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AD546 Circuit Board Notes

The AD546 is designed for through hole mount into PC boards. Maintaining picoampere level resolution in that environment requires a lot of care. Since both the printed circuit board and the amplifier’s package have a finite resistance, the voltage difference 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 negative supply voltage pin (Pin 4). The negative input (Pin 2) is next to the balance adjust pin (Pin 1) which is biased at a potential 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 of the board. In addition the input trace should be guarded on both of its edges along its entire length.

Figure 26. Sources of Parasitic Leakage Currents

Figure 27a. Guarding Scheme—Inverter

Figure 27b. Guarding Scheme—Follower



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