OPA348-Q1_16 Datasheet, PDF (15/32 Pages) Texas Instruments – CMOS Rail-to-Rail Operational Amplifier

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OPA348-Q1, OPA2348-Q1, OPA4348-Q1
SBOS465C – JANUARY 2009 – REVISED JANUARY 2016
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 ability of the amplifier 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 loop, 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 printed
circuit board (PCB) 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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