= Baseline drift
\b[Baseline drift] is a change with time in the non-signal level of a measurement readout. For a <photodetector>, write the measured <electric current> as
$$
i_{\rm meas}(t)=i_{\rm signal}(t)+b(t)+n(t),
$$
where $b(t)$ is the changing baseline and $n(t)$ represents fluctuations around that level. <Detector baseline subtraction> using one fixed value can leave a time-dependent residual even when it correctly removes the initial offset.
If a <detection time gate> uses a constant baseline estimate $\widehat b$, its baseline contribution to the charge error is
$$
\Delta Q_b=\int_{t_0}^{t_0+T_g}[b(t)-\widehat b],dt.
$$
A proposed control brackets the signal in time with signal-absent measurements. For baseline values at equally spaced times $t_c-h$ and $t_c+h$, estimate the central baseline by
$$
\widehat b(t_c)=\frac{b(t_c-h)+b(t_c+h)}2.
$$
This reproduces the central value exactly for a baseline linear in time. If that linearity holds across a gate centred at $t_c$, its integrated baseline is also exactly $T_g\widehat b(t_c)$.
For an original example, choose baseline readings $2$ and $6\,\mathrm{nA}$ before and after the signal, with linear drift between them. The central baseline is $4\,\mathrm{nA}$. A centred gate of width $10\,\mathrm{\mu s}$ contains $40\,\mathrm{fC}$ of baseline charge. Subtracting only the earlier reading removes $20\,\mathrm{fC}$ and leaves a false positive contribution of $20\,\mathrm{fC}$.
Bracketing is not an unconditional drift correction. For $b(t_c+s)=b_0+vs+cs^2/2$, the two-point estimate of the central level is $b_0+ch^2/2$, so curvature introduces an error. Random baseline-readout errors also contribute <measurement uncertainty>. Taking reference readings close to the signal limits some drift effects, but does not remove a source-correlated artifact or compensate for all changes in the measurement conditions.
A real gated-integrator implementation alternates signal-present and signal-absent triggers and reverses the latter contribution before averaging. The manufacturer's stated purpose is active baseline subtraction.<SR250 Gated Integrator by Stanford Research Systems>[PDF p. 2, Polarity Control and Active Baseline Subtraction]{ref} Its effectiveness depends on whether the signal-absent readings represent the baseline during the signal. Plotting those readings against time, as well as reporting their average, can distinguish a changing zero from uncorrelated <measurement noise>.
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