 Switch on-resistance flatness

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.