OPA373_16 Datasheet, PDF (10/37 Pages) Texas Instruments – CMOS Operational Amplifier

OPA373, OPA2373
OPA374
OPA2374, OPA4374
SBOS279E − SEPTEMBER 2003 − REVISED MAY 2008
I_OVERLOAD
10mA max
R
V_IN
V+
OPA 37 3
V_OUT
Figure 3. Input Current Protection for Voltages
Exceeding the Supply Voltage
RAIL-TO-RAIL OUTPUT
A class AB output stage with common-source transistors
is used to achieve rail-to-rail output. For light resistive
loads ( > 100kΩ), the output voltage can typically swing to
within 18mV from the supply rails. With moderate resistive
loads (5kΩ to 50kΩ), the output can typically swing to
within 100mV from the supply rails and maintain high
open-loop gain. See the Typical Characteristic curve,
Output Voltage Swing vs Output Current, for more
information.
CAPACITIVE LOAD AND STABILITY
OPA373 series op amps can drive a wide range of
capacitive loads. However, under certain conditions, all op
amps may become unstable. Op amp configuration, gain,
and load value are just a few of the factors to consider
when determining stability. An op amp in unity-gain
configuration is the most susceptible to the effects of
capacitive load. The capacitive load reacts with the op amp
output resistance, along with any additional load
resistance, to create a pole in the small-signal response
that degrades the phase margin. The OPA373 series op
amps perform well in unity-gain configuration, with a pure
capacitive load up to approximately 250pF. Increased
gains allow the amplifier to drive more capacitance. See
the Typical Characteristics curve, Small-Signal Overshoot
vs Capacitive Load, for further details.
One method of improving capacitive load drive in the
unity-gain configuration is to insert a small (10Ω to 20Ω)
resistor, R_S, in series with the output, as shown in Figure 4.
This significantly reduces ringing while maintaining dc
performance for purely capacitive loads. When there is a
resistive load in parallel with the capacitive load, R_S must
be placed within the feedback loop as shown to allow the
feedback loop to compensate for the voltage divider
created by R_S and R_L.
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. However, when large valued
resistors cannot be avoided, a small (4pF to 6pF)
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capacitor, C_FB, can be inserted in the feedback, as shown
in Figure 5. This significantly reduces overshoot by
compensating the effect of capacitance, C_IN, which
includes the amplifier input capacitance and printed circuit
board (PCP) parasitic capacitance.
V+
R_S
10Ωto 20Ω
OPA373
V_OUT
V_IN
R_L
C_L
Figure 4. Series Resistor in Unity-Gain
Configuration Improves Capacitive Load Drive
R_I
V_IN
C_FB
R_F
V+
OPA373
C_IN
V_OUT
C_L
Figure 5. Improving Capacitive Load Drive
For example, when driving a 100pF load in unity-gain
inverter configuration, adding a 6pF capacitor in parallel
with the 10kΩ feedback resistor decreases overshoot from
57% to 12%, as shown in Figure 6.
60
G = −1V/V
50 ^RFB ^{= 10kΩ}
40
30
20
10
C_FB = 6pF
0
10
100
1k
10k
Load Capacitance (pF)
Figure 6. Improving Capacitive Load Drive