OPA348-Q1 Datasheet, PDF (13/30 Pages) Texas Instruments – OPAx348-Q1 1-MHz 45-μA CMOS Rail-to-Rail Operational Amplifier

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OPA348-Q1, OPA2348-Q1, OPA4348-Q1
SBOS465B – JANUARY 2009 – REVISED DECEMBER 2014
Feature Description (continued)
7.3.9 Capacitive Load and Stability
The OPAx348-Q1 family of devices in a unity-gain configuration can directly drive up to 250-pF pure capacitive
load. Increasing the gain enhances the amplifier’s ability to drive greater capacitive loads (see Figure 13 in the
Typical Characteristics section). In unity-gain configurations, capacitive load drive can be improved by inserting a
small (10 Ω to 20 Ω) resistor, R_S, in series with the output, as shown in Figure 24. This resistor significantly
reduces ringing while maintaining DC performance for purely capacitive loads. However, if a resistive load exists
in parallel with the capacitive load, a voltage divider is created, introducing a direct current (DC) error at the
output and slightly reducing the output swing. The error introduced is proportional to the ratio R_S/R_L and is
generally negligible.
V+
1/2
R_S
OPA2348
V_OUT
V_IN
10W to
20W
R_L
C_L
Figure 24. Series Resistor in Unity-Gain Buffer Configuration Improves Capacitive Load Drive
In unity-gain inverter configuration, the phase margin can be reduced by the reaction between the capacitance at
the op-amp input and the gain setting resistors, thus degrading capacitive load drive. The best performance is
achieved by using small-valued resistors. For example, when driving a 500-pF load, reducing the resistor values
from 100 kΩ to 5 kΩ decreases overshoot from 55% to 13% (see Figure 13 in the Typical Characteristics
section). However, when large valued resistors cannot be avoided, a small (4 pF to 6 pF) capacitor, C_FB, can be
inserted in the feedback, as shown in Figure 25. This small capacitor significantly reduces overshoot by
compensating the effect of capacitance, C_IN, which includes the input capacitance of the amplifier and PC board
parasitic capacitance.
C_FB
R_I
V_IN
R_F
1/2
OPA2348
C_IN
V_OUT
C_L
Figure 25. Improving Capacitive Load Drive
7.4 Device Functional Modes
The OPAx348-Q1 family of devices is powered on when the supply is connected. The device can be operated as
a single-supply operational amplifier or a dual-supply amplifier, depending on the application.
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