Symptom
An analog input reads a steady or wandering offset, saturates at one end of its range, or drops to zero intermittently; a serial link works most of the time and errors when a drive or a large load runs; a scope from either signal conductor to ground shows the same noise on both.
The short answer
Common-Mode Noise
Common-mode noise is a voltage that rides on both conductors of a signal circuit equally with respect to ground, produced by ground potential differences, by capacitive coupling from a power or drive cable onto both wires, or by a drive driving its motor cable and returning current through the earth. A differential input measures only the difference between its two terminals and rejects common-mode voltage until it exceeds the input common-mode range or its rejection ratio runs out, at which point the reading wanders, saturates, or drops out. Confirm it with a scope showing the same noise on each conductor to ground, and fix it with isolation, balanced twisted pair with a single shield ground, common-mode chokes on the cable, and a smaller ground potential difference.
Key points
- Common mode: both wires move together relative to ground. Normal mode: the wires move relative to each other.
- Differential inputs reject common mode only within their common-mode range; beyond it the reading is meaningless.
- Sources: ground potential difference, capacitive coupling onto both wires, drive common-mode current.
- Filters do not remove common mode; isolation, balance, and common-mode chokes do.
- A scope from each conductor to ground is the test: the same waveform on both is common mode.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Ground potential difference between the transmitter and the input | Measure AC and DC volts between the two ground points. The signal conductors carry that voltage as common mode. |
| Capacitive coupling from an adjacent power or drive cable onto both conductors | Twisted pair keeps the coupling equal on both wires, which makes it common mode; a scope shows it on both conductors and the noise tracks the source. |
| Drive common-mode current returning through the earth | Noise at the drive carrier frequency, present only when the drive runs, worse with an unshielded motor cable. |
| Single-ended input measuring a signal referenced elsewhere | A single-ended input references its own common; a signal referenced to a different ground brings the difference in as error. Check the input type in the module manual. |
| Common-mode voltage beyond the input range | Read the specification; many inputs tolerate only around 10 V of common mode, and some serial transceivers less. Measure the voltage between the signal common and the input common. |
| Shield grounded at both ends carrying current that couples into the pair | Clamp the shield for current; lift the field end and re-measure. |
| Non-isolated power supply shared by devices at different ground potentials | Trace the supply commons. A common that is grounded in two places carries the difference. |
Common mode and normal mode
A signal is a voltage between two conductors, or a current through them. Normal-mode noise, also called differential noise, appears between the conductors and adds directly to the signal; it is what a low-pass filter at the input removes. Common-mode noise appears on both conductors together, with respect to ground. In a perfectly balanced circuit it adds nothing to the difference between the conductors and the input never sees it. Real circuits are not perfectly balanced, real inputs reject common mode only up to a limit, and above that limit common-mode noise turns into normal-mode error or drives the input out of its operating range. That conversion is why common mode matters and why the usual filter does not help.
Where the input gives up
An analog input module specifies a common-mode voltage range, the voltage between either terminal and the module common within which it will measure correctly, and a common-mode rejection ratio, how much of the common-mode voltage leaks into the reading. A typical range is a few volts to a few tens of volts; a typical rejection is enough that a volt of 60 Hz common mode produces a small but measurable error. When the common-mode voltage exceeds the range, the input amplifier saturates, and the reading pins, drops, or jumps in a way that has no relation to the signal. Serial transceivers are the same: RS-485 is specified for about minus 7 to plus 12 volts of common mode, and a link with more than that between the grounds of its two ends errors and eventually fails.
Telling it apart
| Test | Common mode | Normal mode |
|---|---|---|
| Scope, each conductor to ground | Same waveform on both conductors | Different, or opposite, on the two conductors |
| Scope, between the conductors | Little or none of the noise visible | The noise is visible |
| Low-pass filter at the input | No improvement | Improves |
| Isolator in the loop | Removes it | Little effect |
| Twisting the pair or improving balance | Reduces conversion to error | Reduces magnetic pickup |
| Voltage between the two grounds | Present and tracks the noise | May be zero |
Diagnostic procedure
- 1
Measure the ground potential
AC and DC volts between the ground at the transmitter or device and the ground at the input. This is the common-mode voltage the circuit has to tolerate at minimum.
