= Analog switch
An \b[analog switch] uses a control signal to connect or disconnect an analog signal path. The control chooses the conducting state; the transmitted <electric potential difference> is not restricted to a binary logic value. Real devices have finite <analog switch on-resistance>, leakage and parasitic capacitance. The primary tutorial describes a bilateral CMOS switch formed from complementary transistors.<MT-088: Analog Switches and Multiplexers Basics>[CMOS Switch Basics, pp2–3, Figures1–3]{ref}
For an original loading example, connect an ideal $1\,\mathrm V$ source through a stipulated $100\,\Omega$ on-resistance to a $10\,\mathrm{k\Omega}$ load. The output is
$$
v_o=1\,\mathrm V\frac{10000}{10000+100}\approx0.9901\,\mathrm V.
$$
The closed switch therefore changes the amplitude even before any switching transient is considered. This is a chosen resistor model, not a specification for a particular part. Variation of on-resistance with signal level can additionally produce distortion.
An open switch is also not an ideal infinite impedance. For example, a stipulated $10\,\mathrm{nA}$ <analog switch leakage current> into $100\,\mathrm{pF}$ changes the node potential at $I/C=100\,\mathrm{V/s}$, or $0.10\,\mathrm V$ in a millisecond if that model remains valid. A high-impedance measurement can be sensitive to this small current.
On a bench, compare the transmitted level and the open-state drift with the same source, load and <oscilloscope> conditions. <Analog switch charge injection> concerns the control transition itself, while <settling time> concerns how long the complete circuit needs to approach its final value. Those effects are separate from the static conducting-state error.
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