. Stay inside common-mode range and linear output headroom; record temperature, supply, gain and load.
- Prove measurement floor. Swap paths, use a known reference or change fixture balance so instrument leakage and source asymmetry are visible.
The simplified output ratio is valid only when the two tests use compatible input definitions. Tying inputs together, changing the fixture or using a different amplitude can alter source impedance and invalidate a direct output-only comparison.
An Installed-System Test
The system question is broader: how much unwanted differential output appears when the installed antenna and cable are exposed to a common-mode disturbance?
- Draw the sensor, both input legs, enclosure, mast, feedline exterior, power/control leads and environmental returns.
- Measure complex balance or mode conversion at the intended reference plane.
- Map exterior cable current at several positions; one point may coincide with a current minimum.
- Inject or identify a repeatable common-mode source without also changing the wanted differential field.
- Measure the declared output response, wanted-signal SNR and blocker behaviour.
- Change one path—route, bond, choke, transformer or source balance—then restore A/B/A to check repeatability.
| Quantity | Unit | What it answers |
|---|---|---|
| Amplifier CMRR | Ratio or dB | Common-input to unwanted-output response relative to differential gain at declared pins and conditions |
| System common-mode rejection | Defined response ratio or dB | Installed response including source balance, conversion, cables, structure and electronics |
| Exterior cable current | A or dB relative to a declared current | Residual whole-cable current at one location within probe limits |
| Common-mode impedance | Complex ohms | Voltage/current relation at a declared port or series path |
| Mixed-mode conversion | Complex S-parameter or dB magnitude | Conversion between declared logical modes in a defined fixture |
The final rule is direct: do not ask one CMRR number to certify an antenna installation. Measure the active device, the source balance, the mode conversion and the exterior current as separate quantities. Then connect them with the complete current path.
Primary and authoritative references
- Analog Devices MT-042 — Op Amp Common-Mode Rejection Ratio
- Texas Instruments AN-1447 — CMRR in Fully Differential Amplifiers
- Texas Instruments INA148-Q1 — Source-impedance imbalance and CMR
- Keysight — Balanced and mixed-mode measurements
- ITU-T K.136 (2022) — Receiver EMC, converted common-mode current and test boundaries
- TDK — Common- and differential-mode choke impedance fixtures
Mini-FAQ
- Are CMR and CMRR always different terms? Not consistently. CMR can be descriptive or a numeric dB result. CMRR is a dimensionless response ratio often expressed in dB, so read the document's definition.
- Does 100 dB device CMRR mean 100 dB rejection in an antenna system? No. Source-impedance imbalance, cable and mast paths, transformer asymmetry, layout and overload can dominate the installed result.
- Can CMRR remove noise already converted to differential mode? No. Once imbalance has created a differential input, the differential signal path treats it like any other differential signal.
- Is common-mode choke impedance the same as rejection? No. Choke impedance is complex ohms. Current or voltage reduction depends on the full common-mode source, return and load network.
- Does one clamp-current reading measure system CMRR? No. It measures residual whole-cable current at one position within probe limits; it does not directly measure common-to-differential output conversion.
- What must accompany an RF CMRR result? State ports, reference, modal convention, source impedances, frequency, level, common-mode range, terminations, calibration plane and uncertainty.