OPA2320-Q1_15 Datasheet, PDF (14/31 Pages) Texas Instruments – Precision, 20-MHz, 0.9-pA, Low-Noise, RRIO,CMOS Operational Amplifier

OPA2320-Q1
SLOS884 – SEPTEMBER 2014
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Feature Description (continued)
7.3.4 Output Impedance
The open-loop output impedance of the OPA2320-Q1 common-source output stage is approximately 90 Ω. When
the op amp is connected with feedback, this value is reduced significantly by the loop gain. For example, with
130 dB (typical) of open-loop gain, the output impedance is reduced in unity-gain to less than 0.03 Ω. For each
decade rise in the closed-loop gain, the loop gain is reduced by the same amount, which results in a ten-fold
increase in effective output impedance. While the OPA2320-Q1 output impedance remains very flat over a wide
frequency range, at higher frequencies the output impedance rises as the open-loop gain of the op amp drops.
However, at these frequencies the output also becomes capacitive as a result of parasitic capacitance. This in
turn prevents the output impedance from becoming too high, which can cause stability problems when driving
large capacitive loads. As mentioned previously, the OPA2320-Q1 device has excellent capacitive load drive
capability for an op amp with the bandwidth.
7.3.5 Capacitive Load and Stability
The OPA2320-Q1 device 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 OPA2320-Q1 device 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 OPA2320-Q1 device 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 32. 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 31.
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 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 OPA2320-Q1 device is able to drive, the error increases to 7.5%.
V_(V+)
R_(S)
OPA320-Q1
V_O
V_I
10 Ω to
20 Ω
R_(L)
C_(L)
Figure 31. Improving Capacitive Load Drive
14
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