Baseline drift

Baseline drift is a change with time in the non-signal level of a measurement readout. For a photodetector, write the measured electric current aswhere $b(t)$ 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 $\hat{b}$, its baseline contribution to the charge error isA 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 byThis 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 $T_g\hat{b}(t_c)$.

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 $10\mu \mathrm{s}$ 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 $b(t_c+s)=b_0+vs+c{s}^2/2$, the two-point estimate of the central level is $b_0+c{h}^2/2$, 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.^[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.