SECTION 1 SINGLE-SUPPLY AMPLIFIERS - Analog Devices

RAIL-TO-RAIL OUTPUT STAGE SWINGIS LIMITED BY Vcesat, Ron, AND LOAD CURRENTFigure 1.7In summary, the following points should be considered when selecting amplifiers forsingle-supply/rail-to-rail applications:First, input offset voltage and input bias currents can be a function of the appliedinput common-mode voltage (for true rail-to-rail input op amps). Circuits using thisclass of amplifiers should be designed to minimize resulting errors. An invertingamplifier configuration with a false ground reference at the non-inverting inputprevents these errors by holding the input common-mode voltage constant. If theinverting amplifier configuration cannot be used, then amplifiers like theOP284/OP484 which do not exhibit any common-mode crossover thresholds shouldbe used.Second, since input bias currents are not always small and can exhibit differentpolarities, source impedance levels should be carefully matched to minimizeadditional input bias current-induced offset voltages and increased distortion.Again, consider using amplifiers that exhibit a smooth input bias current transitionthroughout the applied input common-mode voltage.Third, rail-to-rail amplifier output stages exhibit load-dependent gain whichaffects amplifier open-loop gain, and hence closed-loop gain accuracy. Amplifierswith open-loop gains greater than 30,000 for resistive loads less than 10kohm aregood choices in precision applications. For applications not requiring full rail-railswings, device families like the OPX13 and OPX93 offer DC gains of 0.2V/µV ormore.Lastly, no matter what claims are made, rail-to-rail output voltage swings arefunctions of the amplifier’s output stage devices and load current. The saturationvoltage (V CESAT ), saturation resistance (R SAT ), and load current all affect theamplifier output voltage swing.10