AD795KN Analog Devices, AD795KN Datasheet - Page 9

• Manufacturer Part Number: AD795KN
• Description: Low Power/ Low Noise Precision FET Op Amp
• Manufacturer: Analog Devices

REV. A
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.
GUARD TRACES PARALLEL
TO ANALOG
COMMON
TRACE
INPUT
TRACE
INPUT
TO BOTH EDGES OF
–V
GUARD TRACES
S
INPUT TRACE
–V
S
1
2
3
4
7
6
5
1
2
3
4
8
("N" PACKAGE)
PACKAGE)
BOTTOM
TOP VIEW
AD795
VIEW
("N"
TOP VIEW
AD795
Figure 31. Guarding Scheme–lnverter
Figure 32. Guard Scheme–Follower
2
3
1
4
7
6
5
8
7
8
6
5
CONNECT TO JUNCTION
OF R
FOR UNITY GAIN.
F
AND R
–9–
+
–
I
, OR TO PIN 6
I
S
V
GUARD
S
1
4
2
3
GUARD
Figure 30. Sources of Parasitic Leakage Currents
("R" PACKAGE)
TOP VIEW
R
P
V
S
2
3
I
3
2
S
AD795
I
P
AD795
7
8
6
5
R
F
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.
V
C
E
F
C
6
P
6
2
3
V
AD795
OUT
I
R
P
R
–
+
–
F
I
=
R
V
V
S
P
OUT
R
+
F
dC
+
dT
C
F
P
6
V
S
+
dV
dT
AD795
S
V
OUT
C
P
+
–