Common-Mode Rejection Ratio (CMRR) — Overview
CMRR quantifies how effectively a current-sense amplifier rejects common-mode input voltage variations, expressed as the ratio between differential voltage gain and common-mode voltage gain. Practically, it links changes in input common-mode voltage to resulting changes in input offset voltage, which can impact current-measurement precision.
Key Parameters Mentioned in the Document
| Parameter | Where/How It Is Used in the Guide |
|---|---|
| CMRR | Defined as the ratio of differential voltage gain to common-mode voltage gain; typically provided in datasheets in dB (DC focus discussed). |
| Input offset voltage (Vio / IO) | Datasheet value is valid for a specified datasheet common-mode voltage (icmDS); must be re-estimated when application common-mode voltage (icm) differs. |
| Datasheet common-mode test condition (icmDS) | Unless otherwise specified, parameters are guaranteed for a common-mode voltage icmDS (example uses icmDS = 12 V). |
| Example device conditions | TSC2010 bidirectional current-sense amplifier example: datasheet test condition icmDS = 12 V. Example offset value mentioned for TSC2012 at room temperature and Vicm = 12 V: Vio is 500 µV max at 25 C. |
Setup and Use: Estimating CMRR-Induced Offset Error
- Identify the datasheet common-mode voltage icmDS used to specify the input offset voltage IO/Vio.
- Determine your application common-mode voltage icm.
- If icm differs from icmDS, estimate the additional offset due to CMRR using: Offset due to CMRR = ((icm – icmDS) * 10^(CMRR/20)).
- Add this CMRR-induced offset term to the datasheet IO/Vio to obtain total current-measurement error contribution.
- When evaluating accuracy, compare the CMRR-related error with the error due to IO (ideally same order of magnitude).
Troubleshooting CMRR-Related Measurement Issues
- Symptom: Output/current reading error increases when the application common-mode voltage differs from the datasheet test condition. Cause: Vio/IO is only guaranteed at icmDS, and CMRR causes additional input offset variation with common-mode voltage. Action: Recalculate the CMRR-induced offset using your icm and the datasheet CMRR, then include it in the overall accuracy calculation.
- Symptom: More error at low current (for a chosen device). Cause: CMRR has the most significant impact at low current (as stated in the CMRR error summary). Action: Mitigate CMRR-related error by either increasing shunt resistance or selecting a device with lower input offset voltage and higher CMRR.
- Symptom: Unexpected degradation in rejection of common-mode signals. Cause: Common-mode rejection degradation can originate from internal input-stage mismatch and from common-mode gain caused by gain-resistor mismatch. Action: Prefer designs/components emphasizing resistor matching and good CMRR, as highlighted in the guide.
FAQ
- What does CMRR mean for a current-sense amplifier?
- CMRR is the ratio of differential voltage gain to common-mode voltage gain, and in practice it indicates how input offset voltage changes when the input common-mode voltage changes.
- Why is CMRR specified in dB in datasheets?
- The guide notes that CMRR is usually given in dB to emphasize that it represents a ratio; the dB value is converted to a linear gain ratio when estimating offset variation.
- When does CMRR affect current-measurement accuracy the most?
- According to the summary, CMRR has the most significant impact at low current.
- How do I account for CMRR if my application common-mode voltage differs from the datasheet?
- Use the datasheet reference common-mode voltage icmDS for the specified IO/Vio, then estimate the additional offset from CMRR using Offset due to CMRR = ((icm – icmDS) * 10^(CMRR/20)) and add it to the datasheet offset.
- What strategies does the guide suggest to minimize CMRR-related offset error?
- The guide suggests two ways: increase the shunt resistance value, and choose a device with lower input offset voltage and higher CMRR.
Documents / Resources
 | Exploring Current Sense Amplifiers |
References
- st.comwww.st.com
- st.com/www.st.com
- st.com/trademarkswww.st.com
- st.com/en/amplifiers-and-comparators/current-sensing.htmlwww.st.com
- st.com/en/amplifiers-and-comparators/tsc2010.htmlwww.st.com
- st.com/en/amplifiers-and-comparators/tsc2011.htmlwww.st.com
- youtube.com/watch?v=AWcZ3SpFf-M&list=PLTJzs51NlEIAbbaIen42VfvXpeW3-Yh-M&index=6www.youtube.com
- youtube.com/watch?v=mwiyJ3Y4ULo&list=PLTJzs51NlEIAbbaIen42VfvXpeW3-Yh-M&index=4www.youtube.com
- User Manualmanual.tools
Published: August 8, 2026