AD795

REV. A

–9–

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.

6

3

2

AD795

+

–

+

dT

dT

Figure 30. Sources of Parasitic Leakage Currents

6

3

2

1

2

3

4

8

7

6

5

TOP VIEW

("N" PACKAGE)

AD795

8

7

6

5

1

2

3

4

BOTTOM

VIEW

("N"

PACKAGE)

GUARD TRACES PARALLEL

TO BOTH EDGES OF

INPUT TRACE

INPUT

TRACE

TO ANALOG

COMMON

AD795

C

F

V

OUT

GUARD

R

F

I

S

+

–

1

2

3

4

8

7

6

5

–V

S

("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.

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