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LMC6022IM Datasheet(PDF) 9 Page - Texas Instruments |
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LMC6022IM Datasheet(HTML) 9 Page - Texas Instruments |
9 / 22 page LMC6022 www.ti.com SNOS622D – NOVEMBER 1994 – REVISED MARCH 2013 APPLICATION HINTS AMPLIFIER TOPOLOGY The topology chosen for the LMC6022 is unconventional (compared to general-purpose op amps) in that the traditional unity-gain buffer output stage is not used; instead, the output is taken directly from the output of the integrator, to allow rail-to-rail output swing. Since the buffer traditionally delivers the power to the load, while maintaining high op amp gain and stability, and must withstand shorts to either rail, these tasks now fall to the integrator. As a result of these demands, the integrator is a compound affair with an embedded gain stage that is doubly fed forward (via Cf and Cff) by a dedicated unity-gain compensation driver. In addition, the output portion of the integrator is a push-pull configuration for delivering heavy loads. While sinking current the whole amplifier path consists of three gain stages with one stage fed forward, whereas while sourcing the path contains four gain stages with two fed forward. Figure 26. LMC6022 Circuit Topology (Each Amplifier) The large signal voltage gain while sourcing is comparable to traditional bipolar op amps for load resistance of at least 5 k Ω. The gain while sinking is higher than most CMOS op amps, due to the additional gain stage; however, when driving load resistance of 5 k Ω or less, the gain will be reduced as indicated in the Electrical Characteristics. The op amp can drive load resistance as low as 500 Ω without instability. COMPENSATING INPUT CAPACITANCE Refer to the LMC660 or LMC662 datasheets to determine whether or not a feedback capacitor will be necessary for compensation and what the value of that capacitor would be. CAPACITIVE LOAD TOLERANCE Like many other op amps, the LMC6022 may oscillate when its applied load appears capacitive. The threshold of oscillation varies both with load and circuit gain. The configuration most sensitive to oscillation is a unity-gain follower. See the TYPICAL PERFORMANCE CHARACTERISTICS. The load capacitance interacts with the op amp's output resistance to create an additional pole. If this pole frequency is sufficiently low, it will degrade the op amp's phase margin so that the amplifier is no longer stable at low gains. The addition of a small resistor (50 Ω to 100Ω) in series with the op amp's output, and a capacitor (5 pF to 10 pF) from inverting input to output pins, returns the phase margin to a safe value without interfering with lower-frequency circuit operation. Thus, larger values of capacitance can be tolerated without oscillation. Note that in all cases, the output will ring heavily when the load capacitance is near the threshold for oscillation. Figure 27. Rx, Cx Improve Capacitive Load Tolerance Copyright © 1994–2013, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LMC6022 |
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