5962-9063302MCA Ver la hoja de datos (PDF) - Analog Devices

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5962-9063302MCA

Quad Precision, Low Cost, High Speed, BiFET Op Amp

Analog Devices 

AD713

Figure 9. A D/A Converter Circuit for Digital Audio

Figure 10. Harmonic Distortion as Frequency for the

Digital Audio Circuit of Figure 9

Driving the Analog Input of an A/D Converter

An op amp driving the analog input of an A/D converter, such

as that shown in Figure 11, must be capable of maintaining a

constant output voltage under dynamically changing load condi-

tions. In successive approximation converters, the input current

is compared to a series of switched trial currents. The compari-

son point is diode clamped but may vary by several hundred

millivolts, resulting in high frequency modulation of the A/D

input current. The output impedance of a feedback amplifier is

made artificially low by its loop gain. At high frequencies, where

the loop gain is low, the amplifier output impedance can ap-

proach its open loop value.

Figure 11. The AD713 as an ADC Buffer

Most IC amplifiers exhibit a minimum open loop output imped-

ance of 25 Ω, due to current limiting resistors. A few hundred

microamps reflected from the change in converter loading can

introduce errors in instantaneous input voltage. If the A/D con-

version speed is not excessive and the bandwidth of the amplifier

is sufficient, the amplifier’s output will return to the nominal

value before the converter makes its comparison. However,

many amplifiers have relatively narrow bandwidths, yielding

slow recovery from output transients. The AD713 is ideally

suited as a driver for A/D converters since it offers both a wide

bandwidth and a high open loop gain.

REV. C

–9–

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