AD795
CIRCUIT BOARD NOTES
The AD795 is designed for throughhole mounting on PC
boards, using either mini-DIP or surface mount (SOIC).
Maintaining picoampere resolution in those environments
requires a lot of care. Both the board and the amplifier’s
package have finite resistance. Voltage differences between the
input pins and other pins as well as PC board metal traces will
cause parasitic currents (Figure 30) larger than the AD795’s
input current unless special precautions are taken. Two methods
of minimizing parasitic leakages are guarding of the input lines
and maintaining adequate insulation resistance.
Figures 31 and 32 show the recommended guarding schemes for
follower and inverted topologies. Note that for the mini-DIP,
the guard trace should be on both sides of the board. On the
SOIC, Pin 1 is not connected, and can be safely connected to
the guard. The high impedance input trace should be guarded
on both edges for its entire length.
C
F
R
F
V
E
2
I
S
3
AD795
6
+
V
OUT
–
I
P
R
P
C
P
I
P
=
V
S
V
S
R
P
+
dC
P
dT
V
S
+
dV
S
dT
C
P
Figure 30. Sources of Parasitic Leakage Currents
GUARD TRACES PARALLEL
TO BOTH EDGES OF
INPUT TRACE
1
INPUT
TRACE
TO ANALOG
COMMON
–V
S
2
3
4
8
7
GUARD
2
I
S
R
F
C
F
AD795
TOP VIEW
("N" PACKAGE)
6
5
AD795
3
6
+
V
OUT
–
8 BOTTOM
1
VIEW
2
7
("N"
PACKAGE)
6
3
5
4
1
2
3
4
TOP VIEW
("R" PACKAGE)
8
7
6
5
NOTE:
ON THE "R" PACKAGE
PINS 1, 5 AND 8 ARE OPEN
AND CAN BE CONNECTED
TO ANALOG COMMON OR
TO THE DRIVEN GUARD TO
REDUCE LEAKAGE.
Figure 31. Guarding Scheme–lnverter
GUARD TRACES
1
2
INPUT
TRACE
–V
S
3
4
8
7
–
6
5
GUARD
3
AD795
TOP VIEW
+
V
S
2
AD795
6
R
F
+
V
OUT
CONNECT TO JUNCTION
OF R
F
AND R
I
, OR TO PIN 6
FOR UNITY GAIN.
R
I
–
Figure 32. Guard Scheme–Follower
REV. A
–9–
AD [ ANALOG DEVICES ]
