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The AD713 is an excellent output amplifier for CMOS DACs.It can be used to perform both 2 and 4 quadrant operaTIon. Theoutput impedance of a DAC using an inverted R-2R ladderapproaches R for codes containing many “1”s, 3R for codes containinga single “1” and infinity for codes containing all zeros.For example, the output resistance of the AD7545 will modulatebetween 11 kΩ and 33 kΩ. Therefore, with the DAC’sinternal feedback resistance of 11 kΩ, the noise gain will varyfrom 2 to 4/3. This changing noise gain modulates the effect ofthe input offset voltage of the amplifier, resulTIng in nonlinearDAC amplifier performance. The AD713, with its guaranteed1.5 mV input offset voltage, minimizes this effect achieving12-bit performance.Figures 16 and 17 show
