AD711
Figures 8 and 9 show the AD711 and AD7545 (12-bit CMOS
DAC) configured for unipolar binary (2-quadrant multiplication)
or bipolar (4-quadrant multiplication) operation. Capacitor C1
provides phase compensation to reduce overshoot and ringing.
R2
*
V
DD
GAIN
ADJUST
V
IN
R1
*
+15
C1
33pF
R
FB
OUT1
0.1 F
compared to a series of switched trial currents. The comparison
point is diode clamped but may deviate several hundred millivolts
resulting in high frequency modulation of A/D input current.
Figures 10a and 10b show the settling time characteristics of the
AD711 when used as a DAC output buffer for the AD7545.
V
DD
V
REF
AD7545
DGND AGND
AD711K
C
F
0.1 F
V
OUT
DB11-DB0
*
FOR VALUES R1 AND R2,
REFER TO TABLE 1
ANALOG
COMMON
–15
Figure 8. Unipolar Binary Operation
a. Full-Scale Positive
Transition
b. Full-Scale Negative
Transition
R1 and R2 calibrate the zero offset and gain error of the DAC.
Specific values for these resistors depend upon the grade of
AD7545 and are shown below.
Table I. Recommended Trim Resistor Values vs. Grades
of the AD7545 for V
DD
= 5 V
TRIM
RESISTOR JN/AQ/SD KN/BQ/TD LN/CQ/UD GLN/GCQ/GUD
Figure 10. Settling Characteristics for AD711 with AD7545
R1
R2
500
W
150
W
200
W
68
W
100
W
33
W
20
W
6.8
W
compared to a series of switched trial currents. The comparison
point is diode clamped but may deviate several hundred milli-
volts 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
W
due to current limiting resistors.
A few hundred microamps reflected from the change in con-
verter loading can introduce errors in instantaneous input
12/8
CS
A
O
R/C
GAIN
ADJUST
R2
100
+15V
0.1 F
OFFSET
ADJUST
10V
ANALOG
INPUT
R1
100
CE
REF IN
REF OUT
BIP OFF
10V
IN
20V
IN
0.1 F
–15V
ANA COM
LOW
BITS
+5V
+15V
–15V
DIG COM
HIGH
BITS
STS
NOISE CHARACTERISTICS
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 guaranteed
maximum noise levels to realize the full accuracy of their equipment.
The AD711C grade is specified at a maximum level of 4.0
mV
p-p,
in a 0.1 Hz to 10 Hz bandwidth. Each AD711C receives a 100%
noise test for two 10-second intervals; devices with any excursion
in excess of 4.0
mV
are rejected. The screened lot is then submitted
to Quality Control for verification on an AQL basis.
All other grades of the AD711 are sample-tested on an AQL
basis to a limit of 6
mV
p-p, 0.1 to 10 Hz.
DRIVING THE ANALOG INPUT OF AN A/D CONVERTER
AD574
MIDDLE
BITS
AD711
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 conditions.
In successive-approximation converters, the input current is
R2
*
C1
33pF
OUT1
V
REF
AD7545
AGND
DGND
DB11-DB0
12
–15V
*
FOR VALUES R1 AND R2,
REFER TO TABLE 1
DATA INPUT
ANALOG
COMMON
V
DD
R
FB
+15V
0.1 F
ANALOG COM
Figure 11. AD711 as ADC Unity Gain Buffer
V
DD
GAIN
ADJUST
V
IN
R1
*
R4
20k
1%
R3
10k
1%
R5
20k
1%
+15V
0.1 F
AD711K
0.1 F
AD711K
0.1 F
–15V
V
OUT
Figure 9. Bipolar Operation
–10–
REV. E
AD [ ANALOG DEVICES ]
