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LMC6082 Datasheet(PDF) 11 Page - Texas Instruments |
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LMC6082 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 22 page LMC6082 www.ti.com SNOS630D – AUGUST 2000 – REVISED MARCH 2013 Figure 26. LMC6082 Noninverting Gain of 10 Amplifier, Compensated to Handle Capacitive Loads In the circuit of Figure 26, R1 and C1 serve to counteract the loss of phase margin by feeding the high frequency component of the output signal back to the amplifier's inverting input, thereby preserving phase margin in the overall feedback loop. Capacitive load driving capability is enhanced by using a pull up resistor to V+ Figure 27. Typically a pull up resistor conducting 500 μA or more will significantly improve capacitive load responses. The value of the pull up resistor must be determined based on the current sinking capability of the amplifier with respect to the desired output swing. Open loop gain of the amplifier can also be affected by the pull up resistor (see Electrical Characteristics). Figure 27. Compensating for Large Capacitive Loads with a Pull Up Resistor 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 LMC6082, typically less than 10 fA, it is essential to have an excellent layout. Fortunately, 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 acceptably 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 LMC6082's inputs and the terminals of capacitors, diodes, conductors, resistors, relay terminals, etc. connected to the op-amp's inputs, as in Figure 28. 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 1012 Ω, which is normally 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 100 times degradation from the LMC6082's actual performance. However, if a guard ring is held within 5 mV of the inputs, then even a resistance of 1011 Ω would cause only 0.05 pA of leakage current. See Figure 29 for typical connections of guard rings for standard op-amp configurations. Copyright © 2000–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LMC6082 |
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