INPUT VOLTAGE
CAPACITIVE LOAD AND STABILITY
The OPA703 and OPA704 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”).
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 OPA703
features no phase inversion when the inputs extend beyond
supplies if the input current is limited, as seen in Figure 4.
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.
+V
RS
20Ω
IOVERLOAD
OPA703
VOUT
10mA max
VIN
VOUT
OPA703
CL
RL
VIN
R
V–
FIGURE 3. Input Current Protection for Voltages Exceeding
the Supply Voltage.
FIGURE 5. Series Resistor in Unity-Gain Buffer Configura-
tion Improves Capacitive Load Drive.
APPLICATION CIRCUITS
VS = ±5.0V, VIN = 11Vp-p
Figure 6 shows a G = 5 non-inverting amplifier implemented
with the OPA703 and OPA704 op amps. It demonstrates the
increased speed characteristics (bandwidth, slew rate and
settling time) that can be achieved with the OPA704 family
when used in gains of five or greater. Some optimization of
feedback capacitor value may be required to achieve best
dynamic response. Circuits with closed-loop gains of less
than five should use the OPA703 family for good stability
and capacitive load drive. The OPA703 can be used in gains
greater than five, but will not provide the increased speed
benefits of the OPA704 family.
20µs/div
The OPA703 series op amps are optimized for driving
medium-speed sampling data converters. The OPA703 op
amps buffer the converter’s input capacitance and resulting
charge injection while providing signal gain.
FIGURE 4. OPA703—No Phase Inversion with Inputs
Greater than the Power-Supply Voltage.
Figure 7 shows the OPA2703 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 combination ideal for automatic test equip-
ment, data acquisition systems, and other low-power space-
limited applications.
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”).
OPA703, OPA704
11
SBOS180A
TI [ TEXAS INSTRUMENTS ]
