App note: Fast Rail-to-Rail operational amplifiers ease design constraints in low voltage high speed systems

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Migration to lower rail voltages considerations on operational amplifier designs an Application note from Analog Devices. Link here (PDF)

Movement towards lower power supply voltages is driven by the demand that systems consume less and less power coupled with the desire to reduce the number of power supply voltages in the system. Lowering power supply voltages and reducing the number of supplies has obvious advantages. One such advantage is to lower system power consumption. This has the additional benefit of saving space. Lowering overall power consumption has a residual benefit in that there may no longer be a need for cooling fans in the system.

However, as the traditional system power supply voltages of ±15 V and ±12 V give way to lower bipolar supplies of ±5 V and single supplies of +5 V and +3.3 V, it is necessary for circuit designers to understand that designing in this new environment is not simply a matter of finding components that are specified to operate at lower voltages. Not all design principles used in the past can be directly translated to a lower voltage environment.

Reducing the power supply voltage to a typical op amp has a number of effects. Obviously, the signal swings both at the input and output are reduced. The required headroom between signal and rail (typically 1 V to 2 V in conventional amplifiers), which is of lesser importance with power supplies of ±15 V, now drastically reduces the usable signal range. While this reduction does not normally increase noise levels in the system, signal-to noise ratios will be degraded. Because the designer can no longer use techniques such as increasing power supply voltages and signal swings in order to “swamp” noise levels, greater attention must be paid to noise levels in the system.

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