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LMC6572BIM Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LMC6572BIM Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 13 page Applications Hints (Continued) nent of the output signal back to the amplifier’s inverting in- put, thereby preserving phase margin in the overall feedback loop. 4.0 PRINTED-CIRCUIT-BOARD LAYOUT FOR HIGH-IMPEDANCE WORK It is generally recognized that any circuit which must operate with less than 1000 pA of leakage current requires special layout of the PC board. When one wishes to take advantage of the ultra-low bias current of the LMC6574/2, typically less than 20 fA, it is essential to have an excellent layout. Fortu- nately, the techniques of obtaining low leakages are quite simple. First, the user must not ignore the surface leakage of the PC board, even though it may sometimes appear accept- ably low, because under conditions of high humidity or dust or contamination, the surface leakage will be appreciable. To minimize the effect of any surface leakage, lay out a ring of foil completely surrounding the LMC6574/2’s inputs and the terminals of capacitors, diodes, conductors, resistors, re- lay terminals, etc. connected to the op-amp’s inputs, as in Figure 3. To have a significant effect, guard rings should be placed on both the top and bottom of the PC board. This PC foil must then be connected to a voltage which is at the same voltage as the amplifier inputs, since no leakage current can flow between two points at the same potential. For example, a PC board trace-to-pad resistance of 10 12 Ω, which is nor- mally considered a very large resistance, could leak 5 pA if the trace were a 5V bus adjacent to the pad of the input. This would cause a 250 times degradation from the LMC6574/2’s actual performance. However, if a guard ring is held within 5 mV of the inputs, then even a resistance of 10 11 Ω would cause only 0.05 pA of leakage current. See Figure 4 for typi- cal connections of guard rings for standard op-amp configurations. The designer should be aware that when it is inappropriate to lay out a PC board for the sake of just a few circuits, there is another technique which is even better than a guard ring on a PC board: Don’t insert the amplifier’s input pin into the board at all, but bend it up in the air and use only air as an in- sulator. Air is an excellent insulator. In this case you may have to forego some of the advantages of PC board con- struction, but the advantages are sometimes well worth the effort of using point-to-point up-in-the-air wiring. See Figure 5. DS011934-8 FIGURE 3. Example of Guard Ring in P.C. Board Layout DS011934-9 Inverting Amplifier DS011934-10 Non-Inverting Amplifier DS011934-11 Follower FIGURE 4. Typical Connections of Guard Rings DS011934-12 (Input pins are lifted out of PC board and soldered directly to components. All other pins connected to PC board). FIGURE 5. Air Wiring www.national.com 9 |
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