Guarding vs. Shielding: What to Consider When Specifying a Triaxial Vacuum Feedthrough

Direct Answer

Shielding keeps outside electrical interference from coupling into a measurement. Guarding keeps current from leaking through insulation by holding a surrounding conductor at nearly the same voltage as the sensitive signal. A triaxial vacuum feedthrough can support both functions, with the guard on the middle conductor and a grounded shield on the outside.

Whether it does so in a given system depends on the conductor mapping, the voltage each insulation layer must withstand, and every cable, adapter, and in-vacuum connection along the path. This article explains both functions and gives a checklist for specifying the connection.

Why Small Currents Are Hard to Measure

Many vacuum experiments measure very small currents. Ion collectors, Faraday cups, photodiodes, and semiconductor test structures often produce picoamps (trillionths of an ampere) or less.

At those levels, insulation stops behaving like a perfect barrier. Suppose a signal wire sits at 10 V and its insulation has a resistance of 1 teraohm (10¹² Ω) to a grounded conductor. Ohm’s law gives a leakage current of 10 pA. If the signal of interest is 1 pA, the leakage is ten times larger than the measurement.

Two separate problems cause most of the trouble. Interference is picked up from outside the circuit, and leakage seeps through the insulation inside it. Shielding addresses the first. Guarding addresses the second.

What Shielding Does

An electrostatic shield is a conductive enclosure, such as a cable braid, a metal box, or a chamber wall, that surrounds sensitive circuitry. Nearby charged objects, moving people, and electrical equipment create changing electric fields. The shield intercepts those fields and carries the resulting current away before it reaches the signal conductor. A shield works like a roof: it keeps outside weather off what is underneath.

Shielding should extend all the way to the fixture or device, because any unshielded section can still pick up interference.

The shield’s electrical reference depends on the instrument. It may connect to measurement common (the instrument’s internal reference, often labeled LO), or to chassis ground (the instrument’s metal case, normally tied to the building’s safety earth). The two are not automatically the same point.

Follow the instrument manufacturer’s documentation. Protective-earth connections exist for safety and must never be removed to reduce noise.

Shielding has a limit. A grounded metal sleeve does nothing to stop current from leaking through the insulation between the signal and the sleeve. In the example above, that leakage path is exactly what produced the 10 pA error.

For standard RF or higher-level signal paths where interference rejection is the primary concern, standard coaxial feedthroughs like BNC and SMA provide effective Faraday shielding without the added complexity of a driven guard conductor.

What Guarding Does

A guard is a conductor held at nearly the same voltage as the high-impedance signal it surrounds, and supplied from a low-impedance point in the circuit. “High impedance” describes a sensitive node that a tiny stray current can disturb. “Low impedance” describes a source that can supply current without its voltage changing noticeably.

Leakage needs a voltage difference to flow. If the guard holds the voltage across the insulation to about 1 mV instead of 10 V, the leakage through that 1 TΩ path falls from 10 pA to about 1 fA (a thousandth of a picoamp). Water does not flow between two tanks filled to the same level, and current does not flow between two conductors at the same voltage.

Current still leaks from the guard to ground. That current is supplied by the guard’s own source, so it never passes through the measurement input. A well-designed guard also intercepts leakage from other parts of a fixture and routes it away from the input.

A driven guard is one that an instrument actively holds at the signal’s voltage, usually with a unity-gain buffer amplifier. Source measure units (SMUs), which combine a precision voltage or current source with a meter, drive their guard terminal at the same potential as the output HI terminal. Because the guard also reduces the effective capacitance of the cable, a guarded connection can settle to a stable reading faster.

Guarding does not make insulation perfect, and guard is not another name for ground or signal return. Follow the specific instrument’s connection diagrams rather than copying a wiring convention from another system.

When a Grounded Conductor Already Acts as a Guard

The guard’s job is to sit at the same voltage as the signal. In some measurements, that voltage is ground.

Picoammeters, electrometers in current mode, and SMUs measuring current use a feedback ammeter circuit. This circuit holds its input HI terminal within a fraction of a millivolt of input LO. If LO is grounded, a grounded conductor surrounding the signal is already at nearly the signal’s potential, so very little current leaks between them.

In that configuration, a coaxial or triaxial feedthrough with a grounded shield can be appropriate, and the leakage that matters is usually elsewhere in the circuit, such as across the fixture insulation on the voltage-source side. A driven guard is needed when the sensitive node sits at a voltage well away from ground: sourcing voltage through the same conductor, or measuring a high-impedance voltage or resistance.

Coaxial Versus Triaxial Connections

Coaxial and triaxial describe how conductors are arranged, not a complete measurement setup. A coaxial cable has a center conductor and one surrounding conductor. A triaxial cable adds a second surrounding conductor, which makes it possible to separate the guard from the outer shield.

