AD712AH Analog Devices, AD712AH Datasheet - Page 10

• Manufacturer Part Number: AD712AH
• Description: Dual Precision/ Low Cost/ High Speed/ BiFET Op Amp
• Manufacturer: Analog Devices

AD712
D/A CONVERTER APPLICATIONS
The AD712 is an excellent output amplifier for CMOS DACs.
It can be used to perform both 2 quadrant and 4 quadrant op-
eration. The output impedance of a DAC using an inverted
R-2R ladder approaches R for codes containing many 1s, 3R for
codes containing a single 1, and for codes containing all zero,
the output impedance is infinite.
For example, the output resistance of the AD7545 will modu-
late between 11 k and 33 k . Therefore, with the DAC’s
internal feedback resistance of 11 k , the noise gain will vary
from 2 to 4/3. This changing noise gain modulates the effect of
the input offset voltage of the amplifier, resulting in nonlinear
DAC amplifier performance.
The AD712K with guaranteed 700 V offset voltage minimizes
this effect to achieve 12-bit performance.
Figures 31 and 32 show the AD712 and AD7545 (12-bit
CMOS DAC) configured for unipolar binary (2-quadrant multi-
plication) or bipolar (4-quadrant multiplication) operation.
Capacitor C1 provides phase compensation to reduce overshoot
and ringing.
V
*FOR VALUES OF
R1 AND R2 SEE TABLE I
IN
ADJUST
GAIN
R1*
DATA INPUT
V
DB11–DB0
REF
V
V
DD
DD
12
AD7545
DGND
R
Figure 32. Bipolar Operation
FB
AGND
OUT1
R2*
C1
33pF
ANALOG
COMMON
–10–
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.
Trim
Resistor
R1
R2
V
V
AD712
+15V
IN
*REFER TO
TABLE I
IN
*REFER TO
TABLE I
Table I. Recommended Trim Resistor Values vs. Grades
of the AD7545 for V
1/2
ADJUST
ADJUST
GAIN
GAIN
0.1 F
R1A*
R1B*
10k
Figure 31. Unipolar Binary Operation
R3
V
V
DB11–DB0
DB11–DB0
JN/AQ/SD
500
150
REF
REF
1%
V
V
V
V
DD
DD
DD
DD
R4
20k
AD7545
AD7545
AD712
DD
1%
20k
–15V
1/2
DGND
DGND
R5
= +5 V
R
R
FB
FB
1%
0.1 F
AGND
AGND
OUT1
OUT1
KN/BQ/TD
200
68
R2A*
R2B*
COMMON
COMMON
ANALOG
ANALOG
V
OUT
C1A
33pF
C1B
33pF
LN/UD
100
33
AD712
AD712
+15V
–15V
1/2
1/2
0.1 F
0.1 F
GLN/GUD
20
6.8
REV. B
V
V
OUTA
OUTB