ad713knz Analog Devices, Inc., ad713knz Datasheet - Page 9

• Manufacturer Part Number: ad713knz
• Description: Quad Precision, Low Cost, High Speed, Bifet Op Amp
• Manufacturer: Analog Devices, Inc.

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.
Most IC amplifiers exhibit a minimum open loop output imped-
ance of 25 , 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.
AD713