All CMOS integrated circuits have some form of input protection circuit, because otherwise the gate insulation is easily destroyed (see the later section on handling precautions). The usual protection network is shown in Figure 3.50: Although it may use distributed diodes, the network is equivalent to clamping diodes to Vss and to Vdd, combined with resistive current limiting. If you drive the inputs (or outputs) more than a diode drop beyond the supply voltages, the diode clamps go into conduction, making the inputs (or outputs) look like a low impedance to the respective supplies. Worse still, the chip can be driven into "SCR latchup," a terrifying (and destructive) condition we'll describe in more detail in Section 14.16. For now, all you need to know about it is that you don't want it! SCR latchup is triggered by input currents (through the protection network) of roughly 20mA or more. Thus, you must be careful not to drive the analog inputs beyond the rails. This means, for instance, that you must be sure the power supply voltages are applied before any signals that have significant drive current capability. Incidentally, this prohibition goes for digital CMOS ICs as well as the analog switches we have been discussing.