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Analog switch on-resistance is the conducting path's effective electrical resistance at a specified signal level, supply and temperature. In a locally resistive model, $R_{\mathrm{on}}=\Delta v/i$. It is neither an ideal short circuit nor necessarily constant as the signal moves. Complementary switch devices reduce signal-dependent resistance variation, as the manufacturer's circuit and curves illustrate.^[ref]

An original example drives a $1.0\mathrm{k\Omega }$ load from an ideal source through $R_{\mathrm{on}}=30\Omega$. The voltage divider givesIf the analog switch on-resistance rises to $50\Omega$ elsewhere in the signal range, the ratio becomes $0.95238$, about $1.90\%$ below its first value. One gain correction cannot make both ratios exact. Switch on-resistance flatness therefore matters separately from the minimum resistance; variation with the signal can create nonlinearity.

The same resistance affects dynamics. A stipulated constant $50\Omega$ discharging $2.0\mathrm{nF}$ has RC circuit time constant $0.10\mu \mathrm{s}$ and requires $\tau \log 100\approx 0.461\mu \mathrm{s}$ to reduce an initial residual to one percent. This is a chosen single-pole model; amplifier dynamics can extend the actual integrator reset settling time.

On a bench, measure the terminal drop at known electric current while sweeping the signal level within the permitted range. Separate a nearly constant series loss from its signal-dependent variation. The source resistance and load belong in that measurement; the switch's control propagation delay is not its conducting-path resistance.

Switch on-resistance flatness describes how little an analog switch on-resistance changes across a stated analog signal range, at specified supply and temperature. A common measure is the peak-to-peak differenceThe signal interval $\mathcal{V}$ is part of the definition. A small minimum resistance does not establish good switch on-resistance flatness. Analog Devices illustrates how complementary transistor paths flatten the resistance curve and explains the distinction between fixed attenuation and signal-dependent distortion.^[ref]

For an original quasistatic model, an ideal source drives a load $R_L$ through a switch withHere the stipulated resistance law uses the source signal $v$; it is not a transistor model or a device specification. For $|\beta v^2|\ll R_L+r_0$, expansion givesWith $v=A\sin \omega t$, the identity ${\sin }^3\theta =(3\sin \theta -\sin 3\theta )/4$ yieldsA constant gain correction cannot remove the generated third harmonic.

Taking $R_L=1000\Omega$, $r_0=10\Omega$, $\beta =2\Omega /{\mathrm{V}}^2$ and $A=1\mathrm{V}$ gives a resistance range10–12ohms and an approximate third-harmonic amplitude0.490millivolts. Its ratio to the fundamental is about0.050percent. This selected low-frequency calculation isolates a static nonlinearity; it omits switch capacitances and amplifier dynamics.

A practical test holds the switch on, applies a small known electric current, and sweeps the terminal signal level while measuring the conducting-path drop. Measuring several temperatures and supply conditions separates flatness over signal from changes between operating conditions. A second test with a sinusoidal source and Fourier analysis checks whether the complete circuit produces harmonics; source and readout distortion must be measured independently.