A cable shield is a Faraday cage that blocks external electromagnetic interference (EMI) from reaching the signal conductors inside. Its effectiveness depends entirely on how the shield is terminated at both ends. Incorrectly terminated shields (or shields that inadvertently create ground loops) can inject more noise into the signal than they remove.
This article establishes the engineering principles of cable shielding for servo systems and provides specific guidance for motor cable shields, encoder cable shields, and the ground loop conditions that arise when both are connected to the same installation.
How Cable Shielding Works
A Faraday cage is a conductive enclosure that attenuates the external electromagnetic field at its interior. A cable shield acts as a tubular Faraday cage for the conductors it encloses. For the shield to be effective:
- The shield must be conductive: braided tinned copper or aluminum foil with drain wire provides good shielding effectiveness
- The shield must be connected to the reference: an ungrounded shield accumulates charge and can re-radiate the collected interference
- The termination must be 360° around the conductor: connection via a pigtail wire (flying lead) reduces the effective shield coverage significantly at high frequencies
Ideal shield termination: Both ends terminated 360° via the cable connector shell to the chassis ground. The connector shell makes full circumferential contact with the chassis or enclosure.
When 360° termination is not possible: Keep any flying lead connection as short as physically possible. A 5 cm flying lead acts as an antenna at frequencies above approximately 600 MHz.
For typical servo PWM noise (switching frequencies 2–20 kHz), shorter leads reduce but do not eliminate radiated coupling.
Inner and Outer Shields for Analog Encoder Signals
Analog encoder signals (sin/cos outputs) are particularly sensitive to crosstalk from adjacent conductors carrying motor phase currents or other high-current signals.
A cable with both an outer shield and inner shields on individual twisted pairs provides two-layer protection:
- Outer shield: Terminates at both ends to chassis. Blocks the large fraction of external EMI.
- Inner shield (per twisted pair): Connected to the servo drive’s signal ground (not chassis). Provides crosstalk isolation within the cable between the sin/cos pairs.
The combined approach: outer shield blocks most external noise; inner shield blocks remaining noise and prevents crosstalk between the differential sin and cos pairs.
Note: Inner shielding is a compromise, it provides some benefit but is not as effective as outer shielding because inner shields are typically grounded at only one end (to avoid creating a ground loop within the cable). Grounding at only one end means the inner shield does not form a closed Faraday cage for low-frequency interference.
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Specific Shielding Configuration for Motor and Encoder Cables
A common installation scenario: encoder signal noise that persists after improving motor case grounding. An effective shielding configuration for this case:
- Motor cable shield: Connected to both the motor case (at the motor end) and to the servo drive chassis (at the drive end). This provides a low-impedance return path for motor phase current noise to the PSU.
- Encoder cable shield: Connected only to the servo drive chassis (at the drive end). The motor end of the encoder cable shield is left unconnected (or connected through a small capacitor).
Critical accompanying requirement: The PSU (power supply unit) return must be connected to the chassis ground. This ensures that noise generated by the drive’s PWM output has a low-impedance return path to the PSU, without routing through the signal ground or encoder cable.
If the PSU return is not connected to chassis, the drive’s switching noise returns through the signal cables, directly injecting PWM frequency interference into the encoder signal path.
Ground Loops: What They Are and Why They Cause Noise
A ground loop occurs when two connected devices have signal grounds at different potentials, creating a closed circuit through which an unwanted current flows.
Two common ground loop formation mechanisms:
1. Connected signal grounds: When the signal grounds of two devices are connected via both the signal cable and a separate path through the building wiring and earth, a closed loop exists. Any potential difference between the earth connection points (VG) drives a current around the loop.
2. Shield terminated at both ends: When a cable shield is connected to chassis at both ends, and each chassis is connected to earth, a closed loop is formed through the shield, the earth wiring, and the building structure.
In both cases, VG (the ground loop voltage) has two components:
- DC component: From physical separation of earth ground connections at different potentials
- AC component: From the loop acting as an antenna coupling 50–60 Hz AC power line fields, a phenomenon known as “hum”
The hum current induced in the ground loop couples into adjacent signal conductors through resistive and inductive means, appearing as a 50/60 Hz noise component on the encoder signal.
Ground Loop Mitigation Techniques
Technique 1: Break the Ground Loop, Single-End Shield Termination
Terminate the cable shield at only one end. This eliminates the closed circuit.
Tradeoff: The shield is less effective because it is not a complete Faraday cage. A single-terminated shield provides good protection against capacitively coupled noise (which needs a reference) but reduced protection against inductively coupled noise.
Technique 2: Capacitor Bridge
Instead of terminating the shield at one end, insert a small capacitor between the shield and chassis at one end:
- Value: 100 pF is typical.
- Effect: Blocks DC potential differences (preventing DC current flow in the loop) and attenuates lower-frequency AC hum currents.
- The capacitor is transparent at high frequencies (EMI frequencies), where the shield remains fully effective.
Some specialized connectors incorporate a built-in capacitor in the connector shell — enabling 360° high-frequency shield termination while blocking the DC ground loop path.
Technique 3: Resistor in Signal Ground Path
For RS-422 and similar differential interfaces that require a shared ground connection to keep common-mode voltages within specification:
- Insert a 100 Ω resistor between the signal ground conductor and the earth/chassis ground.
- This limits the ground loop current to milliampere levels without eliminating the ground reference needed for common-mode voltage compliance.
Installation Best Practices Summary
Beyond shielding, a complete servo motor installation should include:
- Star grounding at the chassis, all PE connections meeting at one point
- Differential signaling for all encoder connections
- Line termination on digital encoder cables where required by data rate
- Motor and signal cables routed separately, do not share cable trays where avoidable
- Power and signal cables crossing at 90° where crossing is unavoidable
- Line filter between AC supply and servo drive power supply input (CE marking requirement in many regions)
- AC supply line reactor to protect against power line surges
- Choke on the drive 3-phase output to reduce PWM switching noise, especially important for cable runs exceeding 10 m
- Do not coil cable excess, excess cable loops act as inductors and antennas
- Ground connections to painted or anodized surfaces must be scraped to bare metal
To expand your understanding, take a look at our guide on
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