The weak-signal bottleneck is usually the switch
Thermocouples, strain gauges, pressure cells and similar transducers output microvolt or millivolt signals, and they are at their most vulnerable before the first gain stage. A switch placed in that path can inject its own thermal offset, add a leakage route to ground, or attenuate the signal outright. In many measurement chains the multiplexer, not the amplifier or ADC, is the element that sets the floor on accuracy.
Thermal EMF is an offset you cannot calibrate away by luck
A junction of two dissimilar metals generates a Seebeck voltage. In an ordinary relay the sum of these junctions can run to tens of microvolts — enough, across a type-T thermocouple, to read as a sizeable temperature error. SHR AUTOSENSOR’s low-thermal line controls contact materials, lead materials and junction symmetry to hold the added thermal offset within ±1 µV, so the relay contributes almost no voltage of its own to the measurement node.
Guard shield and dielectric strength
Low thermal offset addresses one error term; high-impedance circuits add another. These relays include an internal electrostatic shield that can be tied to a guard potential, which diverts leakage away from the measurement node and lifts the signal-to-noise ratio in high-Z front ends. Where the same channel also needs isolation, the family carries high dielectric strength — the BT series, for instance, specifies a minimum of 2000 V dielectric, so low-thermal switching and isolation do not have to be split across two parts.
Picoamp and femtoamp inputs demand TΩ insulation
Switching pA- or fA-range currents raises the insulation requirement to roughly 1014–1015 Ω, because below that range leakage through the switch becomes comparable to the quantity being measured. Hermetically sealed contacts on ceramic or high-grade polymer insulation are what make the figure reachable. The HVFR series addresses this duty with several configurations, keeping the high-insulation architecture at switching voltages up to 1.0 kVDC.
HVFR and BT series at a glance
- Switching voltage: up to 1.0 kVDC
- Insulation resistance, contact-to-contact: typically to 1012 Ω (1014 Ω in special versions)
- Insulation resistance, contact-to-coil: typically to 1013 Ω (1015 Ω in special versions)
- Dielectric strength, contact-to-contact: 1.5 kVDC
- Dielectric strength, contact-to-coil: 4.5 kVDC
- Thermal offset: below 10 mV standard; below 1.0 mV in the BT series
Typical applications
- Thermocouple scanning and temperature calibration
- Pressure and strain transducer multiplexing
- Electrometer and ionization-chamber readout
- Low-level leakage and insulation-resistance measurement
- Semiconductor parametric test at microvolt and picoamp levels
Layout practices that preserve the specification
A low-thermal relay cannot rescue a board that fights it; the surrounding design has to hold up its end.
- Drive the electrostatic shield from a low-impedance guard that tracks the signal, so leakage flows guard-to-guard instead of crossing the measurement node.
- Keep the signal-path junctions isothermal. A temperature gradient across a terminal block or wire generates its own thermal EMF on top of the relay offset.
- Allow a settling interval after actuation before taking a microvolt reading — the contact junction needs a brief moment to reach thermal equilibrium.
- Keep signal-node insulation clean and flux-free; surface contamination forms a leakage resistor that no relay specification can overcome.
Choosing between the families
For a microvolt-level chain, the BT series is the starting point for its sub-millivolt thermal offset and 2000 V dielectric. For channels that route currents down toward femtoamperes at elevated voltage, move to the HVFR series and its 1014–1015 Ω insulation options. Matching the relay to the dominant error term — offset for low voltage, insulation for low current — is what keeps the switch from becoming the accuracy limit.
About SHR AUTOSENSOR
SHR AUTOSENSOR TECH LIMITED (SHR AUTOSENSOR) manufactures high-voltage reed relays, mercury wetted relays and high-voltage DC contactors. Contact: [email protected] | +86 13761571029 | www.reed-relay.com