= Gain ranging
\b[Gain ranging] changes the sensitivity of a measuring signal path to keep its output within a useful interval as the input changes. A high-sensitivity setting resolves smaller inputs; a lower-sensitivity setting accommodates larger ones before <amplifier clipping>. Combining the settings can extend the instrument's usable <dynamic range>, provided their input intervals overlap and their <measurement calibration> is consistent.
A concrete apparatus is a switchable <transimpedance amplifier> converting <electric current> into an output <electric potential difference>. The DLPCA-200 manufacturer diagram separates a current-to-voltage stage from programmable amplification, and its performance tables give gain-dependent noise, input limits and <bandwidth>. The settings therefore change more than the decimal point of a display.<Datasheet DLPCA-200 by FEMTO>[p. 1, block diagram; p. 2, performance depending on gain setting]{ref}
For an original ideal calculation, suppose two settings follow $V_j=G_jI+b_j+n_j$, where $I$ is input current, $b_j$ the offset and $n_j$ zero-mean output noise. Stipulate a usable offset-corrected output from $5\,\mathrm{mV}$ to $1\,\mathrm V$ in either setting. With $G_H=10^8\,\mathrm{V/A}$, the input interval is $50\,\mathrm{pA}$ to $10\,\mathrm{nA}$. With $G_L=10^6\,\mathrm{V/A}$, it is $5\,\mathrm{nA}$ to $1\,\mathrm{\mu A}$. Each setting spans a ratio 200, but their overlapping union spans a current ratio 20000. These are stipulated intervals, not specifications for the cited amplifier.
<Range-overlap calibration> checks that both settings estimate the same current in their common $5$–$10\,\mathrm{nA}$ interval. A <range-switching hysteresis> rule can switch down in sensitivity above $8\,\mathrm{nA}$ and back up below $6\,\mathrm{nA}$, reducing repeated switching near one boundary. The output must settle before it is accepted as a new <measurement indication>.
An <analog-to-digital converter> that merely changes its printed units cannot recover an already clipped upstream signal. Likewise, increasing amplification does not automatically improve <input-referred noise>; the new setting may have different noise sources, bandwidth and drift. A meaningful combined range includes these effects and the required <measurement uncertainty>, rather than multiplying nominal gain settings without checking their usable intervals.
Back to article page