- 2
Scope the conductors
With a scope probe grounded at the input common, look at each signal conductor. The same noise on both is common mode. Note its frequency: 60 Hz says ground potential from power current; a few kilohertz with fast edges says a drive.
- 3
Compare with the input specification
Find the common-mode range and rejection in the module manual, and the common-mode limit of the transceiver on a serial link. A measured voltage near or above the range explains the symptom outright.
- 4
Correlate with the source
Watch the common-mode voltage while the suspected drive or load switches. The one that moves it is the source.
- 5
Check the balance
Shield grounded once at the panel, pair twisted, no single-ended input used on a remote signal, no common grounded in two places.
- 6
Isolate and choke
Put an isolator on the loop or use an isolated input, and add a common-mode choke on the cable at the input. Watch the reading. If the source is a drive, install the shielded motor cable and the drive grounding it needs, which reduces the source for every signal.
Fixes
- Isolation: a loop isolator, an isolated input channel, an isolated transmitter, or an isolated serial converter breaks the common-mode path entirely. It is the reliable fix where the ground potential cannot be reduced.
- Balance: twisted pair, a shield grounded at one end, differential inputs, and no single-ended inputs on remote signals. Balance keeps common mode from becoming error.
- Common-mode chokes: a ferrite core with both conductors passed through it several turns presents impedance to current that flows the same way on both, and none to the signal. Effective against drive-frequency common mode, useless at 60 Hz.
- Reduce the source: shielded motor cable bonded at both ends and a proper drive ground keep drive current out of the earth; correcting the grounding system removes the power-frequency potential difference.
- 4-20 mA instead of voltage: a current loop with a single ground is inherently tolerant of common mode within the transmitter compliance.
Frequently asked questions
- My input is differential. Why does it still see the noise?
- Because the common-mode voltage exceeds the range the input can reject, or because the circuit is unbalanced enough that some of the common mode converts to normal mode. Measure the common-mode voltage against the module specification; if it is anywhere near the limit, isolate.
- Will a ferrite core fix 60 Hz common mode?
- No. Ferrite chokes work at kilohertz and above; at 60 Hz they are a piece of wire. Power-frequency common mode comes from ground potential differences and is fixed by isolation and by correcting the grounding system.
- Is common-mode noise the same thing as a ground loop?
- A ground loop is one way common-mode voltage gets applied to a circuit: two ground connections at different potentials. Common mode also arises without any loop, from capacitive coupling and from drive currents. The tests and the fixes overlap, and isolation cures both.
- Why does a drive produce common-mode current?
- The drive switches its output between the DC bus rails thousands of times a second, and the sum of the three output voltages is never zero at any instant. That non-zero sum drives current through the capacitance between the motor cable and ground and between the motor windings and the frame. With a shielded cable bonded at both ends, that current returns on the shield; without one it returns through the plant ground and every signal circuit that touches it.
Related topics
- Ground Loop SymptomsHow a ground loop shows itself: hum on analog signals, offsets that change when large loads switch, serial links that error, and readings that differ by location. The mechanism, the measurements that prove it, and fixes that never lift a safety ground.
- VFD Noise on Analog SignalsAnalog readings that jump or wander when a drive runs, and speed references that make a drive hunt: how the drive output couples into signal wiring, why the input turns kilohertz noise into a slow wander, the tests that pin it on the drive, and the fixes.
- Modbus Device Intermittently OfflineA device that answers most of the time. Termination, bias, timing, and the physical layer details that produce a bus which almost works.
- Ground Loops in InstrumentationTwo grounds at different potentials, one signal circuit between them, and a reading that will not sit still. How to find it and how to prevent it.
- Shield Grounded at Both EndsA shield connected to ground at the panel and again in the field becomes a conductor between two grounds, and its current becomes noise on the pair inside. How to find the second connection, what to measure before lifting it, and when both ends are correct.
- Filtering Analog InputsHow to damp a noisy analog input in the controller without hiding a real process change: the first-order filter, the moving average, the median, where each belongs, and what never to filter.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.