AD546JN AD [Analog Devices], AD546JN Datasheet - Page 7

• Manufacturer Part Number: AD546JN
• Description: 1 pA Monolithic Electrometer Operational Amplifier
• Manufacturer: AD [Analog Devices]

AD546
Circuit Board Notes
The AD546 is designed for through hole mount into PC boards.
Maintaining picoampere level resolution in that environment re-
quires a lot of care. Since both the printed circuit board and the
amplifier’s package have a finite resistance, the voltage differ-
ence between the amplifier’s input pin and other pins (or traces
on the PC board) will cause parasitic currents to flow into (or
out of) the signal path (see Figure 26). These currents can easily
exceed the 1 pA input current level of the AD546 unless special
precautions are taken. Two successful methods for minimizing
leakage are guarding the AD546’s input lines and maintaining
adequate insulation resistance.
The AD546’s positive input (Pin 3) is located next to the nega-
tive supply voltage pin (Pin 4). The negative input (Pin 2) is
next to the balance adjust pin (Pin 1) which is biased at a poten-
tial close to the negative supply voltage. The layouts shown in
Figures 27a and 27b for the inverter and follower connections
will guard against the effects of low surface resistance of the
board. Note that the guard traces should be placed on both sides
Figure 26. Sources of Parasitic Leakage Currents
of the board. In addition the input trace should be guarded on
both of its edges along its entire length.
Figure 27a. Guarding Scheme—Inverter
Figure 27b. Guarding Scheme—Follower
REV. A
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