These are typical arrangements, not universal wiring instructions. Keithley’s instrument documentation shows the outer triaxial conductor tied to different references depending on the instrument and fixture. A triaxial feedthrough is the right choice when the measurement needs a separate guard. The instrument’s documented connection scheme decides that.

What Changes When the Path Crosses a Vacuum Wall

A vacuum feedthrough is a hermetic seal and an electrical component at the same time. Several requirements apply to it that do not apply to an ordinary bulkhead connector.

Each Insulation Layer Sees a Different Voltage

A triaxial feedthrough has two insulation layers: one between the center conductor and the guard, and one between the guard and the grounded shell. They carry very different voltages. With a driven guard, the center-to-guard voltage is close to zero. The guard-to-shell insulation carries the full test voltage, because the guard follows the signal and the shell is grounded.

Instrument cable makers rate these paths separately. Keithley’s CA-554 high-voltage triax cable, for example, specifies leakage below 100 pA center-to-guard but below 100 nA guard-to-shield, both at 3000 V. Ask which path a feedthrough’s voltage rating covers, and confirm that the guard-to-shell insulation is rated for the highest voltage the instrument can source.

Grounded Shells and Ground Loops

A grounded-shield feedthrough electrically ties the cable’s outer shield to the flange, and therefore to the chamber. If the chamber’s ground and the instrument’s ground sit at slightly different voltages, current can circulate through the shield and add noise. This is called a ground loop.

A single, deliberate grounding point usually prevents it. Decide where the outer shield should be referenced before choosing between grounded-shield and floating-shield (isolated-shell) feedthrough designs.

The Vacuum-Side Insulator Is Part of the Circuit

Thin films from sputtering, evaporation, or other deposition processes can coat the vacuum-side ceramic and create a surface leakage path. Leakage from the center conductor to the guard is mostly harmless, because the guard supplies that current. Leakage from the center conductor directly to the grounded shell or flange is not. Sputter shields and insulator geometry that lengthens the surface path both help.

Temperature and Water Change Insulation Resistance

The insulation resistance of alumina ceramic falls steeply as temperature rises. Readings taken during a bakeout, or before the feedthrough has cooled to its working temperature, can show extra leakage that disappears later. The air-side connector is often rated well below the feedthrough body: the A18379 drawing rates the feedthrough to 450°C but its air-side connector only to 165°C. Remove or protect the connector before any bakeout that would exceed its rating.

Water adsorbed on insulator surfaces after venting can do the same thing until it is pumped away. On the air side, a cryogenic flange can let condensation or frost form on exposed insulators, even while the device itself sits in vacuum.

In-Vacuum Cable Matters as Much as the Feedthrough

The wiring between the feedthrough and the device also needs a guard if the measurement does. In-vacuum coaxial and triaxial cables are commonly insulated with polyimide (Kapton) or PTFE. They differ in outgassing, maximum bakeout temperature, radiation tolerance, and how much triboelectric noise they generate when they flex. Choose the cable alongside the feedthrough, not after it.

Confirming a Specific Feedthrough

A feedthrough must preserve the required electrical paths while maintaining the vacuum boundary. A connector that mates mechanically does not prove that the signal, guard, and shield remain separately accessible through the assembly.

For the exact part and drawing revision, confirm:

• Which contacts continue from the air side to the vacuum side.
• Which conductive elements connect to the shell or mounting flange.
• Which elements are isolated from one another, and the voltage rating of each isolation path.
• Which triaxial connector family is used on each side.
• Which mating connectors and cables preserve those relationships.

MPF Products’ A18379 triaxial feedthrough on a 1.33″ CF flange is documented with a grounded shield, the conventional arrangement for the outer conductor of a triaxial measurement system. It is rated for 500 V DC and 3 A.

For broader component-selection context, see Electrical Feedthrough Selection for UHV.

Specify the Complete Measurement Path

Review the path from instrument terminal to in-vacuum device, not just the chamber penetration:

• Instrument: model, measurement mode, terminal functions, guard implementation, and grounding requirements.
• Air-side cable: conductor assignments, cable construction, length, and suitability for low-current work.
• Connectors and adapters: the electrical mapping through every interface, including any change between connector families.
• Feedthrough: continuity, isolation, voltage rating of each path, shell connections, and accessible contacts on both sides.
• In-vacuum connection: wiring, insulators, fixture supports, and the final device interface.

Triaxial bayonet connectors come in 2-lug and 3-lug versions, and the two do not mate without an adapter. In Keithley’s naming, the female jack is 3-lug and the mating male plug is 3-slot. Some older instruments use the 2-lug style, while present-day SMU and electrometer cables generally use the 3-lug family. Specify the family on both sides of the feedthrough.

