AD795

REV. B

–9–

CIRCUIT BOARD NOTES

The AD795 is designed for mounting on PC boards. 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. 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.

6

3

2

AD795

+

–

+

dT

dT

Figure 30. Sources of Parasitic Leakage Currents

6

3

2

AD795

GUARD

+

–

1

2

3

4

8

7

6

5

("R" PACKAGE)

TOP VIEW

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.

F

Figure 31. Guarding Scheme–lnverter

6

2

3

1

2

3

4

8

7

6

5

AD795

GUARD TRACES

INPUT

TRACE

AD795

GUARD

+

–

+

–

CONNECT TO JUNCTION

FOR UNITY GAIN.

TOP VIEW

Figure 32. Guard Scheme–Follower