Noise in a sample cell. A, The voltage PSD is shown at three holding potentials, with the rms,σ, given in the legend. The noise increased as the cell was depolarized. The best fit to S_V = A_V/1 + (f/f_c)^nV is shown with a dashed curve. Here, n_V = 2.4 ± 0.2. Individual PSD cutoffs, f_c, are shown by inverted triangles. The mean cutoff was at 5 ± 2 Hz, indicated by a vertical line, corresponding to a time constant, τ = 1/2π f_c, of 32 ± 11 msec for the cell. B, The input impedance magnitude was well fit by R/(1 + (f/f_c)²)^nV, as shown with the dashed curve. The cutoff frequency, f_Z = 5.2 ± 0.2 Hz, matched the cutoff of the voltage PSD, with n_Z = 0.345 ± 0.005. The DC component, R, corresponded to input resistances of 205-234 MΩ. C, The current PSD is shown for the same cell at four holding potentials. It was fit to S_I = A_I/1 + (f/f_c)^nI for f > 1 Hz. The high-frequency portion of the graph was confounded by instrumental noise, shown in black and detailed in Materials and Methods, along with visible power line interference at 60 Hz. Otherwise, noise increased at all frequencies with depolarization. The mean cutoff, indicated by the vertical line, was now at 8 ± 1 Hz for three holding potentials, with an outlier at 0.7 Hz for -80 mV. D, The current and voltage PSDs are related by S_I(f) = S_V(f)/| Z_in(f)|² demonstrated at the holding potentials of Figure 4 A.