AD712
APPLICATIONS INFORMATION
GUARDING
The low input bias current (15 pA) and low noise characteristics
of the AD712 BiFET op amp make it suitable for electrometer
applications such as photo diode preamplifiers and picoampere
current-to-voltage converters. The use of a guarding technique,
such as that shown in Figure 38, in printed circuit board layout
and construction is critical to minimize leakage currents. The
guard ring is connected to a low impedance potential at the
same level as the inputs. High impedance signal lines should
not be extended for any unnecessary length on the printed
circuit board.
PDIP (N), CERDIP (Q),
AND SOIC (R) PACKAGES.
4
5
6
3
2
7
00823-038
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
DD
R2A*
C1A
33pF
OUT1
V
REF
+15V
0.1µF
V
IN
R1A*
AD7545
AGND
DGND
ANALOG
COMMON
*REFER TO
TABLE 3
DB11 TO DB0
V
DD
R2B*
C1B
33pF
1
AGND
DGND
ANALOG
COMMON
DB11 TO DB0
DIGITAL-TO-ANALOG CONVERTER APPLICATIONS
The AD712 is an excellent output amplifier for CMOS DACs. It
can be used to perform both 2-quadrant and 4-quadrant
operations. The output impedance of a DAC using an inverted
R-2R ladder approaches R for codes containing many 1s, and 3R
for codes containing a single 1. For codes containing all 0s, the
output impedance is infinite.
For example, the output resistance of the AD7545 modulates
between 11 kΩ and 33 kΩ. Therefore, with an 11 kΩ DAC
internal feedback resistance, the noise gain varies 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 μV offset voltage minimizes
this effect to achieve 12-bit performance.
V
DD
R2*
C1
33pF
OUT1
V
REF
+15V
–15V
Figure 39. Unipolar Binary Operation
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 listed in Table 3.
Table 3. Recommended Trim Resistor Values vs. Grades of
the AD7545 for V
DD
= 5 V
Trim
Resistor
R1
R2
JN/AQ
500 Ω
150 Ω
KN/BQ
200 Ω
68 Ω
LN
100 Ω
33 Ω
GLN
20 Ω
6.8 Ω
R4
20kΩ 1%
0.1µF
R5
20kΩ 1%
AGND
DB11 TO DB0
12
DGND
ANALOG
COMMON
Figure 40. Bipolar Operation
Rev. G | Page 14 of 20
00823-040
DATA INPUT
*FOR VALUES OF
R1 AND R2 SEE TABLE 3
+
+
R1*
AD712
R3
10kΩ 1%
–
V
IN
AD7545
–
GAIN
ADJUST
V
DD
R
FB
1/2
AD712
0.1µF
1/2
V
OUT
–15V
00823-039
*REFER TO
TABLE 3
+
Figure 38. Board Layout for Guarding Inputs
V
IN
R1B*
V
REF
AD7545
–
8
GAIN
ADJUST
V
DD
R
FB
OUT1
–
+
GAIN
ADJUST
V
DD
R
FB
AD712
1/2
V
OUTA
AD712
0.1µF
1/2
V
OUTB
AD [ ANALOG DEVICES ]
