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Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →An instrumentation amplifier is often asked to measure a millivolt-level difference while both input wires sit at several volts—or carry the same mains, inverter, or ground-loop interference. Common-mode rejection ratio (CMRR) describes how effectively it amplifies the wanted difference while suppressing the voltage shared by both inputs. It is essential, but it works only while the inputs and output remain in their specified linear ranges and the external circuit preserves the required symmetry.
Differential voltage versus common-mode voltage
For input voltages V1 and V2, the desired differential signal is:
VD = V2 − V1
The voltage shared by the two inputs is:
VCM = (V1 + V2)/2
For example, if V1 = 10.000 V and V2 = 10.002 V, the signal is 2 mV even though the common-mode level is 10.001 V. Battery monitors, bridge sensors, thermocouples, shunt resistors and remote sensor cables routinely present this combination. Analog Devices discusses small differential measurements in large common-mode environments at this application article.
Common-mode interference can come from 50/60-Hz pickup, ground-potential differences, motor-inverter switching, a battery-stack voltage, or electromagnetic coupling into a balanced twisted pair. A disturbance is rejected only to the extent that it reaches both inputs with nearly equal amplitude, phase and impedance.
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- Low Offset Voltage: 25 µV (Maximum), G ≥ 100
- Low Drift: 0.1 µV/°C, G ≥ 100
- Low Noise: 50 nV/√Hz, G ≥ 100
What CMRR means
A practical instrumentation-amplifier output can be represented as:
VOUT = ADVD + ACMVCM + VREF
AD is differential gain, ACM is common-mode gain, and VREF is an intentional output shift when the device provides a reference pin. CMRR is:
CMRR = AD/ACM
CMRRdB = 20 log10(AD/ACM)
Equivalently, the approximate input-referred error from a common-mode voltage is:
VERR,IN ≈ VCM/10CMRRdB/20
| CMRR | Common-mode conversion | Error from 1 V common mode |
|---|---|---|
| 60 dB | 0.001 | 1 mV input-referred |
| 80 dB | 0.0001 | 100 µV input-referred |
| 100 dB | 0.00001 | 10 µV input-referred |
| 120 dB | 0.000001 | 1 µV input-referred |
With 100 dB CMRR, 1 V of common mode ideally creates about 10 µV at the input. At gain 100 that becomes approximately 1 mV at the output. This calculation excludes offset, drift, bias-current, noise, reference, range and frequency errors.
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TI defines CMRR and its instrumentation-amplifier role in its Precision Labs material.
How an instrumentation amplifier rejects common mode
The three-op-amp architecture
- Input amplifiers buffer each sensor lead, providing high input impedance and establishing most of the gain.
- Differential subtractor subtracts the two amplified paths.
- Reference connection shifts the output level, useful when a bipolar sensor signal must fit a single-supply ADC.
Cancellation depends on the two paths having the same gain and phase. Integrated devices place precision resistors and matched amplifier structures close together, improving tracking over temperature and reducing board-level matching work. TI describes this design emphasis in its instrumentation-amplifier overview.
Why resistor ratios dominate a subtractor
In a simple difference amplifier, the ratios should satisfy:
R2/R1 = R4/R3
Common-mode cancellation is controlled mainly by ratio matching, not by the nominal resistance value. A first-order estimate is CMRR ≈ 1/ε, where ε is fractional ratio error:
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| Ratio mismatch | Approximate CMRR |
|---|---|
| 1% | 40 dB |
| 0.1% | 60 dB |
| 0.01% | 80 dB |
| 0.001% | 100 dB |
These are limits for an idealized circuit, not guarantees. Temperature tracking, amplifier mismatch, source impedance, parasitic capacitance and layout can make the result worse. TI covers resistor and layout effects at sboa422 and in its CMRR measurement note. Matched networks can improve ratio tracking; Analog Devices discusses that approach at this article.
What limits CMRR in a real circuit?
Frequency and transients
CMRR is not one fixed number. DC, 50/60-Hz, 1-kHz and switching-edge rejection can differ substantially. At higher frequency, unequal phase shift, input capacitance, filter poles and trace parasitics convert common mode into differential voltage. A common-mode step can cause a transient error even when steady-state CMRR is excellent. Review CMRR-versus-frequency curves and transient data; see Analog Devices’ frequency discussion and its common-mode transient note.
Input-source and filter imbalance
Unequal source resistance lets input bias currents create unequal voltage drops. An RC filter fitted to only one lead, or built with different resistor or capacitor values, turns common-mode noise into a differential signal. Match both components, their placement and their bias-current return paths. Capacitor tolerance and dielectric behavior matter as well.
Board, cable and connector asymmetry
- Route the pair together in the same layer and surroundings.
- Use twisted-pair wiring for remote sensors.
- Keep both traces away from clocks, motor nodes and high-dv/dt switching copper.
- Place matched protection and filter parts symmetrically.
- Keep the reference return low impedance and quiet.
- Place supply bypass capacitors close to the amplifier pins.
- Use a deliberate shield and grounding strategy rather than relying on CMRR to fix a ground loop.
Reference, supply and downstream errors
Noise on the reference pin appears directly at the output. Supply ripple is a PSRR problem, not a CMRR problem. The ADC driver, anti-alias network and ADC can also convert common mode into error. Therefore an amplifier’s datasheet CMRR is not the CMRR of the complete sensor-to-code system.
