OPA322 Datasheet, PDF (14/36 Pages) Texas Instruments – 20-MHz, Low-Noise, 1.8-V, RRI/O, CMOS Operational Amplifier with Shutdown

OPA322, OPA322S
OPA2322, OPA2322S
OPA4322, OPA4322S
SBOS538E – JANUARY 2011 – REVISED JUNE 2012
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CAPACITIVE LOAD AND STABILITY
The OPA322 is designed to be used in applications where driving a capacitive load is required. As with all op
amps, there may be specific instances where the OPA322 can become unstable. The particular op amp circuit
configuration, layout, gain, and output loading are some of the factors to consider when establishing whether an
amplifier is stable in operation. An op amp in the unity-gain (+1 V/V) buffer configuration and driving a capacitive
load exhibits a greater tendency to become unstable than an amplifier operated at a higher noise gain. The
capacitive load, in conjunction with the op amp output resistance, creates a pole within the feedback loop that
degrades the phase margin. The degradation of the phase margin increases as the capacitive loading increases.
When operating in the unity-gain configuration, the OPA322 remains stable with a pure capacitive load up to
approximately 1 nF.
The equivalent series resistance (ESR) of some very large capacitors (C_L > 1 µF) is sufficient to alter the phase
characteristics in the feedback loop such that the amplifier remains stable. Increasing the amplifier closed-loop
gain allows the amplifier to drive increasingly larger capacitance. This increased capability is evident when
observing the overshoot response of the amplifier at higher voltage gains, as shown in Figure 33. One technique
for increasing the capacitive load drive capability of the amplifier operating in unity gain is to insert a small
resistor (R_S), typically 10 Ω to 20 Ω, in series with the output, as shown in Figure 34.
This resistor significantly reduces the overshoot and ringing associated with large capacitive loads. A possible
problem with this technique is that a voltage divider is created with the added series resistor and any resistor
connected in parallel with the capacitive load. The voltage divider introduces a gain error at the output that
reduces the output swing. The error contributed by the voltage divider, however, may be insignificant. For
instance, with a load resistance, R_L = 10 kΩ and R_S = 20 Ω, the gain error is only about 0.2%. However, when R_L
is decreased to 600 Ω, which the OPA322 is able to drive, the error increases to 7.5%.
70
G = 1, V_S = 1.8 V
60
G = 1, V_S = 5.5 V
50
G = 10, V_S = 1.8 V
G = 10, V_S = 5.5 V
40
30
20
10
0
0
500 1000 1500 2000 2500 3000
Capacitive Load (pF)
Figure 33. Small-Signal Overshoot versus Capacitive Load (100-mV_PP output step)
V+
R_S
OPA322
V_OUT
V_IN
10 W to
20 W
R_L
C_L
Figure 34. Improving Capacitive Load Drive
14
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