AD712
Figures 33a and 33b show the settling time characteristics of the
AD712 when used as a DAC output buffer for the AD7545.
DRIVING THE ANALOG INPUT OF AN A/D CONVERTER
100
90
10
0%
500ns
An op amp driving the analog input of an A/D converter, such
as that shown in Figure 34, 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 resistors.
12/8
CS
A
O
STS
HIGH
BITS
a. Full-Scale Positive Transition
100
90
GAIN
ADJUST
R/C
CE
AD574
MIDDLE
BITS
LOW
BITS
+5V
REF IN
R2
100
+15V
0.1 F
10
0%
R1
100
REF OUT
BIP OFF
10V
IN
20V
IN
ANA
COM
500ns
b. Full-Scale Negative Transition
Figure 33. Settling Characteristics for AD712 with AD7545
NOISE CHARACTERISTICS
10V
ANALOG
INPUT
1/2
AD712
0.1 F
OFFSET
ADJUST
+15V
–15V
DIG
COM
–15V
ANALOG COM
Figure 34. AD712 as ADC Unity Gain Buffer
The random nature of noise, particularly in the 1/f region,
makes it difficult to specify in practical terms. At the same time,
designers of precision instrumentation require certain guaran-
teed maximum noise levels to realize the full accuracy of their
equipment.
The AD712C grade is specified at a maximum level of 4.0
µV
p-p, in a 0.1 Hz to 10 Hz bandwidth. Each AD712C receives a
100% noise test for two 10-second intervals; devices with any
excursion in excess of 4.0
µV
are rejected. The screened lot is
then submitted to Quality Control for verification on an AQL
basis.
All other 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.
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 AD712 is
ideally suited to drive high speed A/D converters since it offers
both wide bandwidth and high open-loop gain.
REV. B
–11–
AD [ ANALOG DEVICES ]