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CMRR is different from range, swing and PSRR
- Input common-mode range: whether each input voltage is within the linear input-stage range. A tiny differential signal does not make an out-of-range absolute voltage safe.
- Output swing: whether G(V2 − V1) + VREF fits the output limits under the actual load.
- PSRR: rejection of supply-rail changes, separate from rejection of signals on the input pins.
- Noise and drift: input voltage noise, 1/f noise, bias-current noise, offset and temperature drift remain even with perfect common-mode cancellation.
- Transient and overload behavior: clipping or recovery after a common-mode step invalidates the small-signal CMRR model.
For example, the TI INA818 page lists CMRR separately from input common-mode headroom. Those specifications must be checked together.
Worked design example
Suppose a bridge produces a 2 mV maximum differential signal while both inputs sit at 10 V. With gain 100, the wanted output is 200 mV. If CMRR is 100 dB, the 10 V common-mode voltage contributes approximately:
10 V / 100,000 = 100 µV input-referred
After gain 100, that is about 10 mV output-referred. Whether 10 mV is acceptable depends on the ADC range and total accuracy budget. Offset, drift, resistor and reference errors may add to it, and a 100-dB figure may apply only at a stated gain, frequency, temperature and common-mode voltage.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to select an instrumentation amplifier
- Calculate the minimum and maximum voltage on each input, including fault and transient conditions.
- Calculate the differential range and choose gain so the largest output, including the reference shift, remains linear.
- Convert the permitted measurement error into an allowed common-mode error, then determine the required CMRR at the actual interference frequency.
- Check guaranteed minimum versus typical CMRR, gain, temperature, supply and common-mode conditions.
- Verify input common-mode range, output swing, reference-pin range, overload recovery and protection ratings.
- Compare input noise, 1/f noise, offset, drift, bias current and source resistance.
- Check bandwidth and settling at the selected gain; high gain commonly reduces bandwidth.
- Evaluate the input filters, cabling, connector, ADC driver and ADC as part of the same error budget.
- Confirm package, supply range, qualification, lifecycle and availability for the intended geography and design life.
Illustrative current devices
| Device | Vendor-listed signal | Selection caveat |
|---|---|---|
| TI INA818 | Minimum CMRR 110 dB; gain 1–10,000; 4.5–36 V supply | Check rail headroom, bandwidth and conditions behind the minimum rating. |
| TI INA630 | Minimum CMRR 130 dB; gain 20–1,000; 4.5–36 V supply; typical 40 nV/√Hz at 1 kHz | Not a unity-gain or very-low-voltage choice. |
| TI INA326 | 2.7–5.5 V supply; rail-to-rail input/output; minimum CMRR 100 dB | Check bandwidth and whether 100 dB is sufficient at the interference frequency. |
| Analog Devices AD8226 | Application material presents a programmable, high-speed, high-CMRR front end | Use the device datasheet for guaranteed electrical limits and the exact gain/bandwidth trade-off. |
| LT5400/LT5401 networks | Matched resistor options including 0.01%, 0.025% and 0.05% | A network does not correct op-amp mismatch, range violations or layout asymmetry. |
Vendor figures are condition-dependent; do not compare them without matching gain, frequency, temperature, common-mode voltage and typical-versus-guaranteed status. Pricing and stock vary by package, quantity, geography and date and are not specified here.
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- Note: 1. There may be slight size deviations due to manual measurement, different measuring methods and tools. 2. The picture may not reflect the actual color of the item because of different photographing light, angle and display monitor.
Measuring CMRR without measuring the fixture
Apply the same controlled AC voltage to both inputs and measure the residual output. A commonly used form is:
CMRRdB = 20 log10(VCM,IN/VOUT,CM)
The exact equation depends on gain and the datasheet test convention. Use a balanced source, equal cables and connectors, matched source impedances, a quiet reference, stable supplies and a known output load. Keep the generator, oscilloscope and amplifier from sharing unintended ground paths. At 100 dB, the residual is one hundred-thousandth of the applied signal, so generator imbalance, cable capacitance, board leakage and instrument pickup can dominate. Test DC, 50/60 Hz and the frequencies or common-mode edges that exist in the application. Report whether the result is device-, board- or system-level.
Common design mistakes
- Choosing the largest headline number: compare the full conditions, not just a typical DC value.
- Ignoring absolute input voltage: calculate both input pins, not only their difference.
- Using unmatched filters: match values, layout and return paths.
- Building a precision subtractor from ordinary resistors: ratio mismatch can set a 40–60 dB ceiling before amplifier errors are considered.
- Leaving the reference pin vulnerable: drive it from a low-noise, low-impedance source within its specified range.
- Confusing CMRR with shielding: unequal pickup is differential and cannot be removed by the amplifier.
- Ignoring the ADC: verify the complete analog front end and conversion reference.
Bottom line
CMRR is the ratio that lets an instrumentation amplifier distinguish a small differential measurement from a much larger shared voltage. High CMRR is necessary for precision sensing, but it is not a license to exceed input or output limits and it cannot repair an asymmetric sensor interface. Select the amplifier at the required frequency and gain, preserve impedance and physical symmetry from cable to ADC, and verify the complete system with a balanced, well-controlled test.
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