Range-switching hysteresis

Range-switching hysteresis uses different thresholds for changing from a more sensitive instrument range to a less sensitive one and for changing back. Within the intervening interval, the current range is retained. The selected range therefore depends on the previous state as well as the present measurement indication. This prevents small fluctuations near a single boundary from repeatedly triggering gain ranging.

A concrete electronic analogue is an electronic comparator with two switching thresholds. The primary circuit example specifies upper and lower thresholds of $2.5\mathrm{V}$ and $2.2\mathrm{V}$ and explains why one threshold can produce unwanted transitions with a noisy or slowly changing input. Those are comparator circuit values, not specifications for an autoranging measurement instrument.^[ref]

For an original two-range current measurement, let $H$ denote high sensitivity and $L$ low sensitivity. Use an input-current estimate $x$, expressed on the same calibrated scale in either range. Switch $H\to L$ when $x\ge 8\mathrm{nA}$, switch $L\to H$ when $x\le 6\mathrm{nA}$, and otherwise keep the previous state. The hysteresis interval has width $2\mathrm{nA}$.

Starting in $H$, the indication sequence $(7.9,8.1,7.9,8.1,5.9)\mathrm{nA}$ selects $(H,L,L,L,H)$. A single threshold at $8\mathrm{nA}$ would instead select $(H,L,H,L,H)$ and switch at every step after the first. At a steady underlying $7\mathrm{nA}$ with bounded readout fluctuations of $\pm 0.2\mathrm{nA}$, neither boundary is crossed; either previous range remains selected.

The thresholds should lie within the usable overlap established by range-overlap calibration. If the two ranges give inconsistent current estimates, a switch can itself move the indication across the opposite threshold and create a cycle. The comparison must therefore use the corrected measurand estimate, not identical raw output voltages from different gains.

Hysteresis is not temporal averaging and does not ensure that the new reading has settled. Reserve headroom for a rapidly rising input, reject readings during the required settling time, and verify that the less sensitive range still meets its noise-limited detection threshold. An input that jumps beyond the current range can cause amplifier clipping before the controller reacts, even when its two-threshold rule is correct.