LMC6484 Datasheet, PDF (26/45 Pages) National Semiconductor (TI) – CMOS Quad Rail-to-Rail Input and Output Operational Amplifier

LMC6484
SNOS675C – AUGUST 2000 – REVISED SEPTEMBER 2015
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Typical Application (continued)
8.2.1 Design Requirements
• For best performance, ensure that the input voltage swing is between V+ and V-.
• Ensure that the input does not exceed the common-mode input range.
• To reduce the risk of de-stabilizing the output, use resistive isolation on the output when driving capacitive
loads (see Capacitive Load Compensation).
• When large feedback resistors are used, it may be necessary to compensate for parasitic capacitance on the
input (see Compensating for Input Capacitance).
8.2.2 Detailed Design Procedure
8.2.2.1 Capacitive Load Compensation
The LMC6484 can typically directly drive a 100-pF load with V_S = 15 V at unity gain without oscillating. The unity
gain follower is the most sensitive configuration. Direct capacitive loading reduces the phase margin of
operational amplifiers. The combination of the output impedance of the operational amplifier and the capacitive
load induces phase lag. This results in either an under-damped pulse response or oscillation.
Capacitive load compensation can be accomplished using resistive isolation as shown in Figure 49. This simple
technique is useful for isolating the capacitive input of multiplexers and A/D converters.
Figure 49. Resistive Isolation of a 330-pF Capacitive Load
Figure 50. Pulse Response of the LMC6484 Circuit in Figure 49
Improved frequency response is achieved by indirectly driving capacitive loads as shown in Figure 51.
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