Fast edges put the analog front end under scrutiny
A digital oscilloscope has to display ever-shorter pulses and faster edges faithfully, and the fidelity of that display is decided in its analog front end rather than on the screen. One of the less obvious decisions is the switch used for time-range selection, which sits in the feedback loop of the vertical amplifier — directly on the measurement signal path, so every imperfection of the switch becomes an error in the waveform.
Range switching happens in the op-amp feedback loop
A scope offers several time ranges, and selecting one switches different resistive or capacitive elements into the feedback network to set gain and bandwidth. The element doing that switching is therefore carrying the measurement signal itself: its leakage becomes a current error, its capacitance becomes a bandwidth limit, and its on-resistance becomes a gain term.
Why the standard options fall short
- A semiconductor switch brings parasitic capacitance and charge injection into a high-impedance feedback loop, where they couple into the measurement and interact with the DUT.
- A conventional electromechanical relay has cleaner contacts but is bulky, wears out and is costly per channel — a poor match for a dense multi-channel front end.
The HVMR miniature reed relay series is SHR AUTOSENSOR’s part for this position, sitting between the two.
HVMR characteristics for feedback duty
- 1000 V dielectric strength between contacts
- Ground pin and external magnetic shield to hold down crosstalk and stray pickup among tightly packed analog channels
- Low, stable contact resistance on long-life sealed contacts
- Insulation resistance above 10 TΩ to keep leakage out of the signal path
- Small outline for multi-channel vertical boards
What the contact does for the measurement
A reed contact in the feedback loop gives the scope a genuine mechanical open — near-zero leakage, stable on-resistance and real open-circuit isolation — without the footprint and cost of a full-size EMR. In practice that shows up as cleaner range changes, a lower noise floor in the vertical chain and less rounding of fast edges. A displayed edge is only as clean as the analog path that acquired it, and the range switch is part of that path.
Board-level notes
The relay still needs a cooperative layout to deliver its specification. Treat the feedback traces as guarded nodes, tie the relay ground pin to the analog star point, and keep coil-drive traces away from the signal path so actuation transients do not couple in. The external shield then contains the coil field when multiple range relays sit side by side, preserving channel-to-channel isolation. The small package is what lets a four- or eight-channel scope keep all range relays on the vertical board itself, avoiding long analog jumpers that would add their own capacitance. Low contact resistance, teraohm insulation and a compact body together buy range flexibility without paying for it in noise floor or board area.
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