OPA365-Q1_16 Datasheet, PDF (14/22 Pages) Texas Instruments – Single-Supply Operational Amplifiers

OPA365-Q1, OPA2365-Q1
SBOS512D – MARCH 2010 – REVISED DECEMBER 2015
9 Application and Implementation
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NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 Application Information
9.1.1 Capacitive Loads
The OPA365-Q1 device may be used in applications where driving a capacitive load is required. As with all
operational amplifiers, there may be specific instances where the OPA365-Q1 device can become unstable,
leading to oscillation. The particular operational amplifier circuit configuration, layout, gain and output loading are
some of the factors to consider when establishing whether an amplifier will be stable in operation. An operational
amplifier in the unity-gain (1 V/V) buffer configuration and driving a capacitive load exhibits a greater tendency to
be unstable than an amplifier operated at a higher noise gain. The capacitive load, in conjunction with the
operational amplifier 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 OPA365-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.
See Figure 15.
One technique for increasing the capacitive load drive capability of the amplifier operating in unity gain is to insert
a small resistor, typically 10 Ω to 20 Ω, in series with the output; see Figure 24. 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 OPA365-Q1
device is able to drive, the error increases to 7.5%.
V+
R_S
OPA365
V_OUT
V_IN
10Ω to
20Ω
R_L
C_L
Figure 24. Improving Capacitive Load Drive
9.1.2 Achieving an Output Level of Zero Volts (0 V)
Certain single-supply applications require the operational amplifier output to swing from 0 V to a positive full-
scale voltage and have high accuracy. An example is an operational amplifier employed to drive a single-supply
ADC having an input range from 0 V to 5 V. Rail-to-rail output amplifiers with very light output loading may
achieve an output level within millivolts of 0 V (or +V_S at the high end), but not 0 V. Furthermore, the deviation
from 0 V only becomes greater as the load current required increases. This increased deviation is a result of
limitations of the CMOS output stage.
When a pulldown resistor is connected from the amplifier output to a negative voltage source, the OPA365-Q1
can achieve an output level of 0 V, and even a few millivolts below 0 V. Below this limit, nonlinearity and limiting
conditions become evident. Figure 25 illustrates a circuit using this technique.
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