AD712
the AD712 when used as a DAC output buffer for the AD7545.
1mV
100
90
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.
12/8
CS
A
O
GAIN
ADJUST
R2
100Ω
R1
100Ω
OFFSET
ADJUST
1/2
STS
HIGH
BITS
10
0%
00823-041
5V
500ns
Figure 41. Positive Settling Characteristics for AD712 with AD7545
1mV
100
90
R/C
AD574A
MIDDLE
CE
BITS
REF IN
REF OUT
BIP OFF
10V
IN
20V
IN
AC
LOW
BITS
+5V
+15V
–15V
DC
00823-043
+15V
10
0%
00823-042
0.1µF
5V
500ns
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.
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.
Rev. G | Page 15 of 20
+
–15V
±10V
ANALOG
INPUT
–
AD712
0.1µF
ANALOG COM
Figure 43. AD712 as An ADC Unity-Gain Buffer
AD [ ANALOG DEVICES ]
