AD795JRZ datasheet(12/20 Pages) AD

12 / 20 page

AD795

Data Sheet

Rev. D | Page 12 of 20

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.

AD795

–

VOUT

IP =

dCP

Figure 33. Sources of Parasitic Leakage Currents

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.

AD795

–

VOUT

GUARD

TOP VIEW

(“R” PACKAGE)

Figure 34. Guarding Scheme—lnverter

GUARD TRACES

INPUT

TRACE

AD795

TOP VIEW

CONNECT TO JUNCTION OF

RF AND RI OR TO PIN 6 FOR

UNITY GAIN.

AD795

VOUT

GUARD

–

Figure 35. Guard Scheme—Follower

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