Adapters deserve particular attention. Some triax-to-BNC adapters, such as Keithley’s 237-BNC-TRX and 7078-TRX-GND, leave the guard disconnected on purpose, so that guard voltage never appears on an exposed shell. Such an adapter mates perfectly and still ends the guard at that point. Likewise, a guard carried to the chamber wall does not protect the fixture beyond it.

Include cable and fixture capacitance in the instrument compatibility review. Long connections slow settling, and instruments can specify separate limits for signal-to-guard, guard-to-shield, and load capacitance.

Safety: The Guard Is Live

A driven guard sits at the same voltage as the signal. If the instrument sources hundreds of volts, the guard conductor carries hundreds of volts too.

That is why the outer conductor of a triaxial system is normally connected to earth ground: it encloses the hazardous conductors. Check that no guard contact is exposed on either side of the feedthrough or on any adapter, and follow the instrument manufacturer’s interlock and grounding instructions.

Engineering Requirements Checklist

Give the component supplier a schematic and a requirements package covering:

• Measurement objective: current, charge, resistance, or high-impedance voltage; expected signal range; allowable offset, leakage contribution, noise, and settling time.
• Instrument configuration: manufacturer, model, measurement mode, connection diagram, guard behavior, maximum source voltage, and any remote-sense connections.
• Electrical path: required independent conductors, contact assignments, signal return, guard, outer shield, and intentional shell or flange connections.
• Voltage and isolation: working voltage for center-to-guard and guard-to-shell paths, and required insulation resistance for each.
• Interfaces: exact cable and connector part numbers, triax connector family, adapter mappings, air-side and vacuum-side terminations.
• Electrical acceptance: continuity and isolation matrix, insulation-resistance or leakage tests, test voltage, test conditions, and documentation.
• Environment: base pressure in Torr, helium leak-rate requirement in atm cc/sec (stated separately), operating and bakeout temperatures, process deposition exposure, cleanliness, and thermal cycling.
• Installation: mounting interface, clearances, cable support, service access, and protection from connector or cable loading.

State the temperature each component will see. A feedthrough on a room-temperature chamber wall faces different conditions from wiring at a cold stage, and the phrase “cryogenic system” does not say which is which.

Common Specification Mistakes

• Selecting by “triaxial” alone. Require the contact mapping, isolation, and voltage ratings the instrument needs.
• Treating every grounded conductor as unsuitable. A grounded shield works as a guard when the input sits near ground, as in a feedback ammeter. It cannot follow a signal held at a voltage away from ground.
• Rating only the center conductor. With a driven guard, the guard-to-shell insulation carries the full test voltage.
• Combining guard, signal return, and protective earth without review. Keep the distinctions the instrument and the safety design require.
• Checking only the feedthrough. Include adapters, in-vacuum cable, and the fixture when evaluating leakage paths.
• Treating hermeticity as electrical qualification. A passing helium leak test does not establish acceptable insulation leakage or measurement performance.

Keep vacuum and electrical acceptance criteria separate. MPF Products’ vacuum feedthrough failure guide explains why a component can remain hermetic while failing to provide its required electrical function.

FAQs

Can cable movement create current even with guarding and shielding?

Yes. When cable insulation rubs against a conductor, friction can generate charge and a small current, called the triboelectric effect. Mechanical stress on some insulators can produce similar piezoelectric currents.

Low-noise cable construction, firm cable support, and time to settle after a cable is moved all reduce these effects. Guarding and shielding do not remove them.

Does an electrically quiet, unbiased connection prove leakage will be acceptable under bias?

No. Leakage depends on the voltage across unintended resistance paths. Evaluate the connection at the intended test voltage and environmental conditions. Some insulators also absorb charge under voltage and release it slowly afterward (dielectric absorption), so allow time after each voltage change.

Can air-side humidity matter when the device is under vacuum?

Yes. Exposed air-side insulators and connectors are still part of the measurement circuit. Moisture and contamination can lower insulation resistance or generate unwanted currents, even though the device itself is inside an evacuated chamber.

How can engineers locate the connection contributing an unwanted offset?

Establish an instrument baseline with the input capped, then add connection elements one at a time. Compare readings as cables, adapters, feedthroughs, and fixtures are introduced. Allow the reading to settle after each change before attributing an offset to a component.

Further Reading

Keithley’s Low Level Measurements Handbook (Tektronix) is the standard engineering reference on guarding, shielding, and low-current error sources.

Closing

A triaxial feedthrough works best when it is specified as one part of a complete measurement path. If you are planning a guarded measurement across a vacuum boundary, MPF Products can review your instrument connection diagram, conductor mapping, voltage requirements, and environmental conditions. The A18379 technical drawing is a useful starting reference for that assembly.

To compare current configurations, browse MPF Products’ triaxial feedthroughs.