AD712

Rev. G | Page 15 of 20

Figure 41 and Figure 42 show the settling time characteristics of

the AD712 when used as a DAC output buffer for the AD7545.

1mV

5V

500ns

100

10

0%

90

Figure 41. Positive Settling Characteristics for AD712 with AD7545

1mV

5V

500ns

100

10

0%

90

Figure 42. Negative Settling Characteristics for AD712 with AD7545

NOISE CHARACTERISTICS

The random nature of noise, particularly in the flicker noise

region, makes it difficult to specify in practical terms. At the

same time, designers of precision instrumentation require

certain guaranteed maximum noise levels to realize the full

accuracy of their equipment. All grades of the AD712 are sample

tested on an AQL basis to a limit of 6 μV p-p, 0.1 Hz to 10 Hz.

DRIVING THE ANALOG INPUT OF AN

ANALOG-TO-DIGITAL CONVERTER

An op amp driving the analog input of an ADC, such as that

shown in Figure 43, must be capable of maintaining a constant

output voltage under dynamically changing load conditions. In

successive approximation converters, the input current is com-

pared to a series of switched trial currents. The comparison

point is diode clamped, but can deviate several hundred millivolts

resulting in high frequency modulation of analog-to-digital

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 resistors.

+15V

1/2

AD712

0.1µF

0.1µF

–15V

ANALOG COM

AD574A

12/8

CS

R/C

CE

REF IN

REF OUT

BIP OFF

STS

+5V

+15V

–15V

±10V

ANALOG

INPUT

OFFSET

ADJUST

R2

100Ω

R1

100Ω

GAIN

ADJUST

HIGH

BITS

MIDDLE

BITS

LOW

BITS

AC

DC

Figure 43. AD712 as An ADC Unity-Gain Buffer

A few hundred microamps reflected from the change in con-

verter loading can introduce errors in instantaneous input

voltage. If the analog-to-digital conversion speed is not excessive

and the bandwidth of the amplifier is sufficient, the amplifier

output returns to the nominal value before the converter makes

its comparison. However, many amplifiers have relatively narrow

bandwidth yielding slow recovery from output transients. The

AD712 is ideally suited to drive high speed analog-to-digital

converters because it offers both wide bandwidth and high

open-loop gain.