OPA340 series op amps are laser-trimmed to the reduce
offset voltage difference between the N-channel and
P-channel input stages, resulting in improved common-
mode rejection and a smooth transition between the
N-channel pair and the P-channel pair. However, within the
400mV transition region PSRR, CMRR, offset voltage,
offset drift, and THD may be degraded compared to opera-
tion outside this region.
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 200fA,
however, input voltages exceeding the power supplies by
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 AND STABILITY
OPA340 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
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, OPA340 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,
it creates a voltage divider introducing 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
OPA340 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 OPA340 series provides an effective means
of buffering the A/D’s input capacitance and resulting
charge injection while providing signal gain.
Figures 5 and 6 show the OPA340 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 OPA340 series, the combina-
tion is ideal for space-limited and low power applications.
For further information consult the ADS7816 data sheet.
With the OPA340 in a noninverting configuration, an RC
network at the amplifier’s output can be used to filter high
frequency noise in the signal (Figure 5). In the inverting
configuration, filtering may be accomplished with a ca-
pacitor across the feedback resistor (Figure 6).
V+
I
OVERLOAD
10mA max
V
IN
5kΩ
OPAx340
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.”
V+
R
S
OPAx340
V
IN
10Ω to
20Ω
R
L
C
L
V
OUT
FIGURE 4. Series Resistor in Unity-Gain Configuration Improves Capacitive Load Drive.
®
9
OPA340/2340/4340
BURR-BROWN [ BURR-BROWN CORPORATION ]
