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LMC660CM Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LMC660CM Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 14 page Application Hints (Continued) 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 7. BIAS CURRENT TESTING The test method of Figure 8 is appropriate for bench-testing bias current with reasonable accuracy. To understand its op- eration, first close switch S2 momentarily. When S2 is opened, then A suitable capacitor for C2 would bea5pFor10pF silver mica, NPO ceramic, or air-dielectric. When determining the magnitude of I b−, the leakage of the capacitor and socket must be taken into account. Switch S2 should be left shorted most of the time, or else the dielectric absorption of the ca- pacitor C2 could cause errors. Similarly, if S1 is shorted momentarily (while leaving S2 shorted) where C x is the stray capacitance at the + input. DS008767-17 (a) Inverting Amplifier DS008767-18 (b) Non-Inverting Amplifier DS008767-19 (c) Follower DS008767-20 (d) Howland Current Pump FIGURE 6. Guard Ring Connections DS008767-21 (Input pins are lifted out of PC board and soldered directly to components. All other pins connected to PC board.) FIGURE 7. Air Wiring DS008767-22 FIGURE 8. Simple Input Bias Current Test Circuit www.national.com 9 |
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