more than 500mV can cause excessive current to flow in or
out of the input pins. Momentary voltages greater than
500mV 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.
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, OPA343 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 performance curve “Small-Signal
Overshoot 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
OPA343 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
converters. The OPA343 series provides an effective means
of buffering the A/D’s input capacitance and resulting
charge injection while providing signal gain. For applica-
tions requiring high accuracy, the OPA340 series is recom-
mended.
Figures 5 and 6 show the OPA343 driving an ADS7816.
The ADS7816 is a 12-bit, micro-power sampling converter
in the tiny MSOP-8 package. When used with the minia-
ture package options of the OPA343 series, the combina-
tion is ideal for space-limited and low-power applications.
For further information consult the ADS7816 data sheet.
With the OPA343 in a noninverting configuration, an RC
network at the amplifier’s output can be used to filter high
frequency noise in the signal (see Figure 5). In the invert-
ing configuration, filtering may be accomplished with a
capacitor across the feedback resistor (see Figure 6).
V+
I
OVERLOAD
10mA max
V
IN
5kΩ
OPAx343
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 performanc curve “Output Voltage
Swing vs Output Current.”
CAPACITIVE LOAD AND STABILITY
OPA343 series op amps can drive a wide range of capacitive
loads. However, all op amps under certain conditions may
become unstable. Op amp configuration, 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
V+
R
S
OPAx343
V
IN
10Ω to
20Ω
R
L
C
L
V
OUT
FIGURE 4. Series Resistor in Unity-Gain Configuration Improves Capacitive Load Drive.
OPA343, 2343, 4343
SBOS090A
9
TI [ TEXAS INSTRUMENTS ]
