In predominantly digital systems it is often necessary to include linear circuit functions. Traditionally， separate power supplies have been used to run the linear components （see Box， “Linear Power Supplies—Past， Present， and Future”）。

　　Recently， there has been increasing interest in powering linear circuits directly from the 5V logic rail. The logic rail is a diffi cult place for analog components to function. The high amplitude broadband current and voltage noise generated by logic clocking makes analog circuit operation diffi cult. （See Box， “Using Logic Supplies for Linear Functions”。）

　　Generally speaking， analog circuitry which must achieve very high performance levels should be driven from dedicated supplies. The diffi culties encountered in maintaining the lowest possible levels of noise and drift in an analog system are challenging enough without contending with a digitally corrupted power supply.

　　Many analog applications， however， can be successfully implemented using the logic supply. Combining components intended to provide high performance from the logic rail with good design can give excellent results （see Box， “High Performance， Single Supply Analog Building Blocks”）。 The examples which follow show this in a variety of precision measurement and control circuits which function from a 5V supply.