Digital Systems: Principles and Design

499ADC, DAC and AnalogDigital Mix Interfaces

FIGURE 20.7 (a) An analog de-multiplexer (b) The circuit and a table (in the truth table like format) for the four channel analog

de-multiplexer.

4. CMOS gates have negligible current drains on the supply provided input is held

steady at 0 or 1 i.e. at V

SS

or at V

DD

(Figure 20.8). In CMOS gates, the current ratio

from supply is negligible as the currents are few mA only if the inputs are not float-

ing but are kept at logic 0 or at logic 1. These circuit inputs can be given directly

from V



or to V

+

or to an output of a logic gate.

5. Currents at logic 1 inputs are much smaller compared to logic 0 inputs ...