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AD795 Datasheet PDF : 20 Pages
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AD795
CIRCUIT BOARD NOTES
The AD795 is designed for mounting on printed circuit boards
(PCBs). Maintaining picoampere resolution in those environ-
ments 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 PCB metal traces causes
parasitic currents (see Figure 33) larger than the AD795’s input
current unless special precautions are taken. Two methods of
minimizing parasitic leakages include guarding of the input lines
and maintaining adequate insulation resistance.
Figure 34 and Figure 35 show the recommended guarding
schemes for noninverting and inverting topologies. 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.
CF
IS
IP
RP
VS
RF
VE
2
AD795 6
3
+
VOUT
–
CP
IP =
VS
RP
+
dCP
dT
VS
+
dV
dT
CP
Figure 33. Sources of Parasitic Leakage Currents
CF
1
8
2
7
3
6
TOP VIEW
4 (“R” PACKAGE) 5
GUARD
IS
RF
2
AD795 6
3
NOTES
1. ON THE “R” PACKAGE PIN 1, PIN 5, AND PIN 8 ARE OPEN
AND CAN BE CONNECTED TO ANALOG COMMON OR TO
THE DRIVEN GUARD TO REDUCE LEAKAGE.
Figure 34. Guarding Scheme—lnverter
+
VOUT
–
GUARD TRACES
1
2
INPUT
TRACE
3
4
AD795 8
TOP VIEW 7
6
5
GUARD
VS
CONNECT TO JUNCTION OF
RF AND RI OR TO PIN 6 FOR
UNITY GAIN.
Figure 35. Guard Scheme—Follower
3
AD795 6
2
+
VOUT
RF
RI
–
Rev. C | Page 12 of 20
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