AD795KN Ver la hoja de datos (PDF) - Analog Devices
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AD795KN Datasheet PDF : 16 Pages
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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.
AD795
CF
RF
VE
2
IS
AD795 6
+
VOUT
3
–
IP
RP
VS
CP
IP
=
VS
RP
+
dCP
dT
VS+ddVTS
CP
Figure 30. Sources of Parasitic Leakage Currents
GUARD TRACES PARALLEL
TO BOTH EDGES OF
INPUT TRACE
1
8
INPUT
TRACE
TO ANALOG
COMMON
–VS
2
7
AD795
3
6
TOP VIEW
4 ("N" PACKAGE) 5
GUARD
IS
CF
RF
2
AD795 6
3
+
VOUT
–
8 BOTTOM 1
VIEW
7 ("N" 2
PACKAGE)
6
3
5
4
1
8
2
7
3
TOP VIEW
6
("R" PACKAGE)
4
5
Figure 31. Guarding Scheme–lnverter
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.
GUARD TRACES
GUARD
INPUT
TRACE
1
8
AD795
2
TOP VIEW
7
3
6
+
VS
–
–VS
4
5
CONNECT TO JUNCTION
OF RF AND RI, OR TO PIN 6
FOR UNITY GAIN.
3
AD795 6
2
Figure 32. Guard Scheme–Follower
+
VOUT
RF
RI
–
REV. A
–9–
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