AD845

REV. D

–8–

Figure 28a. Settling Time of the Three Op Amp Instru-

mentation Amplifier. Horizontal Scale: 200 ns/Div; Vertical

Scale, Positive Pulse Input: 5 V/Div; Output Settling:

1 mV/Div

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 29, 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 deviate several hundred

millivolts resulting in high frequency modulation of A/D input

current. The output impedance of a feedback amplifier is made

artificially low by the loop gain. At high frequencies, where the

loop gain is low, the amplifier output impedance can approach

its open-loop value. Most IC amplifiers exhibit a minimum

open-loop output impedance of 25

Ω due to current limiting re-

sistors. A few hundred microamps reflected from the change in

converter loading can introduce errors in instantaneous input

voltage. If the A/D conversion 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

bandwidth yielding slow recovery from output transients. The

Figure 28b. Settling Time of the Three Op Amp Instru-

mentation Amplifier. Horizontal Scale: 200 ns/Div; Vertical

Scale, Negative Pulse Input: 5 V/ Div; Output Settling:

1 mV/Div

AD845 is ideally suited to drive high resolution A/D converters

with 5

µs on longer conversion times since it offers both wide

bandwidth and high open-loop gain.

Figure 29. AD845 As ADC Unity Gain Buffer

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

Mini-DIP (N) Package

Cerdip (Q) Package