LMC7101_15 Datasheet, PDF (22/33 Pages) Texas Instruments – LMC7101, LMC7101Q-Q1 Tiny Low-Power Operational Amplifier With Rail-to-Rail Input and Output

LMC7101, LMC7101Q-Q1
SNOS719G – SEPTEMBER 1999 – REVISED SEPTEMBER 2015
8 Application and Implementation
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NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers must
validate and test their design implementation to confirm system functionality.
8.1 Application Information
8.1.1 Rail-to-Rail Output
The approximate output resistance of the LMC7101 is 180-Ω sourcing and 130-Ω sinking at V_S = 3 V and 110-Ω
sourcing and 80-Ω sinking at V_S = 5 V. Using the calculated output resistance, maximum output voltage swing
can be estimated as a function of load.
8.1.2 Capacitive Load Tolerance
The LMC7101 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 and the capacitive load of the operational
amplifier induces phase lag, which results in either an underdamped pulse response or oscillation.
Capacitive load compensation can be accomplished using resistive isolation as shown in Figure 64. This simple
technique is useful for isolating the capacitive input of multiplexers and A/D converters.
Figure 64. Resistive Isolation
of a 330-pF Capacitive Load
8.1.3 Compensating for Input Capacitance When Using Large Value Feedback Resistors
When using very large value feedback resistors, (usually > 500 kΩ) the large feed back resistance can react with
the input capacitance due to transducers, photo diodes, and circuit board parasitics to reduce phase margins.
The effect of input capacitance can be compensated for by adding a feedback capacitor. The feedback capacitor
(as in Figure 65), C_f is first estimated by Equation 1 and Equation 2, which typically provides significant
overcompensation.
1
1
³
2pR₁C_IN 2pR₂C_f
(1)
R₁ C_IN ≤ R₂ C_f
(2)
Printed circuit board stray capacitance may be larger or smaller than that of a breadboard, so the actual optimum
value for C_F may be different. The values of C_F must be checked on the actual circuit (refer to CMOS Quad
Operational Amplifier (SNOSBZ3) for a more detailed discussion).
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