OPA341 Datasheet, PDF (10/16 Pages) Burr-Brown (TI) – SINGLE-SUPPLY, RAIL-TO-RAIL OPERATIONAL AMPLIFIER WITH SHUTDOWN

A double-folded cascode adds the signal from the two input
pairs and presents a differential signal to the class AB output
stage. Normally, input bias current is approximately 600fA,
however, input voltages exceeding the power supplies by
more than 300mV can cause excessive current to flow in or
out of the input pins. Momentary voltages greater than
300mV beyond the power supply can be tolerated if the
current on the input pins is limited to 10mA. This is easily
accomplished with an input resistor, as shown in Figure 3.
Many input signals are inherently current-limited to less
than 10mA, therefore, a limiting resistor is not required.
I_OVERLOAD
10mA max
V_IN
V+
OPAx341
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
(> 50kΩ), the output voltage is typically a few millivolts
from the supply rails. With moderate resistive loads (2kΩ to
50kΩ), the output can swing to within a few tens of milli-
volts from the supply rails and maintain high open-loop
gain. See the typical characteristic “Output Voltage Swing
vs Output Current.”
CAPACITIVE LOAD AND STABILITY
OPA341 series op amps can drive a wide range of capacitive
loads. However, all op amps under certain conditions may
become unstable. Op amp configurations, gain, and load
value are just a few of the factors to consider when determin-
ing 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’s output resistance,
along with any additional load resistance, to create a pole in
the small-signal response which degrades the phase margin.
In unity gain, OPA341 series op amps perform well, with a
pure capacitive load up to approximately 1000pF. Increasing
gain enhances the amplifier’s ability to drive more capaci-
tance. See the typical characteristic “Small-Signal Over-
shoot vs Capacitive Load.”
One method of improving capacitive load drive in the unity-
gain configuration is to insert a 10Ω to 20Ω resistor in series
with the output, as shown in Figure 4. This significantly
reduces ringing with large capacitive loads. However, if there
is a resistive load in parallel with the capacitive load, R_S
creates a voltage divider. This introduces a DC error at the
output and slightly reduces output swing. This error may be
insignificant. For instance, with R_L = 10kΩ and R_S = 20Ω,
there is only about a 0.2% error at the output.
DRIVING A/D CONVERTERS
OPA341 series op amps are optimized for driving medium
speed (up to 100kHz) sampling A/D converters. However,
they also offer excellent performance for higher-speed con-
verters. The OPA341 series provides an effective means of
buffering the A/D converter’s input capacitance and result-
ing charge injection while providing signal gain. For appli-
cations requiring high accuracy, the OPA340 series is rec-
ommended.
The OPA341 implements a power-saving shutdown feature
particularly useful for low-power sampling applications. Figure 5
shows the OPA341 driving the ADS7816, a 12-bit micro-power
sampling converter available in the tiny MSOP-8 package. With
the OPA341 in non-inverting configuration, an RC network at the
amplifier’s output is used as an anti-aliasing filter. By tying the
enable of the OPA341 to the shutdown of the ADS7816, addi-
tional power-savings can be used for sampling applications. To
effectively drive the ADS7816, timing delay was introduced
between the two devices, see Figure 5. Alternative applications
may need additional timing adjustments.
Figure 6 shows the OPA341 configured as a speech band-
pass filter. Figure 7 shows the OPA341 configured as a
transimpedance amplifier.
V+
OPAx341
V_IN
V_ENABLE
R_S
10Ω to
20Ω
V_OUT
R_L
C_L
FIGURE 4. Series Resistor in Unity-Gain Configuration Improves Capacitive Load Drive.
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OPA341, 2341
SBOS202A