OPA348_15 Datasheet, PDF (9/32 Pages) Texas Instruments – 1MHz, 45uA, CMOS, Rail-to-Rail OPERATIONAL AMPLIFIERS Value Line

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RAIL-TO-RAIL INPUT
The input common-mode range extends from (V–) –
0.2V to (V+) + 0.2V. For normal operation, inputs
should be limited to this range. The absolute
maximum input voltage is 500mV beyond the
supplies. Inputs greater than the input common-mode
range but less than the maximum input voltage, while
not valid, will not cause any damage to the op amp.
Unlike some other op amps, if input current is limited
the inputs may go beyond the power supplies without
phase inversion, as shown in Figure 21.
V_IN
5V
V_OUT
G = +1V/V, V_S = +5V
0V
10ms/div
Figure 21. OPA348—No Phase Inversion with
Inputs Greater than the Power-Supply Voltage
Normally, input currents are 0.5pA. However, large
inputs (greater than 500mV beyond the supply rails)
can cause excessive current to flow in or out of the
input pins. Therefore, as well as keeping the input
voltage below the maximum rating, it is also important
to limit the input current to less than 10mA. This is
easily accomplished with an input voltage resistor, as
shown in Figure 22.
OPA348
OPA2348
OPA4348
SBOS213G – NOVEMBER 2001 – REVISED MARCH 2013
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 5kΩ loads
connected to any potential between V+ and ground.
For light resistive loads (> 100kΩ), the output voltage
can typically swing to within 18mV from supply rail.
With moderate resistive loads (10kΩ to 50kΩ), the
output voltage can typically swing to within 100mV of
the supply rails while maintaining high open-loop gain
(see the typical characteristic Output Voltage Swing
vs Output Current, Figure 6).
CAPACITIVE LOAD AND STABILITY
The OPA348 in a unity-gain configuration can directly
drive up to 250pF 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, Figure 13). In
unity-gain configurations, capacitive load drive can be
improved by inserting a small (10Ω to 20Ω) resistor,
R_S, in series with the output, as shown in Figure 23.
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+
R_S
OPA348
V_OUT
V_IN
10W to
20W
R_L
C_L
I_OVERLOAD
10mA max
V_IN
5kW
+5V
OPA348
V_OUT
Figure 22. Input Current Protection for Voltages
Exceeding the Supply Voltage
Figure 23. Series Resistor in Unity-Gain Buffer
Configuration Improves Capacitive Load Drive
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