Datasheet

OPA705, 2705, 4705
9
SBOS182A
RAIL-TO-RAIL OUTPUT
A class AB output stage with common-source transistors is
used to achieve rail-to-rail output. This output stage is
capable of driving 1kΩ loads connected to any point be-
tween V+ and ground. For light resistive loads (> 100kΩ),
the output voltage can swing to 40mV from the supply rail.
With moderate resistive loads (20kΩ), the output can swing
to within 75mV from the supply rails while maintaining high
open-loop gain (see the typical performance curve “Output
Voltage Swing vs Output Current”).
CAPACITIVE LOAD AND STABILITY
The OPA705 series op amps can drive up to 1000pF pure
capacitive load. Increasing the gain enhances the amplifier’s
ability to drive greater capacitive loads (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 inside the
feedback loop, as shown in Figure 5. This reduces ringing
with large capacitive loads while maintaining DC accuracy.
FIGURE 5. Series Resistor in Unity-Gain Buffer Configura-
tion Improves Capacitive Load Drive.
INPUT VOLTAGE
Device inputs are protected by ESD diodes that will conduct
if the input voltages exceed the power supplies by more than
approximately 300mV. Momentary voltages greater than
300mV beyond the power supply can be tolerated if the
current 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 always required. The OPA705
features no phase inversion when the inputs extend beyond
supplies if the input current is limited, as seen in Figure 4.
FIGURE 4. OPA705—No Phase Inversion with Inputs
Greater than the Power-Supply Voltage.
FIGURE 3. Input Current Protection for Voltages Exceeding
the Supply Voltage.
2.0V/div
V
S
= ±5.0V, V
IN
= 11Vp-p
20µs/div
APPLICATION CIRCUITS
The OPA705 series op amps are optimized for driving
medium-speed sampling data converters. Figure 6 shows the
OPA2705 in a dual-supply buffered reference configuration
for the DAC7644. The DAC7644 is a 16-bit, low-power,
quad-voltage output converter. Small size makes the combi-
nation ideal for automatic test equipment, data acquisition
systems, and other low-power space-limited applications.
R
OPA705
10mA max
+V
V–
V
IN
V
OUT
I
OVERLOAD
R
S
20Ω
OPA705
C
L
R
L
V
IN
V
OUT