OPA2348-HIREL Datasheet, PDF (11/14 Pages) Texas Instruments – 1-MHz, 45-μA CMOS RAIL-TO-RAIL OPERATIONAL AMPLIFIER

OPA2348-HiRel
www.ti.com
SBOS482 – NOVEMBER 2009
Rail-to-Rail Output
A class AB output stage with common-source transistors is used to achieve rail-to-rail output. This output stage is
capable of driving 5-kΩ loads connected to any potential between V+ and ground. For light resistive loads
(>100 kΩ), the output voltage can typically swing to within 18 mV from supply rail. With moderate resistive loads
(10 kΩ to 50 kΩ), the output voltage can typically swing to within 100 mV of the supply rails while maintaining
high open-loop gain (see the typical characteristic "Output Voltage Swing vs Output Current").
Capacitive Load and Stability
The OPA2348 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 the typical characteristic "Small-Signal
Overshoot vs Capacitive Load"). 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 5. This significantly reduces
ringing while maintaining dc performance for purely capacitive loads. However, if there is a resistive load 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 5. Series Resistor in Unity-Gain Buffer Configuration Improves Capacitive Load Drive
In unity-gain inverter configuration, 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. 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 the typical characteristic "Small-Signal Overshoot
vs. Load Capacitance"). 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 6. This significantly reduces overshoot by
compensating the effect of capacitance, C_IN, which includes the amplifier's input capacitance and PC board
parasitic capacitance.
C_FB
R_I
V_IN
R_F
1/2
OPA2348
C_IN
V_OUT
C_L
Figure 6. Improving Capacitive Load Drive
Copyright © 2009, Texas Instruments Incorporated
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